Weeds and Weed Control


8 Credit
Category: 1A, 1B, 1C, 2, 3A, 6

By Richard and Patricia Kaae

Part 1

Weeds are the number one agricultural pests encountered in the United States, Canada and parts of Mexico. In many of the tropical areas of the world, insects are of equal, if not more, importance as pests.  Worldwide there are approximately 250,000 species of plants in the world and most experts feel about 3 % of these (8000) are considered weeds.  Of these, 0.1%, or 250 to 300, is encountered on a daily basis.  With this in mind the question may arise as to what constitutes a weed.  Depending on frame of mind a few definitions have been coined.  One of the original definitions is “a plant out of place” or, a little later, the Weed Science Society of America defined a weed as “a plant growing where it is not desired”.    Based on these definitions a plant growing in one situation may be considered beneficial while in another it might be considered a weed.  For example a pepper plant growing in the middle of a tomato field might be considered a weed.  Basically the definition of a weed is based on human desires, or how its presence affects crop production, humans or animal health or `aesthetics.  It should be noted that the 8,000 weeds indicated above do not include plants such as pepper, but only those not normally grown for any useful aspect.  For example a plant like dandelion would occur on this list although dandelion greens are consumed by some humans.

From an ecological standpoint, weeds are considered as plants that are competitive and aggressive.   That is, they tend to exploit existing conditions, grow very fast and out-compete desirable plants for nutrients, available light, space and moisture. Weeds are persistent in that they must be managed continuously and, of course, are considered harmful because their presence results in a number of undesirable effects.

Adverse Effect of Weeds-Non-agricultural.  
The adverse effects of weeds are mant, but fall into 2 broad categories, namely agricultural and non- agricultural.  Some of the more obvious non-agricultural effects include fire hazards from dried weeds, aesthetic detractions in recreation areas such as parks, athletic fields and golf courses, obstruction of water flow in rivers (Florida spends $300 million annually in attempting to control  Hydrilla), drainage canals and other waterways and the aesthetic effects on homeowners’ gardens, landscaping and lawns. In addition weeds can have considerable effects on human and animal health.  Several of the more common of these are discussed below.

Of all the things that can cause an allergy, pollen is one of the most pervasive. Many of the foods, drugs, or animals that cause allergies can be avoided to a great extent; even insects and their by-products and household dust are not inescapable. However, short of staying indoors when the pollen count is high (and even that may not help), there is no easy way to evade wind driven pollen.

An allergy can be defined as sensitivity to a normally harmless substance, usually one that does not bother most people. The allergen (the foreign substance that provokes a reaction) can be a food, dust particles, drugs, insect venom, or mold spores, as well as pollen. Allergic people often have sensitivity to more than one substance. 

Most experts feel that people inherit a tendency to be allergic, although not to any specific allergen. Children of allergic parents are much more likely to develop allergies than other children. Even if only one parent has allergies, a child has a one in four chance of being allergic.  People with pollen allergies often develop sensitivities to other troublemakers that are present all year such as dust and mold.

Normally, the immune system functions as the body's defense against invading agents (bacteria and viruses, for instance). In most allergic reactions, however, the immune system is responding to a false alarm. When an allergic individual first comes into contact with an allergen, his or her immune system treats the allergen as an invader and mobilizes to attack. The immune system does this by generating large amounts of a type of antibody (a protein) called immunoglobulin E, or IgE. (Only small amounts of IgE are produced in nonallergic people.) Each IgE antibody is specific for one particular allergen. In the case of pollen allergy, the antibody is specific for each type of pollen: one antibody may be produced to react against oak pollen and another against ragweed pollen and so on.

These IgE molecules attach themselves to the body's mast cells, which are tissue cells and to basophils, which are cells in the blood. When the enemy allergen is again encountered the IgE, the allergen attaches to the antibody like a key fitting into a lock, signaling the cell to which the IgE is attached to release (and in some cases to produce) powerful inflammatory chemicals like histamines, prostaglandins, leukotrienes, and others. The effects of these chemicals on various parts of the body cause the symptoms of allergy.

The types of pollen that most commonly cause allergic reactions are produced by the plain-looking plants (trees, grasses, and weeds) that do not have showy flowers. These plants manufacture small, light, dry pollen granules that are custom-made for wind transport; for example, samples of ragweed pollen have been collected 400 miles out at sea and 2 miles high in the air. Because airborne pollen is carried for long distances, it does little good to rid an area of an offending plant - the pollen can drift in from many miles away. In addition, most allergenic (allergy-producing) pollen comes from plants that produce it in huge quantities - a single ragweed plant can generate a million grains of pollen a day.

Among North American plants, weeds are the most prolific producers of allergenic pollen. Ragweed (Figure 1) is the major culprit, but others of importance are sagebrush, redroot pigweed, lamb's quarters, Russian thistle (tumbleweed), and English plantain.

Grasses are important sources of allergenic pollen. Although there are more than 1,000 species of grass in North America, only a few produce highly allergenic pollen. These include timothy grass, Kentucky bluegrass, Johnson grass, Bermuda grass, redtop grass, orchard grass, and sweet vernal grass. Trees that produce allergenic pollen include oak, ash, elm, hickory, pecan, box elder, and mountain cedar.

As soon as the allergy-causing pollen lands on the mucous membranes of the nose, a chain reaction occurs that leads the mast cells in these tissues to release histamine. This powerful chemical dilates the many small blood vessels in the nose. Fluids escape through these expanded vessel walls, which causes the nasal passages to swell and results in nasal congestion. Histamine can also cause itching, irritation, and excess mucus production. Other offensive chemicals, including prostaglandins and leukotrienes, also contribute to allergic symptoms.

Some people with pollen allergy develop asthma, a serious respiratory condition. While asthma may reoccur each year during pollen season, it can eventually become chronic. The symptoms of asthma include coughing, wheezing, shortness of breath due to a narrowing of the bronchial passages, and excess mucus production. Asthma can be disabling and sometimes fatal. If wheezing and shortness of breath accompany hay fever symptoms, it is a signal that the bronchial tubes also have become involved, indicating the need for medical attention.

Poisonous Plants-Dermatitis
One effect that immediately comes to mind is irritation of the skin caused by certain weed types.  Common examples are poison oak, ivy and sumac.  The actual skin irritant occurring in these plants is uriushiol, which is a mixture of several derivatives of the chemical catechol.  This oil occurs in the sap of most of the parts of these weeds, including roots, stems, leaves and fruit.

All three plants belong to the cashew family. Poison ivy (Figure 2) grows plentifully in parts of the United States and southern Canada.  This plant will forms upright bushes if it has no support to climb but also occurs as a twining vine on tree trunks or straggling other plants over the ground.  Poison oak (Figure 3) occurs commonly in California, the Pacific Northwest and nearby regions of Canada, and poison sumac (Figure 4) is primarily an eastern United States species. Poison oak and poison sumac both are shrubs.

poison ivy plants

Figure 2.  Poison ivy in Autum.  Image courtesy of Famartin CC BY-SA 3.o


Figure 3.  Poison oak, Image courtesy of Publ;ic Domain


Figure 4.  Poison sumac.  Image courtesy of Jim Dunphy.

The irritant may brush onto the clothing when coming in contact with the plants. Many people have been poisoned merely by taking off their shoes or clothing after walking through poison ivy. In cases of extreme sensitivity, the mere presence of the plant in the general vicinity may result in skin irritations.  In addition the disease can be passed from individual to individual if the oil remains on the skin. The eruptions ((Figure 5.) themselves are not a source of infection.

Kim Hill Rash

Exposure to urioshiol. Image Courtesy Abm6868 CC BY-SA 4.0

Control and Treatment. 
If exposed to any of these plants it is important to wash thoroughly several times with soap and water.  Because the oil can be transferred by mere contact, avoid touching any part of the body, as even tiny amounts of the oil will cause irritation.  Blistering and red, itching skin may be treated with dressings of calamine lotion, Epsom salts, or bicarbonate of soda. There is a vaccine that is available (oral or injection) that is effective when taken prior to exposure.

There are a number of other weeds (for example stinging nettle) that can produce less severe dermatitis if exposed to the skin.  In some cases these weeds are much more common than those discussed above but unless allergic reactions are involved, the symptoms are much less severe and short term.

Additionally, there are a number of species of plants, including weeds that are quite toxic (even deadly) if consumed.  The question may arise as to why a plant would contain toxic materials.  Of course the answer is that these toxins are a form of natural protection against plant-eating insects, birds and any of a number of other animals.

Effects of Weeds on Agriculture. 
The total economic impact of weeds on agriculture in the United States is undoubtedly higher than most would suspect and somewhat difficult to analyze.  There are fairly accurate (but certainly not current) estimates of their effects on agricultural crops but their importance on areas such as homeowner properties, recreational areas, public lands (e.g. schools), aquatic situations, animal and human illnesses and other noncropland  situations are much less documented.  A recent survey (1998) indicated that weeds are estimated to reduce yearly crop yields by 12% or approximately $36 billion in lost revenues.  Most of these losses occur in fields (82%) and, to a lesser extent, vegetable (9%) crops.  Weeds are important but less damaging to tree crops.  Partially this is due to the fact that trees tend to “shade out” weeds” and have a deep root system, thus drawing water and soil nutrients from different areas of the soil than do most weeds. 

Besides direct losses of plant crops, weeds reduce livestock yields by competing with pasture or forage crops or by poisonous plants that cause slower growth or death of animals.  In addition many weeds serve as reservoirs of harmful insects or plant diseases.

