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Weeds definition


Common Weeds of Albuquerque
Course objective, To learn what weeds are, how to identify and how to
control them.
Weeds definition, a very successful plant that will grow in a given
location. A weed is person specific. We will only consider only the ones
the generally thought to be a weed. Nature abhors a vacuum.
Weed identification,
common , invasive, weed seeds, transporting, (vectors). pictures, as
seedling, as mature plant,
Control methods and prevention
mulching
solarization
Pulling
herbicides
torching

Annual weeds
Perennial weeds
edible vs. poisonous weeds
Cool weather, hot weather weeds
Bindweed.
Russian thistle. pigweed, dandelion, London rocket, scotch thistle,
purslane, filaree, phacelia, shepherd's purse, knapweed, goatheads,
nutsedge, nutgrass. spurge, downy brome, crabgrass, dallis grass, goose
grass. sandbur knotweed, salsify, gaura, purple asters, globe mallow.
foxtail.
Color photos of some weeds as well as some wild flower in a page just for you.

1. Bindweed flowers. 2. Bindweed plant. 3 Black medic. 4. Buffalo Bur. 5. Cockle bur 6, Dandelion. 7. Dodder. 8. Dodder flowers. 9. Filaree. 10. Oxalis. 11. Plantain. 12. Puncture vine. 13. Tansy

Introduction..  In any discussion on the management of weeds, first consider the definition of a “weed”. How do you define these types of plants. Are they ‘just “plants out of place” or “plants whose virtues have not yet been discovered such as a wild flower” or are they poisonous, injurious, harmful, hard to manage, or pernicious.

Weeds are pollutants to our environment. Their characteristics are: they appear attractive, desirable, or harmless, weaken and may kill native vegetation, can be toxic, painful, or otherwise injurious to humans.  can increase (or decrease) soil erosion and associated water quality problems, multiplies itself exponentially and can generate tons of itself in a matter of months, spreads naturally in water, wind, or soil, is also spread by wildlife, livestock, and recreationists.  Effects are not usually apparent until the spread is already out of control because it can lie dormant and undetected underground for decades, then reappear and spread.

Then "what is a weed?" Consider this definition and see if it fits into most situations:  “A weed is any plant which interferes with the management objectives for a given area of land (or body of water) at a given point in time.”

In the urban setting it is the individual homeowner who sets those management objectives. To one individual, a single dandelion in the front yard may be one too many, whereas the neighbor may feel comfortable with a lawn full of the bright yellow flowered plant. The level of weed management will depend upon the objectives established for the property.

Of the total number of plants in the world (about 250,000 species), only 3% or 8,000 species are thought to behave as weeds in agriculture. Of these, about 250 or 0.1% of the total are recognized as major problems in world agriculture, and only about 0.01 % or 25 species cause the major weed problems in any one crop.

Some of our weedy plant species are:

• Cyperus rotundus, Purple Nutsedge;

• Cynodon dactvlon, Bermudagrass;

• Echinochloa crusgalli, Barnyardgrass;

• Echinochloa colonum, Jungle rice;

• Eleusine indica, Goosegrass;

• Sorghum haleoense, Johnsongrass;

• Portulaca oleracea, Common Purslane;

• ChenoDodium album, Common Lambsquarters;

• Digitaria sanguinalis, Large Crabgrass;

• Convolvulus arvensis, Field Bindweed;

• Cyperus esculentus, Yellow Nutsedge.

 Characteristics of Weeds

 Consider some of the characteristics which make weedy plants so competitive, persistent, and pernicious.

Number of seeds per plant:  Weeds are producers of large number of seeds.  Example:  Barnyard grass, 7000 seeds; Common Purslane, 52, 000 seeds; Common Lambs quarters, 72,000 seeds; Redroot Pigweed, 117,000 seeds; Russian Thistle, 200,000 seeds.

Dormancy:  Dormancy is the ability of seeds to remain viable in the soil for extended periods of time.  For Example:  Johnson grass, 20 years; Field Bindweed, 20+  years; Common Lambs quarter, 40 years; Redroot Pigweed, 40 years;

Special Adaptations or Appendages:  Plants have developed means to assist in their spread and distribution, these include:  Hooks and spines - sandbur, puncture vine; Pappus (parachutes) - musk thistle, milkweeds; Good Looks - spotted knapweed, white top, Dalmation toadflax.