Why Weeds Are Such a Problem? 
Part of the reason is that growing of high yielding crops tends to disrupt the natural ecosystem, thus giving weeds a competitive advantage.  It is very difficult to predict which weeds are going to be a problem from year to year or season to season.  When fields are deeply plowed, weed seeds that are on the top of the soil are buried and those that were previously deep in the soil are brought up to the surface.  Of course this results in a shift and/or mixing of the seed population and eventually in the species of weed in ensuing seasons.  In addition weed seeds or other sources of propagation are moved from field to field on farm equipment or by natural means of dispersal (discussed later).  In turf, seed dispersal becomes even more problematic, especially in homeowner situations.  Gardeners or mowers who move from one yard to another transport a tremendous amount of seeds and propagative clippings from property to property

Most of our cultivated crops are a product of selective breeding and thus can be thought of as a product of domestication.  On the other hand, weeds are wild plants and it is a well documented rule of nature that wild organisms have a competitive advantage for survival when compared to a domesticated organism.  All situations where plants grow have what is termed a carrying capacity, or how many plants can grow in that area.  The carrying capacity of a given area is limited by the availability of sunlight, space, water and soil nutrients.  In simple terms, weeds are highly competitive and typically tend to out-compete cultivated plants for these resources.

As previously stated weeds are extremely aggressive and tend to out-compete cultivated plants for available nutrients, space, water, and sunlight.  Even if weeds do not out-compete cultivated plants for these essentials of growth, they compete with weeds for each requirement and all are in limited amounts. Nitrogen, phosphorous and potassium are the primary plant nutrients, with nitrogen the most important of these. In general weeds have greater nutrient requirements and typically absorb more than most crops. Nitrogen is typically the first of these nutrients to occur in short supply in weed-crop competition. 

The presence of water is the most critical of the plant growth requirements and in many cropping situations is of limited supply.  Generally speaking weeds do not necessarily require more or less water than the crops with which they compete.  Nor do they utilize it more efficiently than crops.  However numerous experiments have indicated that weeds are typically more efficient than crop in obtaining water.  Many weeds possess deeper rooting depths and larger root mass than crops.

A big part of the reason why weeds are so competitive with cultivated plants has to do with carbohydrate production through the process of photosynthesis.  Carbohydrates are used both for production of cells (cell structure, cell walls) and as a source of energy or food.  The carbohydrates that are not used for cells structure are referred to as TNC, or total nonstructural carbohydrates.  TNC are stored in plants and could be thought of as a “saving account.”  The more TNC a plant has the better adapted it is for survival and the more competitive it is.  In any growing situation there is a set amount of resources (carrying capacity) and weeds tend to “take away” TNC from cultivated plants.  One of the reasons why weeds “take away” TNC is because most weeds utilize a more efficient photosynthetic process than do most cultivated plants.  The so called C3 type of photosynthesis produces TNS more rapidly and efficiently than does the C4 type of photosynthesis that occurs in many cultivated plants.

Weed Biology. Sexual Reproduction and Seed Biology. 
Weeds basically exhibit 2 types of survival mechanisms.  One is referred to in the botanical world as the R strategy.  With this strategy all resources are aimed toward production of massive amounts of seed.  In some cases the number of seeds per plant can be astronomical.  For example witch weed and muellin reportedly produce approximately one-half and one quarter million seeds per plant, respectively. With most annual weeds (weeds that complete life cycle in one growing season)) the main means of reproduction utilize the R strategy.  With thousand or tens of thousands of weed seeds produced per plant many more seeds are produced than are needed to sustain the species in a given environment.  As a result typically more weed seedling develop in an area than can survive resulting in eventual death of the weaker and survival of the genetically stronger individuals.  This is a means of natural selection thus producing stronger weeds which are competing with cultivated plants (the crop).

Seed Dispersal. 
Weeds would be much less of a problem if their seeds just dropped to the ground but their seeds are dispersed in a variety of ways including wind, animals, water, contaminants in crop seeds, grains, hay and straw.   Many have special adaptations that aid in their dispersal (Figure 6).  Puncturevine seeds are equipped with large strong spines that become entangled in clothing, shoes, hair or fur.  These spines are so strong that they can puncture a tire.  Of course dandelion, Canadian thistle, sowthistle and wild lettuce, to name a few, are equipped with a parachute-like apparatus, or fluffy hair, to aid in wind dispersal.  Curly dock seeds pods have bladder-like float which also aids in water dispersal in streams, irrigation ditches and other bodies of moving water.

Burclover seed pods readily cling to clothing, fur and almost every one has had numerous hare barley burs stuck in their socks after walking in a weedy field.

Every year new weed species are introduced into the United States from other countries and to other countries from the United States. Of course the most obvious avenue of the movement of weed species around the world is through shipment of various grains for human and animal consumption.  Even if weed seed contaminated grain is consumed by animals, weed seeds may survive the digestive process.  In one experiment a cow was fed 6 pounds of flax seeds containing over one million seeds.  Upon defecation the seeds found in her feces decreased in viability by about 80%. Finally, it is well established that many types of farm equipment readily moves weed seeds from field to field.

Figure 6.  Seeds From left to right- burclover, curly dock, hare barley and dandelion. Images courtesy left to right USDA. Dr. Kaae

Seed Viability, Germination and Dormancy. 
A seed is determined as viable if it is capable of germinating and producing offspring.  When compared to seeds of agriculture crop plants, weed seeds exhibit much longer periods of viability. In one experiment weed seeds of 107 different species were placed in clay pots and buried.  After 20 years the seeds of 57 different species still germinated.  After 38 years, 91%, 48% and 38% of the jimsonweed, mullein and velvetleaf seeds germinated, respectively.

A problem associated with weed control is the predictability of factors that initiate germination in many weed seeds.  These factors are fairly well established and there are models that can be used to predict germination in crop seeds. These types of models have not been successfully developed for most weed seeds, thus making weed control much more difficult as we never know exactly when certain species might germinate and become a problem.

There are a number of factors that influence how and when germination occurs, the most important of which is the availability of water.  Seeds are normally quite dry, a factor that is important in their ability to remain dormant for long periods of time.  Typically, in order for a seed to germinate, water must penetrate the seed coat (process called imbition).  The presence of this water activates enzymes that can initiate the process of germination provided that other significant parameters are present. 

Temperature is the second most important critical factor for seed germination. With each plant species there is a range of temperatures within which this process can occur.  Namely there is a minimum temperature below which a seed will not germinate and a maximum temperature above which a seed will not germinate.  Within this range there is also an optimum temperature for germination. The botanical formula Q10, indicates that for every 10 ° F increment increase within the range, the enzymatic activity that initiates germination doubles.  The presence of oxygen is also important in enzyme activity.  Seeds do not normally germinate in water-locked conditions (thus the lack of available oxygen).  The importance of light in this process is less predictable with some weeds requiring light while others require absence of light for germination. Finally some seeds produce chemicals that actually suppress the process of germination.

Seed dormancy is the failure of seeds to germinate because of factors associated with their embryo, seed coat, and/or environment.  Dormancy may determine the time of year when a seed germinates or it may delay germination for years.  Changes in soil temperature, soil water content, the percentage of oxygen or carbon dioxide in the soil and even soil disturbances, such as plowing, can cause or break dormancy in weed seeds.  Soil microbes can be of importance as they play a role in regulation of oxygen and carbon dioxide levels in the soil.  As a result, due to the complexity of these factors, it is quite difficult to predict when and if dormancy is broken and germination can occur. Of course the ability of weed seeds to go dormant is greatly beneficial to the survival of weed seeds and to our disadvantage in controlling them.

Seed Bank.  
The seed bank extends several inches into the soil but the top 1-to-3 inches of soil contains the most weed seeds. It contains an enormous reservoir of dormant and non-dormant seeds with some studies recording from 120 million to 1.4 billion seeds per acre. This area is continually changing with the addition of new seed each season and the mixing of these seeds by agricultural practices (e.g. plowing). In addition the number of seeds in the bank is continuously depleted due to any of a number of factors, including germination, decay and subsequent death, predation by insects, birds and other organisms, parasitism by microorganisms and the accumulation of thatch on the soil surface.  Thatch is the accumulation of materials like leaf litter on the soil surface and does not have sufficient integrity to support the formation of roots. Even though these factors do cause a depletion of the seed bank they will rarely result in significant effects on ensuing weed populations.  Of course unless weeds are controlled they produce far more seeds than are needed to replenish those seeds that are depleted by natural means.

Vegetative or Asexual Reproduction. 
Most perennial weeds (weeds that require 2 or more growing seasons to complete life cycle) reproduce by vegetative means (referred to as the K strategy) (asexual) as well as seed.  Most plants that spread by vegetative means do so quite slowly.  For example weeds such as quackgrass, field bindweed, Johnsongrass, and Bermudagrass if only utilized vegetative reproduction without human intervention would only spread less than 10 feet a year.  However when we start disking, mowing, tilling fields and cutting asexual structures into small parts, each of these segments is typically capable of rooting and forming new plants and ensuing spread is greatly increased.

Some perennial weeds reproduce by both vegetative and sexual means which is unfortunate from the standpoint of weed control.  Simple and creeping perennials also utilize seed production which may or may not be of significant importance depending on the species.  Slender speedwell, brackenfern, field horsetail and Canadian thistle rarely or do not produce seeds.  On the other hand yarrow, redtop, field bindweed, leafy spurge, creeping buttercup, curly dock, common nettle rely on seeds as well as vegetative means for reproduction.

Vegetative means of reproduction includes stolons, rhizomes, tubers, roots, bulbs, and bulblets (Figure 7).  A rhizome is a root-like structure that originates from the main root and typically travels below ground forming new shoots and roots at various intervals.  These structures are typically found in quackgrass, horsetail, redtop and hedge bindweed.  On the other hand a stolon is similar to a rhizome but is distinguished by originating from the stem and traveling above ground to form new roots and shoots. Stolons are found in such common weeds as Bermudagrass, yarrow, creeping bent and mouse-eared chickweed.  Bulbs are common in wild onion and garlic.  Of course bulbs and bublets reproduce by duplication.  Dandelion of course can reproduce vegatatively by its deep root system.  Weeds which utilize vegetative reproductive strategies are especially difficult to control with herbicides.  In such cases, successful control is primarily based on using a chemical that is systemic in action and is translocated to the below ground structures.


Figure 7.  Various asexual reproductive organs Left to right rhizome Frank Vincentz GFDL, Stolon JonRichfield CC BY-SA 3.0

Weed Control. 
Weed control strategies can vary form totally organic to relying to a great extent on chemicals.  The option or variation of an options depends on a number of factors including cropping vs. non-cropping, size and destination of a crop and variations of a crop.  There is little doubt that the use of nonchemical techniques can be an extremely important part of any overall approach and some of the most successful approaches to weed control rely on the use of both of these broad categories.  Some of the more important nonchemical techniques are discussed below.

Mechanical and Cultural Control. 
There are a variety of techniques that can be classified as mechanical, including the use of heat or burning but pulling and hoeing of weeds are the most common. Annuals and tap-rooted plants are particularly susceptible to control by hand-pulling and hoeing; however, perennials with deep tap roots or rhizomes are difficult to control with these techniques as some of these parts are frequently left behind and will readily resprout. There are a number of tools that are available.  Weed wrenches and other tools are surprisingly powerful and with their use, large saplings and shrubs that are too big to be pulled by hand can be removed; however, these tools are not as effective against many perennial weeds with deep underground stems and roots. 

Soil disturbance can be minimized by pulling out weeds slowly and carefully, and replacing soil to disturbed areas wherever possible.  Trampling can be reduced by limiting the number of people in the site and the amount of time spent there.  Whenever a manual technique is used, it is wise to wear gloves, a long-sleeved shirt, and long pants.  Some plants can cause moderate to severe skin irritation, especially when their stems and leaves are crushed and broken.  Even the flimsiest weeds can leave hands raw and bleeding after several hours of pulling.

The advantages of pulling and hoeing include its minimal ecological impact, minimal damage to neighboring plants, and low (or no) cost for equipment or supplies.  Pulling is extremely labor intensive and is effective only for relatively small areas, even when abundant volunteer labor is available.

Reduce Weed Seed Bank or Other Source of Weed Seeds.  
As previously indicated, limiting the size of the weed seed bank is of extreme important in any weed control strategy.  Although it is essentially impossible to eliminate this primary source of infestation there are a number of techniques that can be used to reduce the size of this area.  Of most importance in cropping or noncropping situations, keeping any existing weeds from going to seed by using any available chemical or nonchemical technique is essential.

If manure is used it should be composted to destroy any weed seed or, better yet, avoid the use of manure as it is likely to be full of weed seeds.  Seeds can stay viable even after passing through the gut of cattle or birds.

It is important in many cropping situations to reduce the influx of weed by mowing or harrowing.  Finally weed seeds can easily be brought into a field, orchards or even lawns by agricultural equipment and lawn mowers.  Such equipment should be power washed, especially when used in an area heavily infested with noxious weeds.  Similarly weed seed can be introduced into a field by irrigation water areas where irrigation canals and ditch-banks are a source.  This water can be screened to remove a possible source.

A concept that may seem unnecessary to point out is to avoid whenever possible growing in fields that have a history of excessive weed problems.  In these cases, the seed banks may be too large to overcome.  This may be difficult in many areas where agricultural land is at a high premium, but this capability does exist in other places.  For example, one of the professors at Cal Poly is involved in a rather large organic farming operation in northern Mexico.  When asked how he manages the weed problem the answer is “when it gets to be a problem, we move to other fields, some of which have never been used for agriculture.”

Rotate Crops When Possible. 
Different crops favor the growth of different weeds.  If weeds and cultivated crops belong to the same family, they will all favor and thrive under the same or similar environmental conditions (soil moisture, temperature, fertility and even the cultural practices used on the crop). For example tomatoes, potatoes, eggplants and peppers all belong to the nightshade family, as do black nightshade and horsenettle, both of which are important weed pests of these crops.  As a result, it is worth considering rotating between crops belonging to different families.  As a consequence, the weeds associated with any one crop will not be continuously exposed to favorable conditions and the resulting seed bank will be minimized.

Another factor to consider when using crop rotation for weed control purposes is crop rooting depth. Shallow-rooted crops are less competitive with weeds for soil moisture and nutrients than deep rooted crops.  As a result a degree of weed control can be accomplished by rotating between crops with shallow and then deep rooted systems. 

Enhancing Crop Competition with Weeds. 
Crops that exhibit rapid growth and produce a dense canopy will shade out and starve weeds for nutrients. For example buckwheat, sorghum, Sudangrass, or millet work well in the summer heat while ryegrass, oats and other small grains provide fall cover and winter erosion and then the ground can be reworked to plow under weed escapees and subsequently planted to a summer crop.  With cover crops it is worthwhile to sow at high rates, drill the seed and even irrigate if necessarily to assure the establishment of thick stands and rapid establishment.

Planting schemes can be manipulated to enhance crop competition with weeds. The use of varieties that are adapted or developed for a specific area often grow more rapidly and tolerate pests better than those that are not. In addition crops should be planted at a time when they will grow most rapidly and consequently competed with weeds for sunlight water, nutrients and space.

Fertilize the Crops Not the Weeds. 
It is important to apply fertilizers at a time or location that benefits the crop rather than weeds.  High fertility levels, particularly nitrates and nitrites, can stimulate germination of some dormant weed seeds. Pre-plant broadcasted soluble nutrients may be more readily available to fast growing weeds than the slower growing crop and should be avoided.  In turf fertilization should be avoided during those times when peak weed growth occurs. If possible fertilizers should be positioned where they are most likely to be captured by the crop rather than weeds.  This can be accomplished by side-dressing and any of a variety of other techniques.

Cultivate When Possible. 
There are a variety of cultivating techniques that can add to an effective weed control program.  Blind “over the top” cultivation refers to shallowly working the entire surface of a field with flex-tine cultivators or rotary hoes.  This is usually done just after small weed emerge and before and sometimes after planting.  Timing is obviously important and the success of this technique partially relies on the quick germination of annual weeds as opposed to many crops.

Shallow between-row cultivators such as basket-weeders, beet-hoes or small harp sweeps can be use to cut off or digs up small weed once crops have emerged. These can get very close to the crop when it is small without moving much soil into the row. Once the crop is growing vigorously and becomes somewhat larger and soil can be thrown into the row and bury in-row weeds using a variety of cultivators including rolling cultivators, large sweeps and hilling disks.

Mulching. Mulching is one of the most important ways to maintain healthy landscape plants. A mulch is any material applied to the soil surface for protection or improvement of the area covered. Mulching is really nature’s idea. Nature produces large quantities of mulch all the time with fallen leaves, needles, twigs, pieces of bark, spent flower blossoms, fallen fruit and other organic material.

Benefits of Mulching

o        When applied correctly, mulch has the following beneficial effects on plants and soil:

o        They prevent loss of water from the soil by evaporation.

o        They reduce the growth of weeds, when the mulch material itself is weed-free and applied deeply enough to prevent weed germination or to smother existing weeds.

o        Mulches keep the soil cooler in the summer and warmer in the winter, thus maintaining a more even soil temperature.

o        prevent soil splashing, which not only stops erosion but keeps soil-borne diseases from splashing up onto the plants.

o        Organic mulches can improve the soil structure. As the mulch decays, the material becomes topsoil. Decaying mulch also adds nutrients to the soil.

o        Mulches prevent crusting of the soil surface, thus improving the absorption and movement of water into the soil.

o        Mulches prevent the trunks of trees and shrubs from damage by lawn equipment.

o        Mulches help prevent soil compaction.

o        Mulches can add to the beauty of the landscape by providing a cover of uniform color and interesting texture to the surface.

o        Mulched plants have more roots than plants that are not mulched, because mulched plants will produce additional roots in the mulch that surrounds them.

Types of Mulches. 
There are basically two types of mulches: organic and inorganic. Both types may have their place in the garden.

An organic mulch is a mulch made of natural substances such as bark, wood chips, leaves, pine needles, or grass clippings. Organic mulches attract insects, slugs, cutworms and the birds that eat them. They decompose over time and need to be replaced after several years.  Inorganic mulches, such as gravel, pebbles, black plastic and landscape fabrics, do not attract pests and they do not decompose.

Mulch Materials

Organic Mulch Materials. Your yard "trash" can be recycled as mulch with the advantage of retaining the nutrients found in these organic materials, in addition to saving money otherwise spent in transporting and disposing of the yard trash.

Grass Clippings . 
The best use for grass clippings is to leave them on the lawn. Grass clippings will decompose rapidly, adding nutrients back into the soil. A two-inch layer of grass clippings provides weed control if they are not full of weed seeds. It is best to build up the layer gradually using dry grass, not fresh clippings, to prevent the formation of a solid mat. Be careful not to use clippings from lawns that have been treated with herbicides.