Vegetative Reproductive Capabilities:  Vegetative reproductive structures are those asexual portions of the plant which allow for new plants to arise without the fertilization of the flower. Examples include the following:  Roots with adventitious buds Ex.: Leafy Spurge & Canada Thistle; Rhizome  Ex. Johnson grass & Bermuda grass; Tubers Ex.: Yellow & Purple Nutsedge; Crowns Ex.:  Dandelions & Plantains; Stolons Ex.:  Bermuda grass; Special Characteristics:  “Weeds have the greater will to live.”

 Classification and Identification of Weeds

The first step to affective weed management is the correct identification of the plant or plants in question.

Learn:
Correct name of the plant whether the locally accepted common name or scientific name - this helps when discussing the problem with someone else.

Able to learn about the plant and it’s characteristics, such as:  Life cycle - annual vs. perennial.  Are there different principles involved when dealing with an annual versus a perennial?   How does it spread and reproduce.  How does this affect the management plan?  Find out if a particular herbicide can be used to manage the problem.

Weeds are usually characterized according to their general growth types:

 Grasses which are characterized by a single embryonic leaf (monocot), fibrous root systems, and parallel veins on the leaves.

Broadleaves are characterized as having two embryonic leaves (dicot), taproot systems, and a netted leaf venation.

Each of these two general groupings have been further subdivided into classification classes according to their life cycle, which begins with the germination of the seed, is followed by vegetative growth, and is completed with the maturation of the seed. Time of germination and means of spread and reproduction help to create a more clear picture of the different classes of weeds encountered. Generalized management statements can be made as a result of this type of classification, as well. The classification scheme is as follows:

 Annual weeds - those which complete their life cycle in one year.  This group of weeds has been further subdivided according to its time of germination into the following groups:

Winter Annual - weeds which germinate in the fall of the year, mature and set seed in the spring of the second year.  Examples of winter annual weeds include:  Rescuegrass;  Wall or Little Barley;  Annual Bluegrass; the Mustards: Flixweed, Tansymustard, Shepherdspurse, London Rocket.

Summer Annual - weeds which germinate in the spring of the year, mature and set seed in the same year.  Examples of summer annual weeds include:   Barnyardgrass, Crabgrass, Field Sandbur, Foxtail, Stinkgrass or Lovegrass, Common Purslane, Common Lambsquarters, Kochia, Pigweed Species, Puncturevine, Prostrate Knotweed, Prostrate Spurge, Russian Thistle.

Regarding annual weeds, their only means of spread and reproduction is through the formation and dispersion of seed.  This means that the principle of management is simple -don’t let them set seed.

Any management plan is going to have to deal with multiple germination, since these weeds will germinate in the soil whenever the conditions favor the process to get started. This means that once you have managed the first flush of annual weeds, through cultivation, etc., there will be another group of seeds waiting in the wings for their chance to germinate and come up.

Biennial weeds - these are those weeds which take two years to complete their life cycle, or they live two years, and, are often, referred to as short-lived perennials.  Examples of biennial weeds include:  Musk Thistle - not found in turf, but is a good example of a true biennial with it’s rosette of leaves the first year, followed by the bolting of the seed stalk, flowering, and seed set during the second year.  Common Mallow - is another biennial, but in this case the plant does not produce a rosette of leaves followed by bolting, rather it has a life span of two years. With biennial weeds the management principle is the same as with the annual weeds since their only means of spread and reproduction is through the formation of seed. Not letting the plant set seed is what is required if these types of plants are to be effectively managed.

Perennial Weeds - these are the plants which have the vegetative reproductive structures already mentioned.  The management of these weeds requires control of these weeds requires not only stopping their seed production, but also the management of the vegetative structure under the soil, or above the soil in the case of the stolon.  Like the annuals, the perennial weeds have been subdivided into two groups, but not according to their time of germination, rather according to their type of root system.

Simple Perennial - these plants are capable of coming back year after year from the same root, but there is no underground lateral branching.  Examples of simple perennial weeds include:   Dandelion, Plantain.

Creeping Perennial - these plants are capable of coming back year after year from the same root system and also have the creeping vegetative reproductive structures. Examples of creeping perennial weeds include:  Bermuda grass, Johnsongrass, Nutsedge - Yellow and Purple, Creeping Woodsorrel, Creeping Chafweed or Khakiweed, Field Bindweed, Silverleaf Nightshade, Texas Blueweed.