Hay and Straw . 
Never use hay for mulch since it contains too many weed seeds. Straw decomposes rapidly, so you will have to replenish it to keep the weeds down. Straw is not very ornamental and is best for a vegetable garden or over newly sown lawns. Straw will improve the soil as it decays.

Leaf Mold . 

Leaf mold has a tendency to form a crust, preventing water from penetrating into the soil. It is better to use leaf mold as a soil amendment than as a mulch.

Leaves .
A 2- to 3- inch layer of leaves provides good weed control. It is best to shred the leaves coarsely, using a shredder or your lawn mower. Whole leaves have a tendency to blow away, while finely shredded leaves do not allow water to penetrate. Oak and beech leaves help to acidify the soil for acid-loving plants. Leaves are usually easy to get, attractive as a mulch, and they will improve the soil once they decompose. After the leaves decompose, dig them into the soil and add a new layer of mulch on top.

Pine Bark .
A 2- to 3- inch layer of pine bark is good for weed control. Pine bark makes an attractive, usually dark-colored mulch. It can be purchased in various particle sizes, from shredded to large-sized particles, called nuggets. Large pine bark nuggets float in water and may not stay in place during a heavy rain. They may also attract termites and other insects.

Pine Needles .
A 2- inch layer of pine needles makes an excellent mulch for acid-loving trees and shrubs. This mulch is very attractive and allows water to penetrate easily.

Shredded Hardwood Mulch 
This mulch is good at suppressing weeds. It does not wash away easily. It decomposes relatively slowly, and it is very attractive.

Wood Chips. 
This material contains bark and pieces of wood of various sizes and makes an attractive mulch. A 2- to 3- inch layer of wood chips provides good weed control. Small wood chips decompose very rapidly using nitrogen from the soil, which needs to be replaced by nitrogen fertilizer. Wood chips may attract termites and other insects.

Pecan Shells . 
Pecan shells make a long-lasting, attractive, dark brown mulch that is effective in retaining moisture in the soil. Availability is usually limited to areas where pecans are processed.

Ground Cover . 
Many perennial ground cover plants, such as ivy, periwinkle, pachysandra, mondo grass and liriope, will cover the soil and act as a mulch.

Inorganic Mulch Materials:

Gravel, Pebbles and Crushed Stone 
These materials are permanent and are best used for permanent plantings such as foundation plants. A 1- inch layer of small rocks will provide good weed control. Do not use them around acid-loving plants since the rocks may add alkaline elements and minerals to the soil. These materials reflect solar radiation and can create a very hot landscape environment during the summer months.

Black Plastic.  
Black polyethylene film is very effective in preventing weed growth. It also holds water in the soil. Therefore, plastic is not recommended for poorly-drained areas as it may cause the soil to remain too wet, which could result in root disease problems. You may have to cut holes in the plastic if water does not go through it. There is black plastic available that has small holes in it to help with drainage. If exposed to sunlight, black plastic is broken down fast, losing its effectiveness as a mulch. However, if you bury black plastic in the soil, it will last for many years. Covering the black plastic with a layer of wood chips or pine needles will reduce heat absorption and mask its artificial appearance.

Clear Plastic. 
Clear plastic will not suppress weed growth because light penetrates the film and raises the soil temperature, which may result in an increased growth of weeds in early spring.

Landscape Cloth or Woven Ground Cloth. 
Materials woven of fabric, plastic or paper are available in various lengths and widths. The materials are treated to resist decomposition. Unlike plastic films, woven materials allow water and air to move through them. They are very effective in controlling most weeds, although some grasses may grow up through the holes in the fabric. Landscape cloth needs to be fastened down so it will not be pushed up by perennial weeds. Better moisture, temperature and weed control will be obtained by adding several inches of another mulching material on top of the landscape cloth.

Aluminum-coated plastic and foil. 
One layer of either one of these materials provides excellent weed control. These materials decompose very slowly, but they are very expensive and quite unattractive mulches.

Ground Rubber Tires. 
Mulches made of ground rubber tires do not decompose and therefore, never need to be replaced. The use of ground rubber tires is relatively new and its effectiveness as a mulch is still being evaluated.

Where to Use Mulch. Mulching is a very important practice for establishing new plantings, because it helps to conserve moisture in the root ball of the new plant until the roots have grown out into the surrounding soil. The growth rate and health of trees and shrubs increases when there is no competition for water and nutrients from weeds. Mulch also helps to prevent tree trunk injury by mowers and trimmers. Newly planted trees require a circle of mulch 3 to 4 feet in diameter. Maintain this for five years. Mulch entire beds of shrubs, trees, annuals, herbaceous perennials and ground covers.

Mulch can also be used to cover trails, driveways, and play and natural areas.

Light-weight mulch such as dried grass clippings and pine straw can be used temporarily to cover low-growing tender plants to protect them from frost injury.

When and How Often to Mulch. 
The best time to mulch new plantings is right after you plant them. Around established plants mulch is best applied in early spring. This is when plants are beginning to grow and before weed seeds start to germinate. How often mulch needs to be replenished depends on the mulching material. Grass clippings and leaves decompose very fast and need to be replenished frequently. Inorganic mulches such as gravel and pebbles rarely need replenishing. As the plants grow and fill in the bed areas, less and less mulch is needed.

How to Apply Mulch. 
Before applying any type of mulch to an area, it is best to weed the area. Spread a layer of mulching materials over the entire plant bed. Keep mulch 2 to 3 inches away from the stems of woody plants. This will prevent decay caused by wet mulch and rodent damage during the winter. Keep mulch 6 to 12 inches away from the walls of buildings.. Subterranean termites nest in the soil and feed on materials that contain cellulose. Termite treatments are applied to the soil around buildings, so keeping mulch away from walls will prevent termites from using it as a bridge to cross treated soil.. Newly planted trees require a circle of mulch 3 to 4 feet in diameter. Maintain this for at least three years. Do not pile mulch against the trunk. For established trees in lawns create a circle of mulch about 2 feet in diameter for each inch of trunk diameter. Increase the size of the mulched area as the tree grows. Try to apply the mulch at least 6 to 12 inches beyond the drip-line of the tree. Because the root system can extend two to three times the crown spread of the tree, mulch as large an area as possible.

How Deep to Mulch. 
The amount of mulch to apply depends on the texture and density of the mulch material. Many wood and bark mulches are composed of fine particles and should not be more than 2 to 3 inches deep. Excessive amounts of these fine-textured mulches can suffocate plant roots, resulting in yellowing of the leaves and poor growth.. Coarse-textured mulches such as pine bark nuggets allow good air movement through them and can be as deep as 4 inches..Mulches composed of grass clippings or shredded leaves should never be deeper than 2 inches, because these materials tend to mat together, restricting the water and air supply to plant roots.

How to Calculate the Amount of Mulch Needed. 
To determine how many cubic feet of mulch is needed, you need to calculate the surface area and the desired depth of coverage. There are 27 cubic feet in a cubic yard. One cubic yard will cover a 324-square-foot area with an inch of mulch. Figure out the square footage of your bed, that is the width times the length for square or rectangular shaped beds. The square footage of a circular bed is the distance from the middle of the circle to the outside, multiplied by itself and then multiplied by 3.14 (which is pi).. Multiply your square footage by the depth desired (in inches) and divide by 324 square feet. This will tell you how many cubic yards you will need.

Mulch Toxicity-Though mulch benefits plants, "sour" mulch can quickly damage plant tissue and lower the soil pH causing injury or death. Bedding and low-growing woody plants are most easily damaged. Symptoms include yellowing of the leaf margins, scorching or dropping of leaves and occasionally entire plant death. Although it may be several days before symptoms appear, spreading sour mulch can damage plants immediately. Sour or "acid" mulch is caused by poor handling or storing of mulch resulting in anaerobic (without air) conditions. Mulch piles need to "breathe" to prevent anaerobic conditions from occurring. In the absence of air, microbes in the mulch (mostly bacteria) produce toxic substances such as methanol, acetic acid, ammonia gas, and hydrogen sulfide gas. Sour mulch smells like vinegar, ammonia, sulfur or silage. Good mulch smells like freshly cut wood or has the earthy smell of a good garden soil. Another way to determine if mulch is sour is to test its pH. Toxic mulch will have a pH of 1.8 to 2.5..To prevent mulch from turning sour or to cure sour mulch, you need to turn your pile once or twice a month, more frequently if the pile is very wet. Do not let the pile get larger than 4 feet thick in any dimension if you are not turning the pile regularly. A good aeration will eliminate the toxic compounds in 24 hours, but to be safe allow three days.

Occasionally, micro- organisms in mulches can become a nuisance. 
The shotgun or artillery fungus (Sphaerobolus) may cause serious problems. While it decays the mulch, it also produces fruiting structures that resemble tiny cream or orange-brown cups that hold a spore mass resembling a tiny black egg (1/10 inch in diameter). This fungus shoots these spore masses high into the air. They stick to any surface and resemble small tar spots on leaves of plants, on cars or on the siding of homes. They are very difficult to remove.

To avoid damage to cars and houses do not use mulches that contain cellulose (wood). Use pure bark mulches, especially pine, or if the mulch is already in place, cover the hardwood mulch with pine needles.

Slime molds are another type of nuisance fungus. They first appear as bright yellow or orange slimy masses that may be several inches to a foot or more in diameter. They are harmless but unsightly. Some fungi in mulches produce toad stools (mushrooms), and some of these are toxic to humans. It is a good idea to destroy them when small children have access to the mulched area.

Plastic mulch has been used for growing vegetables for several decades. Some of the benefits of plastic mulch include earliness, reduced weed pressure in the crop bed, reduced evaporation from the soil surface, reduced splashing of soil onto crops, etc. Plastic mulch is now a component of plasticultre crop production. Plasticulture crop production often uses several different tools to increase production. The most common products used are plastic mulch and drip irrigation. Although, other products like floating rowcover, low tunnels, high tunnels, etc. are all used.

Melons are one of the crops that often show a big response to the use of plastic mulch. Black plastic mulch is most commonly used but, clear, white and other colors are common. Plastic mulch helps increase soil temperatures early in the growing season which benefits warm season crops like melons. Different mulches warm the soil in different ways. Black plastic warms the soil through direct contact. The sun heats up the black plastic and where the plastic touches the soil that heat is transferred. Areas where there is a gap between the soil and the plastic heat much slower. The air gap acts like insulation. The air must be heated before the heat is transferred to the soil.

Clear plastic mulch warms soils the most. The clear plastic allows the suns rays to shine through directly warming the soil. The plastic helps to hold the heat. Soils under clear plastic can be 5 degrees, or more, warmer than bare soil in the same field. The disadvantage of clear plastic is that the sunlight passing through the mulch also allows weeds to grow. A crop that will quickly cover the plastic can reduce weed growth. Or, some herbicides can be used underneath the plastic to reduce weed growth.

Another option is the use of wavelength selective mulches. The wavelength selective mulches try to combine the benefits of both black and clear plastic mulch. The idea is that the wavelengths that warm the soil pass through the mulch but the wavelengths that allow plant growth are blocked by the mulch. These products fall in-between clear and black plastic in their soil warming properties. They will stop most weeds from growing but there a few weed species that will grow under them. These products are often green or brown in color.

Other colors of mulch. 
There have been several studies looking at different colors, red, yellow, green, blue, etc., for different crops. Some studies have found that different crops do best with certain colors. The results have been variable. One of the problems is that there haven’t been standard colors established. Red is not always red! There is red, pink, rose and of course....scarlet! There are also white mulches to help cool soils during hot periods. To avoid weed growth under white mulches grower should try white on black mulch. There are also reflective mulches which may be used to help insect and disease control. These products work to confuse virus vectors like aphids. These products can be effective. But, only as long as the crop canopy doesn’t cover them.

Other commercial mulch products include photodegradable mulch that is broken down by sunlight. These types of mulch have met with mixed results because of erratic breakdown. Sometimes they break at the desired time. Other times they may break down too early or too late. Also, the buried edges will not break down until they are exposed to sunlight. There is also work being done to develop paper mulch. This would be a big benefit because it could be worked into the ground instead of having to be picked up after the growing season. So far paper mulch has not been perfected. The product we tested was difficult to lay, often tearing. Also, the roll is much larger in diameter and heavier than plastic. Hopefully, a paper mulch product will be on the market soon.

As with all things there are some down sides to plastic mulch. The biggest problem is removal from the field and disposal. Plastic can be difficult to pick up and is quite dirty which limits is ability to be recycled. Along with potential contamination by pesticides. The most common means of disposal is the landfill.

While plastic mulch can inhibit weed growth it also can create some difficult to control weed problems. Weeds tend to grow through holes in the plastic. Yellow nutsedge will grow right through plastic mulch. Weeds also tend to grow right along the edge of the plastic where they escape mechanical cultivation. Some herbicides can be used under plastic mulch while other cannot.

If there were one sure way to destroy virtually every kind of harmful insect egg and larvae in your garden soil, would you be interested? How about if the process were easy, cheap, and carried a host of other benefits along with it?  Solarization is a simple, five-step process that kills insects, plant diseases, nematodes, harmful fungi, and weed seeds. At the same time, helpful microorganisms within the soil apparently benefit, possibly form the lack of competition. Soil that has been solarized allows plants to draw on the nutrients, especially nitrogen, calcium, and magnesium more readily. Seeds germinate more quickly. Plants grow faster and stronger, often maturing earlier with substantially higher yields than in unsolarized soil.

Chemical Control

Herbicide Absorption and Movement in Plants. In order for an herbicide to kill a weed (or crop as far as that goes) it must be absorpted or move into the living plant parts.  In the case of postemergence (applied after the weeds have emerged) herbicides absorption typically occurs through the leaves and, to a lesser extent, other above ground parts of the plant.  Once absorbed the herbicide may remain localized (e.g. limited movement in the leaves) or in the case of systemic herbicides may translocate to the roots via the phloem vessels (vessels that carry the products of photosynthesis to the roots).  This movement is referred to as apoplastic movement.  There are a variety of mulches available but all fall into two categories, namely natural and synthetic. Examples of materials that are used as natural mulches are cardboard and other paper materials, dried leaves, chipped or shredded bark, soil, compost and straw.  The advantage of using natural mulches is that they add nutrients and organic matter to the soil and are relatively inexpensive and environmentally friendly. There use is most effective against those weeds that require light for germination and typically are not effective against perennials. A rather thick layer (4 inches or more) of these materials is required for maximum effect. The disadvantages are that they are bulky and difficult to apply over a large area.

There are a number of factors that can influence the degrees of postemergence herbicide absorption via the plant parts and their subsequent effect on weeds. Some of these include surface tension of the spray solution (inherent in formulation of the product), droplet size and carrier volume of the application and texture, waxiness, hairiness and other characteristics of the leaf surface and leaf orientation in respect to application of a herbicide.  Reduced effectiveness of an application do to lack of absorption of an herbicide due to any of these factors can typically be avoided if label directions are closely followed, namely are the target weeds listed on the herbicide label weeds, and are they in a susceptible stage of growth and were the proper dilution and amount of chemical used. Loss of effectiveness can typically be avoided if label directions are closely followed: however, there are some circumstances that cannot be avoided. 

With preemergence herbicides, absorption occurs primarily in the roots and subsequently is typically carried in the xylem vessels to the leaves.   Xylem vessels in plants function to carry water from the roots to above ground plant parts.  Root absorption can also occur with postemergence herbicides if the herbicides reach the roots in the soil.

Selectivity of Herbicides. 
The use of some herbicides results in a great degree of selectivity (only killing certain types of plants) while others like Roundup are extremely nonselective.  Selectivity with herbicides can be associated with any of several reasons.  These are discussed below:

Placement of herbicides.  
This technique is based on applying the herbicide (which may be harmful to a crop) at a time when the crop is not present or in a location where it is not harmful to the crop.  The term preplant application refers applying an herbicide weeks or even months prior to when the crop is planted.  In some cases this can even be accomplished prior to when a seed bed is prepared.  In these cases the crop may have a degree of tolerance to a particular herbicide but due the natural degradation (break down) of the herbicide over time it will typically have little effect on the crop.

Residual preemergent herbicides that are absorbed by the roots can be placed in soil so they affect weeds but not specific crops.  A classical example would be applying an herbicide on the soil surface or at shallow levels in the soil while the seeds of annual crops or roots of perennial crops remain below the herbicide zone.  Obvious this type of application would have little effect on most tree crops, woody ornamentals, coffee and cotton. 

Herbicide Metabolic Difference between Weeds and Crops.  
In many cases there are distinct differences in how weeds and crop metabolize a given herbicide.  In some cases certain crops possess inherent mechanisms that detoxify or break down certain herbicides while weeds do not.  With genetic engineering scientists have also developed the ability to introduce certain genes into a crop’s genetic makeup that increases its ability to withstand the presence of certain herbicides.  A great example is Roundup Ready Soybeans.  In this case it is a genetically engineered variety of soybeans that can withstand significant levels of Roundup, a normally nonselective herbicide.

Anatomical Differences of Crops and Weeds.  Crops and weeds obviously differ drastically in size, shape, texture, root structure and any of a number of different ways.  In many situations these differences can be used to attain a degree of selectivity with herbicide use.  For example the leaf surfaces of certain crops differ drastically from than that of certain weeds.  For example the crop may have a waxy or hairy surface that tends to repel spray applications of postemergence herbicides thus preventing absorption of the chemical by the leaves.  Examples of a waxy leaf surface would include onions, certain cereals, cole crops, peas and conifers. 

In addition difference in leaf shape, size and orientation between crops and weed can result in desired selectivity.  For example the difference in leaf structure of broadleaf dicots growing in monocots (grains, turf) can result in selectivity.  Monocot leaves differ drastically from broad leaf plants in orientation, size shape, and waxyness.  These differences basically result in less contact and therefore less absorption of an applied herbicide.

Differences in Crop and Weed Susceptibility to Herbicides at Different Stages of Growth.  
In some cases selectivity can be obtained by applying a particular herbicide at a time when a crop is in a stage of dormancy or little growth while weeds are actively growing and therefore more susceptible.  In many cases this may be a matter of age difference between weeds and a crop.  Many annual crops become more tolerant to herbicides as they mature.  A good example would be tomatoes which become considerably tolerant to certain herbicides once they develop several leaves while associated weeds are still succulent and readily absorb and translocate these herbicides.

Absorbents Applied to Crops. 
Activated charcoal when applied to the roots of certain transplants (e.g. strawberries) or in narrow bands over rows of seedlings will confer a degree of selectivity. The charcoal works by absorbing herbicides prior to reaching the roots of these crops.   

Top Reasons Why Herbicide Applications Fail.