The first step towards effective management is the correct identification of the plant species. Besides the Internet here are some books that will be helpful in identification. 

“Weeds of the West”- Published by Pioneer of Jackson Hole,- Author: Tom D. Whitson, et. al.  - This publication is an excellent identification guide using colored pictures of 300 selected weeds common to the western portion of the United States. It is available through the Cooperative Extension Service and costs $21.50.

“Southwestern and Texas Wildflowers”- Published by Houghton Mifflin Company,- Author: Theodore F. Niehaus, et. Al.  This is one of the Peterson Field Guides and is series number 31. It is a good publication, using a combination of line drawings and colored illustrations. It is broken down according to flower color which is nice if there is a flower present. It is available through many bookstores at a cost of +/- $12.95.

“The Audubon Society Field Guide of North American Wildflowers -Western Region”- Published by Alfred A. Knopf, - Author: Richard Spellenberg - a professor of botany from New Mexico State University.- This is a nice publication which gives colored pictures of the flowering portion of the plant separately from the description of the plants themselves. The descriptions are excellent and provide useful information. It should be readily available through bookstores at a cost of $18.00.

Weed Management Options

The management of weeds involves taking the time to consider all of the options available. This has been referred to as Integrated Pest Management. Consider this definition and see if it applies to the management of weeds. 

“A control strategy in which a variety of biological, chemical, and cultural control methods are combined to give stable long-term pest control at levels below those causing economic injury.”

That definition has some of the same central themes that our definition of weed had, that of the establishment of economic injury levels. It is the homeowner that will determine the level to which they will allow injury, whether real competition, death, or aesthetic.

The management of weeds is usually discussed under the following groupings:  Biological, Cultural, Mechanical, Chemical, Educational, Preventive. 

Biological Weed Management - the use of living organisms to manage or control other living organisms.  Examples of this form of management include:  You and me - all weeds are edible once, some will just kill you.  Puncturevine weevil - for the management of puncturevine.  Geese - manage grassy weeds in strawberries. 

Chemical Weed Management - the use of herbicides.  A discussion on the use of herbicides will be discussed later, it is sufficient to note at this time that they are a member of the “total options” package for the management of weeds.

Cultural Weed Management - with this type of management, the central theme is “giving the turf, or turf situation, the competitive edge”. It is important to remember that “Weeds are not the cause of poor turf, but rather the result”. Taking time to manage the other components of turf and turf situation management will allow the turf to be more competitive.  Consider this, if a weed is removed form a turf site, unless the turf is healthy and competitive, what will take the place of that removed weed - another weed.

It can not be stressed too much the importance of allowing the turf to be competitive through such factors as:  Correct irrigation practices; managing the fertility; mowing the correct height; keeping diseases and insects under control; etc.

Educational Weed Management - this is one I threw in to point out the need to tell the story of the importance of correctly managing the weeds and using all the available tools at our disposal. Whether it is the use of herbicides, identification of the weed species, or how to establish a mulch for the management of perennial weeds, the presentation of accurate information is critical.

Mechanical Weed management - this is the physical removal of the weedy species. Examples of this form of management include:

Cultivation - with cultivation there are a couple of questions that need to be considered:  How effective is cultivation going to be on the management of annual weeds?  Answer: Quite good if the weeds are small.  How effective is cultivation going to be on the management of perennial weed?  Answer: Quite poor - because cultivation assists in the spread and dispersal of these weeds through the breaking up and movement of the underground vegetative structures. 

Mowing - let’s consider a couple of questions here as well:  How effective will mowing be on the management of puncturevine? (Do you know what puncturevine looks like?)  Answer: Poor due to the prostrate growth habit of this particular plant.  How effective will mowing be on the management of sandbur in the pre-seed head formation stage?  Answer:  Fair, if the flowering stalk is erect and not prostrate on the turf.  Can we use mowing as a means of spreading weeds?  Answer:  Yes, we do it all the time with dandelion when we mow our lawns when the plant is flowering with it’s white balls of seeds.

Hand pulling – Plowing

Mulches - the use of mulches, both living and nonliving, can be quite effective in the management of annual weeds. If using a mulch program to management perennial weeds, the effort may be futile unless the entire patch of perennial weeds is under the mulch. There are good descriptions on how to establish a mulching program for the management of weeds in the publications listed at the end of this section.