It should be emphasized that the pesticide (herbicide) label is the overwhelming source of information as to what a particular product is capable of performing.  In order to develop that information on the labels it takes at least 10 million dollars worth of research and 10 years to do so.  All the information as to what the product will control, how it should be applied, which environmental factor will affect its success and so on is backed by this extensive research.  Suffice it to say that the best chance of attaining maximum effect from any particular product is to follow the directions on the label.

Preemergence Herbicides-#1 Reason. 
The most common reason why applications of preemergence herbicides fail is that existing weeds were present prior to application of the chemical.  While a few preemergence herbicides have some effect on emerged weeds (e.g. Goal, Ronstar, Casoron, Kerb) most don’t. Many of the preemergence herbicides work by inhibiting root growth.  Even small weeds may have developed a deep tap root that extends well below the top 1 or 2 inches where soil applied preemergence herbicides should be placed for effectiveness against germinating weed seeds.  Generally speaking preemergence herbicides should be applied soon after planting.  Many weeds germinate within days of receiving optimal environmental conditions (water, heat, etc.) and many weeds can germinate within several days.

#2 Reason. 
Improper calibration of application equipment is a close second in reasons why these applications fail.  Regular calibration is needed to insure the right amount of chemical is applied.  Clean, properly operating nozzles all of which emit the same amount of chemical are essential.  If one nozzle emits more chemical that other streaking may result with too much chemical applied in one area and less than effective amounts too others.

Although calibration of granular applicators is simple uniform applications can still be difficult.  One method of attaining more uniform applications with this type of equipment is to apply the total amount of recommended chemical to an area in 2 or 3 passes.

#3 Reason. 
Topdressing fertilizers over or with herbicides leads to poor control. It is thought that high concentrations of nitrogen can lead to increased microbial actives and ensuing herbicide degradation.  There are also some indications that increased salinity may lead to reduced herbicidal activity.

#4 Reason.  
Proper sanitation is essential for efficient chemical weed control.  It should be remembered that some weeds produce tens of thousands to hundreds of thousands of seeds.  If sanitation and other control techniques are ignored, chemical control may not work.  Even under the best of circumstances, utilizing the most effective preemergent at the right time and proper application, the chemical barrier will not be perfect.  There are always small areas where the barrier is not sufficient to attain control and the chemicals immediately begin to breakdown after application and continue to degrade over time.  The point is, if a huge seed bank has developed due to poor sanitation, some seeds will undoubtedly find the weak areas of the herbicide barrier.

#5 Reason. 
With preemergents, when the proper application rate is applied and all other factors are considered and followed, there will be a concentration of the chemical in the soil that will inhibit weed growth for 3-to-5 months (depending on the product used). However, if lower than recommended rates are used, there will obviously be less chemical in the soil which may or may not initially control the desired weeds, but will degrade more quickly to the level where control is no longer effective.

#6 Reason. 
Overwatering, especially in container production can lead to reduced herbicide activity.  The exact reason for this is not well documented but could occur to any of a number of different reasons, including leaching of the chemical from the upper areas of the soil, increased microbial activity and increased weed viability.

#7 Reason
Failure to properly incorporate the herbicide into the soil is another possible reason.  This is especially a problem with drip irrigation and other forms of micro-irrigation in large containers.  Both of these do not supply adequate moisture over the entire soil surface to be effective.  Finally the use of rainfall as a means of incorporation is listed on some product labels.  Ideally, enough rainfall at one time will function to properly incorporate the herbicide.  But if the same amount occurs over a few weeks and the soil dies between these episodes, the herbicide will tightly bind to the top layer of the soil (especially sandy soil) and subsequently will then not move to the lower layer where the seeds are mostly found.

#8 Reason.  
Sometimes a combination of herbicides is needed to control all weed species present.  No single preemergence herbicide controls all weeds.

#9 Reason.  
Disruption of the chemical barrier can lead to lack of control with preemergents.  Most seeds occur in the top 1-inch of the soil and these chemicals control the weeds as they germinate within the chemical barrier.  Simple activities such as walking or dragging something across a treated field will readily break this barrier.

#10 Reason. 
Temperature has a critical effect on a few herbicides.  Casoron, Treflan and Kerb are examples which do better in cool season and won’t last long into summer unless incorporated into the soil. Very high temperatures may possibly reduce the longevity of all preemerence herbicides as they speed up the biological and physical processes degradation in the soil.

#11 Reason. 
Soil characteristics can affect the effectiveness of some preemergence herbicides. Some require higher application rates in heavy clay soil because the chemicals are more strongly bound by the soil.

Postemergence Herbicides

#1 Reason. 
The reasons listed above in numbers 2, 4 and 8 hold true for postemergence herbicides.

#2 Reason. 
Climatic factors can influence the effectiveness of some postemergence herbicides.  Rainfall or sprinkler irrigation too soon after application can severely decrease control.  Most postemergence herbicides are absorbed through the leaves and require some degree of rain-free period subsequent to application.  The required period varies depending on the product.  For example Roundup requires about 6 hours while Fusilade and Vantage only require one hour.  Once an herbicide is absorbed by a weed, cool cloudy weather conditions may slow down the effectiveness of these chemicals.

#3 Reason.  
Adequate leaf coverage is essential for successful control.  Besides the characteristics of the leaf coverage is influenced mainly by the volume of the spray and the presence of surfactants which may or may not be required by the label.  The require volume will vary depending on the product and can vary considerable.  For example Roundup, Vantage and Fusilade may be used in low volume applications (10 to 20 GPA) while products like Finale and Paraquat require high volume applications (20 to 60 GPA).

#4 Reason. 
Weed characteristics vary and can influence the effectiveness of control.  The rate and timing of the application has to match the targeted weed problem. Label directions typically specify the rate of application based on which weeds are present.  Annuals and small weeds may be controlled with lower rates than perennials and large weeds. The stage of growth can also influence the success of control.  For example Roundup generally kills annual weeds at any stage of growth but perennials are best controlled close to the flowering stage.

Classification of Herbicides by Family Groups or Chemical Structure and Mode of Action

Herbicides can be classified in any of a number of different ways including mode of action, chemical structure, and application method to name a few. Most scientists, agronomists and others who work with herbicides consider  mode of action the most useful and easiest to understand although a deep analysis of some of these modes is quite confusing and beyond the realm of this synopsis.  The mode of action as defined in its simplest terms means how an herbicide kills plants.  It should be remembered that the exact mode of action of some chemicals is not completely understood and that some herbicide have more than one mode.  In the case of the former where exact modes are not know these chemical are grouped on what is known and in all cases chemicals are grouped on their primary mode as discussed below.  Following the discussion of each mode of action are listed some of the common products exhibit that particular mode.  It should be kept in mind that these listing are by no means a recommendation of chemical or product use.

Photosynthesis Inhibitors. 
In the presence of sunlight, green plants produce sugar from carbon dioxide and water by the process called photosynthesis. Photosynthesis is a two phase process that occurs in the leave’s chloroplasts (structures where chlorophyll is found).  During the light dependent phase (when light is present), the plant transforms the energy from sunlight into biological energy in the form of 2 chemicals, namely ATP and NADPH2.    In the light independent phase these chemicals supply energy for the conversion of carbon dioxide into sugars.  Plants subsequently convert these sugars in more complex carbohydrates which in essence are the main food supply of plants.  Herbicides that inhibit photosynthesis essentially prevent the formation of ATP and NADPH2 thus cutting off the food supply to the plant.  However when these type of herbicides are used symptoms occur to quickly to be totally caused by the slow process of starvation of the plant.  It is thought that chlorosis (see below) of the plant is due to photo-destruction (damage from excessive light) of chlorophyll and other plant pigments.  It is thought that photosynthesis inhibitors block the transfer of energy from the chloroplast to ATP and NADPH2.  Thus chloroplasts self destruct due to the absorption of excessive energy (sunlight) and form a radical type of oxygen that is highly destructive to cell membranes and other cell structures.  As a result, the cells begin to leak essential fluids, resulting in drying up, or desiccation, of the plants.  Typical symptoms of preemergence herbicides that fall into this category include chlorosis to necrosis of leaf margins and injury occurs after true leaf initiation.  For the few products that work as posteemergence herbicides, symptoms include a contact-like burn, bronzing or speckling of leaf tissue and leaf necrosis.

Some of the more important herbicides fall into this category including compounds in several chemical groups (Triazines, substituted phenols, ureas, uracils and bicarbamates).  These chemicals are typically applied to the soil and move through the plant xylem to the leaves.  Movement from the leaves to the root via the phloem vessels does not typically occur with these chemicals; However there are some chemical with this mode of action where the chemicals can be successfully applied to the leaves.

Common products that are photosynthesis inhibitors include: preemergence herbicides=atrazine (Atrazine, Aatrex), simazine (Princep), prometon (Pramitol), hexazinone (Velpar), metribuzin (Sencor), linuron (Lorox), tebuthiuron  (Spike), diuron (Karmex), bromacil (Hyvar), bromacil + diuron (Krovar).  Posteemergence herbicides= bentazon (Basagran),  bentazon + atrazine (Prompt), bromoxynil (Buctril).

Amino Acid and Protein Inhibitors. 
The use of proteins in plants falls into three major roles, namely functional, storage and structural. Functional protein is just another word for enzymes. Storage proteins are typically found in the seeds and cotyledon and supply the initial nutritional requirements or amino acids when the seeding first germinates. In the absence of these proteins and amino acids plants cannot complete the chemical reactions necessary for growth and development.  As indicated by their name these types of herbicides block amino acid and protein synthesis.  Initial symptoms typically include a slow chlorosis and purpling of sensitive plant tissues.  The developing plant gradually dies over a period of time, ranging from one to several weeks.  These chemical belong to the chemical families including amino acid derivatives, imidazolinones, sulfonylureas and sulfonamides.  Common products on the market include glyphosate (Round-up), sulfosate  (Touchdown),  glufosinate (Finale, Ignite, Lightening),  metsulfuron (Ally, Amber), triasulfuron (Amber), sulfometuron methyl (Oust), Imazaquin (Image),  imazapyr  (Arsenal / Chopper), imazapic (Plateau) and many more.

Pigment Inhibitors. 
The primary pigments found in the chloroplasts of plants are chlorophyll (green in color) and carotenoids (yellow in color) both of which absorb light in the photosynthetic process.  An additional important role of carotenoids is to protect chlorophyll from photooxidation or destruction by excessive light.  When applied to plants the herbicides in this group result in an inhibition of carotenoid biosynthesis.  The most obvious symptom with the use of these chemicals is bleaching or removals of color from the leaves (due to the destruction of chlorophyll from excessive sunlight).  The plants sometime recover if only 50% white. Growth continues for a period but without the presence of chlorophyll growth cannot be maintained.  Chemical family groups include isoxazolidinones and pyridazinones.  Common products are clomazone (Cornmand), norflurazon (Zorial) and fluridone (Sonar).

Growth Regulators. 
Auxins are natural plant hormones that regulate plant growth and are under direct metabolic control by the plants.  At low concentrations they promote normal growth and development.  However at abnormally high concentrations they inhibit plant growth.  This group of pesticide mimic that of the natural plant auxin (IAA) and act at the same biological cite as auxin.  However, all are much more active that IAA and persist in the plant longer.  Growth regulators are primarily used to control broadleaf weed in corn, cereals and other grass crops.

Symptoms of treated plants include malformed and downward twisted leaves (epinasty), swollen stems, brittleness, deformed roots and necrotic tissue.  New growth or growing tissue is more effected that mature growth.  Growth regulator are transported in the phloem which results in systemic rather than contact activity and good control of perennial weeds.  One problem with these chemicals is drift due to the fact that they exhibit a flat dose rate with effects occurring well below suggested rates. Families of growth regulator herbicides include the phenoxy acids, benzoic acids and picolinic acids.  Common products are 2,4D, MCPA, MCCP, 2,4-DB, dicamba (Banvel), many, many combinations and formulations, picloram (Tordon, Gazon), clopyralid (Reclaim), triclopr and clopyralid (Confront).

Disruptors of Plant Membranes. 
These chemicals work by disrupting membranes (as their name implies), leakage of fluid from cells and rotting of the cells with the typical main symptom of drying of leaves.   Affected plants often look burned initially turning yellow with wilting and desiccation often occurring within hours of application. Maximum efficiency with the use of these types of chemicals results with thorough leaf coverage, and under condition of high heat and light intensity.  In most cases these chemicals are applied posemergence with a few exceptions that are applied preemergence. Common products in the Bipyridvliums and Diphenyl Ethers families include paraquat (Gramoxone extra), difenzoquat (Avenge), diquat (Diquat, Reward), acifluorfen (Blazer), lactofen (Cobra), fomesafen (Reflex), oxyfluorfen (Goal).

Fatty Acid Synthesis Inhibitors. 
Fatty acids are critical components of cell membranes which in combination with the cell walls of plant tissue give the plant structure and serve as a leak-proof compartment of the inner components of the cell.  If fatty acid synthesis is blocked the cell membranes breakdown thus releasing contents of the cells. These herbicides are most effective when applied when grasses are actively growing and unstressed.

These types of herbicides are mainly contact in nature and defer a considerable degree of selectivity affecting the grasses (monocots) but do not affecting the ornamental monocot and dicot broadleaf weeds. They are used for postemergence control of annual and perennial grasses.  Root and shoot growth stops a few hours after they are applied with the formation of red coloration in the leaves soon to follow (typically within 8 hours of application).  Spotty necrosis (rotting of tissues) occurs at the nodes and above other growing points. Death is slow usually requiring more or less than one week for total destruction of the plant.  These materials are typically applied to and absorbed through leaf tissue with optimum absorption and effectiveness occurring when a surfactant or other type of additive is added to the spray solution. Chemical family groups are Aryloxyphenoxypropionates and

Common products include fenoxaprop (Acclaim), diclofop methyl (Illoxon), fluazifop (Fusilade) and sethoxydirn (Poast/Poast Plus/Vantage).

Growth Inhibitors. 
Products in this group vary considerably in vary considerable as far as their specific biochemical mode of herbicidal activity but all share the common characteristic in that they all  cause some type of growth inhibition of seedlings as they emerge from the soil.  The group can be subdivided into those chemicals that disrupt cell division, inhibit root growth of emerging seedlings, shoots or roots.

As is commonly known that plant or animal growth is dependent on cell division where on cells repeatedly reproduce themselves.  Herbicides that interfere with cell division are referred to as mitotic poisons.  When these chemicals block cell division, new cell production decreases and eventually growth stops.  Symptoms include inhibition of early seedling development by lack of shoot and root growth including considerable swelling.  These chemical do not translocate and have no effect when applied to foliage of mature plants and for the most part are effective against germinating annual seedlings.

InhibitoShoots of Seedlings.  
The Family chloroacetamide group includes the most widely used herbicides in this or other groups since the 1950’s. Other family groups include the carbamates/thiocarbamates/dithiocarbamates. These are applied as preemergence or preplant-incorporated treatments and are absorbed through the roots (mainly dicots) and shoots (mainly monocots) for control of seedlings or germinating seeds of many annual grasses and small seeded broad-leaf species.  Treated seeds typically germinate but either do not break the soil surface or do so as distorted seedlings.  If leaves emerge they typically do not unroll completely thus trapping the tip of the next leaf forming a loop.  Common products includemetolachlor  (Pennant/Dual), alachlor  (Lasso),  (EPTC), (Eptam( and  asulam (Asulox (little pre-emergent activity).,  oxadiazon (Ronstar) ethofumesate, Prograss (absorbed by both roots and shoots).

Inhibitors of Root Only of Seedling.  
The two primary chemicals in this group are Napropamide and bensuline.  They are readily broken down by the sun so it is important to follow application methods as closely as possible.  These chemicals fall into the dinitroanilines family group. Common products on the market include pendimethalin (Pendimethalin, Prowl), oryzalin (Surflan), prodiamine, (Baracade), benefin (Balan), trifluralin (Treflan), oryzalin + isoxaben (Snapshot).  There are also anumber of miscellaneous herbicides that have a similar mode of action but belong to various chemical groups.  These include dithiopyr  (Dimension), pronamide (Kerb), (DCPA),  (Dacthal), isozaben (Gallery), napropamide (Devrinol), bensulide (Betasan) and siduron (Tupersan).

Questions for Part 1.

Part 1. T/F

1.  In the United States, insects cause more damage to crops than what is caused by weeds.

2.  Worldwide there are approximately 250,000 species of plants and most experts feel about 3 %, or 8,000, of these are considered weeds.  Of these 0.1% or 250 to 300 are encountered on a daily basis.

3. Most experts feel that people inherit a tendency to be allergic, although not to any specific allergen.

4.  The symptoms associated with allergies are due to mast cells in the body releasing histamines.

5.  Poison ivy, oak and sumac all belong to the same family, namely the cashew family.

6.  Uriushiol is the main chemical that irritates skin upon exposure to poison ivy, oak and sumac.

7.  Poison oak symptoms can be spread by touching or rubbing the blisters caused by exposure to this  


8.  Plowing an agricultural field can actually add to weed problems as it tends to mix the seed bank, thus making it less predictable as to which weeds can cause problems the following season.

9.  Phosphorous is typically the first of the essential plant nutrients to occur in short supply in weed-crop competition.

10. The so-called C3 type of photosynthesis found in most weeds produces TNS more rapidly and efficiently than does the C4 type of photosynthesis that occurs in many cultivated plants.

11. Some weeds can produce over 250,000 seeds per plant.

12. There are a number of factors that influence how and when germination occurs, the most important is temperature.

13.  The botanical formula Q10 indicates that for every 10° F (within a range) the enzymatic activity initiating germination doubles.

14.  A seed is determined as viable if it is capable of germinating and producing offspring.  When compared to seeds of agriculture crop plants, weed seeds exhibit much longer periods of viability.

15. Thatch is the accumulation of materials like leaf litter on the soil surface and is important and well-suited in the formation of roots.

16.  Most perennial weeds reproduce by seeds.

17. Changes in soil temperature, soil water content, the percentage of oxygen or carbon dioxide in the soil and even soil disturbances, such as plowing, can cause or break dormancy in weed seeds.

18.  One of the main differences between stolons and rhizomes is that the former produces a vegetative root-like structure that travels below ground, producing new roots and shoots. The latter produces a similar structure that travels above ground performing the same function.

19. Annuals and tap-rooted plants are particularly susceptible to control by hand-pulling and hoeing; however, perennials with deep taproots or rhizomes are difficult to control with these techniques as some parts are frequently left behind and will readily resprout.

20. The seed bank is the top 1 to 3 inches of soil which contains the majority of the weed seeds.

21. High fertility levels, particularly nitrates and nitrites, can stimulate germination of some dormant weed seeds.

22. Pre-plant broadcasted soluble nutrients may be more readily available to fast growing weeds than the slower growing crops and should be avoided. 