Preventive Weed Management - this type of management involves keeping the weeds out from the very beginning. It requires the homeowner to be actively involved in the recognition of the common weedy species and a desire to keep them out of their turf situation. There are several ways this can be done, here are a few examples of how weeds can be kept out of the turf situational areas:

Certified seed - when establishing a turf area using seed rather than sod, make sure the seed is certified weed free.

Clean along fence rows - weeds can and do move through and under fences. It is much easier to manage the weeds outside the turf or garden area.

Clean weeds along ditches - weeds are not afraid of the water and are also good swimmers.

Watch your topsoil - if establishing a landscape that requires the addition of topsoil, consider where that topsoil is coming from. It may be coming from an area that is heavily infested with perennial weeds, which means those vegetative structures may be getting a free ride into your yard.

The key to this, and the other options as well, is “common sense”.

There are several books available which have good information dealing with the management of “pests” in turf and garden areas. A couple of ones I have found, and use, include: 

 “Rodale’s All-New Encyclopedia of Organic Gardening”-   Published by Rodale Press,  Edited by Fern Marshall Bradley and Barbara W. Ellis

This is a good reference book which will provide a lot of excellent information dealing with the principles of gardening. There is a good description on how to establish a mulch program for perennial weeds in this publication.

Principles of Herbicide Use

In leading into this discussion it might be well to ask ourselves the following question and make the following point.

How much training does it take to effectively train someone on the proper use of a hoe or how to use a pair of gloves, recognizing that there is a right and a left hand and there may be some confusion?  If you make a mistake using a hoe or pair of gloves, the effects may be small, since few on us can pull up a tree or hedge or even hoe one out.

Such is not the case when considering the use of an herbicide. If the applicator is not properly trained, the product will not be properly applied, the wrong application equipment may be used, the material may be applied at the wrong time or the control of the wrong weeds and when herbicide injury occurs, it is not temporary, and unlike you and your hoe, an herbicide can kill a mature tree.  In dealing with the use of herbicides, discussions will be centered on:  herbicide labels, classification of herbicides, and herbicides and the environment - plant and soils interactions.

Herbicide Labels

When considering the use of an herbicide, the most critical component is the ability to be able to read, understand, and follow the label. When looking at an herbicide label the homeowner should be able to identify the following pieces of information:  Trade Name - name the manufacturer gives the product.  Ingredients Statement:  Active Ingredient - the name of the compound that is managing the weeds also referred to as the “common name” of the herbicide.  Inert Ingredient - non-herbicidal compounds in the formulation.

How much active ingredient is in a particular container in terms of lbs./gal or % of the products overall weight.  Why is it important to know, it helps understand that all herbicides are not created equal, even when they have the same active ingredients because the amount of active ingredient found in the container will directly influence the amount of product to be added to the spray tank in order to achieve the desired active ingredient level in the sprayer.  EPA Registration Number, Company who manufactured the product.  Signal Toxicity Word - which will be one of the following:

Toxicity Word -- EPA Category -- Meaning

• “Danger”  -- I --  Highly Toxic

• “Warning” --  II --  Moderate Toxicity

• “Caution” --  Ill --  Slightly Toxic

• “Caution” --  IV --  Very Low Toxicity

Precautionary Statements which include:  Statement of Practical Treatment - what to do if the herbicide gets in your eyes, is swallowed, is inhaled, or gets on your skin.  Note to the physician - telling him what to do.  Container Storage and Disposal - how to store the herbicide and what to do once the container is empty.  Listing of weeds that will be controlled with the particular herbicide.  Timing and Rates of application - this is critical, since some herbicides have to be applied before the weeds emerge while others can be applied following emergence. It is also against the law to apply an herbicide in any fashion that is not in harmony with the labeled directions.  Turf and Ornamental Sensitivity - some herbicides are safe for certain turf and ornamental species while others may injure the turf or ornamental species in question.

The following is an example of a turf herbicide label, with the parts

KEEP OUT OF REACH OF CHILDREN.