23.  The presence of light is essential to the germination of all weed seeds.

24.  Black plastic mulches will prevent the germination of all types of weed seeds.

25.  Preemergence herbicides are typically applied to the soil, absorbed through the roots and move via the xylem vessels to the leaves.

26.  Selectivity of a herbicide (kills certain weeds but not certain types of cultivated plants) may be due to differences in texture, waxiness, hairiness and other characteristics of the leaf surfaces and leaf orientation in respect to application. 

27. Many annual crops become more susceptible to herbicides as they mature. 

28. The main reason why preemergence herbicides fail in the field is overwatering.

29. Topdressing fertilizer either over or with herbicides leads to poor control. It is thought that high concentrations of nitrogen can lead increased microbial actives and ensuing herbicide degradation

30. Ideally enough rainfall at one time will function to properly incorporate a preemergence herbicide into the soil. However, if the same amount occurs over a few weeks and the soil dries between these episodes, the herbicide will tightly bind to the top layer of the soil (especially in sandy soil) and subsequently will not move to the lower layer where the seeds are mostly found.

31. Most seeds occur in the top 1-inch of the soil and simple activities such as walking or dragging something across a treated field will readily break this barrier.

32. Generally speaking once applied postemergence herbicides quickly dry on weeds and rainfall or sprinkler irrigation within an hour after application will not affect results.

33.  The volume of the spray of a postemergence herbicide application has little effect on the desired results, as long as the right amount of the product is applied.

34. The main reason photosynthesis inhibitor type herbicides kill plants is that they destroy chloroplasts; this results in starvation of the plant.

35.  Amino acid inhibitor type herbicides function by interfering with the production of enzymes that are essential to plant growth and development.

36. The primary pigments found in the chloroplasts of plants are chlorophyll (green in color) and carotenoids (yellow in color), both of which absorb light during photosynthesis.

37. The mode of action of paraquat and Roundup aredisruptors of plant membranes.


Click Here for Part 2 and Weed List


Part 2.  Weeds and Weed Control

For answers to the following refer to weed list on the opening page. T/F-Matching

1. Annual bluegrass (also known as Poa) is one of the most important turf pests causing serious  problems in golf courses.

2. Bermudagrass identification can be confused with that of crabgrass.  The most obvious morphological difference between the two is the inflorescence. With Bermudagrass some of the finger-like spikes arise below the tip of the flower stem, while with crabgrass they all arise  evenly from the tip of the stem.

3.  Parts of the nightshade plant are poisonous, including the mature fruit. They contain high levels of a bitter, toxic alkaloid called solanine.

4.  In recent years the presence of bristly oxtongue has increased considerably in California. One of the key identifying characteristics of this weed is the blister-like structures found on the leaves.

5.  Dallisgrass grows faster than other species of turf grasses and the flowering shoots are often bent over rather than being cut by lawn mowers and springs back up again.

6.  Dandelion has a deep tap root and is difficult to physically remove.  If any part of the root of this perennial is left the plant will readily regrow.  Chemical applications are the most effective for its control.

7.  Canadian thistle leaves are prickly-hairy on the upper surfaces and cottony below while bull thistle leaves are glabrous on the upper surface and either glabrous or hairy on the lower surface.

 8. Allowing dodder to spread in a field or garden area is asking for an increase in the plant diseases  this parasite is capable of spreading. It has been shown to spread the yellows disease peardecline, aster yellows, tomato big bud and elm phloem necrosis.

 9.  Field bindweed has a shallow root system and its control is readily achieved by physical removal.

10.The symptoms of human poisoning from castor beans begin within a few hours of ingestion and include: abdominal pain, vomiting and diarrhea, sometimes bloody. Within several days severe dehydration results accompanied by a decrease in urine and a decrease in blood pressure is  typical. 

11. In California and some of the other western state netseed lambsquarter serves as a host for the beet leafhopper which is the vector of the curly top virus in sugar beets. It is very competitive

     due to its rapid growth and corresponding low use of water.      

12.  Black tipped flower bracts are characteristic of common groundsel.

13. Netseed goosefoot and lambsqurarter are quite similar in overall appearance but can be separated 

      in that lambsquarter leaves are light green in color and those of goosefoot are much darker green  

      and shiny on the upper side and exhibit a lighter mealy appearance on the under surface.     

14. Dry conditions are conducive to the development of Italian thistle as it has a deep taproot.

15. Nutsledge grows in most types of soil but seems to prefer damp or poorly drained sandy soils and is characterized by solid v-shaped stems.

16. Prostrate spurge forms dense mats and is characterized by stems that exude a milky substance and large white flowers.

17. The hairy foliage of red stemmed filaree is divided into narrow, smooth feather-like lobes that lack petioles.

18. Redroot pigweed is very tolerant to frost, contains nitrates that can be toxic to livestock when

      consumed and is characterized by red roots.

19. Water hyacinth reproduces sexually by seeds and vegetative by budding and stolen production.

20. Western wild cucumber is a perennial that regenerates from a huge bubbous taproot and can be

       easily controlled by most types of postemergent herbicides.


 T/F. 21.  The image below exhibits an example of pinnately divided leaves (see glossary).       


T/F. 22.  Omit

T/F. 23.  The sharp elongate structures radiating out from the tips of the seeds illustrated below are examples of awns.



Multiple Choice.  Match the following inflorescence, seed(s) or seed heads with appropriate weed; (a) Wild oats. (b) Burclover. (c) Curly dock. (d) Annual bluegrass. (e) Fiddleneck. (f) Yellowstar thistle. (g) Rabbitfoot polypogon. (h) Soft broom.  (i) Rescuegrass  (j) Littleseed canarygrass.  (k) Bermudagrass, (l) Crabgrass.  (m) Dallisgrass  (n) Bristly foxtail. (o) Italian thistle.

24.  Annual bluegrass

25.  Soft brome

26. MVC-139S

27. MVC-072S


29. fidl                                          

30. Omit

31.       ffxxx

32.  MVC-028S

33.    MVC-008S

34.   nnn


36.   Rabbit foot polypogon                                      

 37.  MVC-120S                       

      38.  rescue

Click Here for Part 2 and Weed List


Short definitions below. 

Alternate.  Used to describe arrangement of leaves or buds on a stem.  In this case one leaf or bud at a node as opposed to opposite where two leaves or buds arise opposite each other.

Annual.  A plant or weed that completes its development during one season.

Ascending.  Referring to growth that is upward in nature or upward turned. 

Axil. The angle or point where a leaf or bud and stem meet.  Commonly used as a reference to where flowers or buds may arise.   

Auricle. Commonly used in grass identification.  Referring to an ear-shaped lobe at the junction of the node and leaf base.

Awn. A slender terminal bristle.  Typically occurs with some grasses and is located at the tip of a spikelet.

Biennial.  A plant requiring two growing seasons to complete its life cycle.

Bract.  A leaflike structure (typically small) that occurs below a flower.   

Clasping. Term used to describe where the base of a leaf extends beyond and surrounds the stem.

Corolla-The combined petals of a flower.   

Cotyledon.  The first leaf-like structure to appear above ground in a germinating seedling.  Not a true leaf.

Elliptical.  Oval to oblong in shape with rounded ends and at least twice and long as wide.

Entire.  Refers to a leaf margin that is not toothed, serrate or cut.  

Inflorescense.  The flowering part of a plant. 

Glabrous.  Smooth without hairs.

Glume. Bract at the base of a grass spiklet.

Lancolate or lance shaped.  Referring to leaf shaped like the head of a lance.

Point where two stems attach.  

Opposite. Used to describe the arrangement of leaves or bud on a stem.  Two arising at the same node but on opposite sides of the stem. 

Ovate.  Egg-shaped.

Palmate.  Spreading out like the fingers of a human palm.  

Pappus. Bristles, scales, awns or short crowned the tip of an achenne.   

Perennial.  A plant or weed surviving more than two years.

Petals.  The showy parts of a flower. 

Petiole.  Stem or stalk of a leaf.

Pinnate or pinnately divided.  Typically refers to leaf shape.  Arising from several different points along a midvein.   

Prostrate. Lying on the ground, usually describing the growth habits of a plant.   

Pubescences.  Hairy or covered with hairs.

Raceme. An arrangement of flowers along a stem on individual stock of equal length.   

Sessile. Without a stalk. 

Simple and compound leaf.  Simple referring to a leaf that is not compounded into sections. 

Spatulate.  Spatula shape.  

Stamen.   The male organ of a flower bearing the pollen. 

Stolons.  An above ground stem that is prostrate and sends out new root at its nodes.  

Succulent.  Fleshy and typically thick.  Frequently refers to leaves.  

Rhizome.  An underground stem that typically runs parallel below ground and sends out roots and new above ground stems.   

Rosette.  A stage of growth frequently found in the early growth of a weed.  Appears as a low growing cluster of leaves radiating out form a central point forming a circular pattern.   

Serrate.  Typically used to describe the margins of leaves.  Saw-toothed with teeth pointing forward

Spike.  Typically a long inflorescence comprised of many spikelets and typical of grasses.

Spiklets-  These are small or secondary spikes typically with many comprising the spikes or inflorescence of grasses.

Toothed Margin.  Sawtoothed projections of margin of leaf blade. 

Trifoliate.  A compound leaf composed of 3 leaflets, most commonly as in a clover leaf.  

Umbel. A flat or rounded cluster of flowers where the stalks radiate out from the same point of attachment (like an umbrella).   

Winged.  A thin extension of a leaf blade or stem.