CAUTION

Precautionary Statements 

Associated with the storage and disposal statement is the question of how to dispose of unused herbicides. The situation has occurred where you mixed up a gallon of solution and only used one quart of the mixture. What are you going to do? The best way to deal with this situation is to not let it occur in the first place. When mixing herbicides, and any other pesticide, mix up only that which you will use. It will require more effort on your part as the actual area to be treated will have to be determined and from there the amount of herbicide to be used can be determined, mixed and applied with no unused product left. Remember - the best way to use unused herbicides is to not create unused herbicide mixtures, and should it happen, dispose of it according to the label.

Herbicide Classification

When dealing with the classification of herbicides, the following terms are used to explain when and how the herbicide is to be used:

Preemergence - this means the application is to be made prior to emergence of the weeds, or in some postemergence labels, it stipulates that it needs to be made prior to emergence of the crop, but postemergence to the weeds.  Examples of preemergence (to the weeds) herbicides:   Benefin, Bensulide, Dithiopyr, Isoxaben, Oryzalin, Metolachlor, DCPA, Trifluralin, Siduron, Napropamide, Prodiamine, Pendimethalin

Postemergence - this means the application is made to emerged and actively growing plants.  Examples of postemergence herbicides:  2,4-D; 2,4—DP; MCPP; MCPA;  Dicamba; Sethoxydim; Glyphosate; Bentazon; DSMA; Triclopyr; Diquat; Dithiopyr; Chlorsulfuron; Imazaquin; Metsulfuron methyl; Paraquat; Fenoxaprop; FIuzaifop-P-butyl; Glufosinate-ammonium.

Systemic - this means the herbicide will move once it gets into the plant. Some herbicides move only in the xylem (water conducting system in the plant), others move only in the phloem (carbohydrate conducting system in the plant), and others will move in both the xylem and phloem.  Examples of systemic herbicides:  2,4-D; 2,4-DP; MCPP; Dicamba; Glyphosate; Prometon; Fluazifop-P-butyl; Sethoxydim.

Nonsystemic - this means the herbicide will not move once it gets into the plant. These herbicides are typically referred to as “contact” herbicides.  Examples of nonsystemic herbicides:  Paraquat; MSMA

Selective - this means the herbicide will manage certain weeds and not others. This selectivity may be as obvious as managing only broadleaf weeds, as is the case with the postemergence turf herbicides (broadleaf weeds are controlled without injury to the turf species). Also, through the reading of the label it can be learned that, though an herbicide will offer control of selected members of a particular plant family, not all members of that family will be effectively managed. An example is the use of pendimethalin for the preemergence control of annual grasses. Crabgrass, foxtails, and other annual grasses are effectively controlled, but sandbur is not.  Examples of selective herbicides: 2,4-D; 2,4-DP; MCPP; MCPA; MSMA; Dicamba; Oryzalin; Benefin; DCPA; Isoxaben; Fluazifop- E-butyl; Chlorsulfuron; Pendimethalin; Prodiamine; Sethoxydim.

Nonselective - this means the herbicide will effectively manage whatever vegetation it comes in contact with.  Examples of nonselective herbicides: Glyphosate; Parequat Glufosinate-ammonium; Diquat.

Residual - another name for residual is persistence - whether the herbicide will persist in the soil - still having it’s herbicidal activity. Herbicides vary in their soil herbicidal activity because there are several factors which are going to influence this persistence, these include:   amount applied; rate and method of breakdown; soil environment conditions - which will influence the previously mentioned factors as well.  It is important to note that just because an herbicide has residual properties, it needs not be considered a “soil sterilent”.

Consider the use of oryzalin (Surflan), it is applied to warm season turf for the preemergence control of annual grasses. The label indicates that 6 to 8 months residual activity can be obtained under certain soil and environmental conditions. The label also points out that it can be applied under trees and shrubs without any injury to the desirable vegetation.

On the other hand, prometon (Triox) offers residual control of annual weeds also, but if it is placed under trees or shrubs injury can be expected.

Both of these examples are residual herbicides, one has the potential to injure trees and the other does not - why? In looking at the different physiological properties of these two herbicides, it is apparent that they are going to perform differently once they come in contact with the plants, either weeds or desirable vegetation. On one hand, oryzalin is not systemic - meaning it will not translocate in the plant, it’s mode of action is to stop cell division in germinating seeds. Prometon, on the other hand, is systemic and will move throughout the plant in the xylem system. It’s mode of action involves photosynthesis and not cell division.

Such information regarding the different characteristics of herbicides is taken into consideration when labels are developed. Herbicides which have not systemic activity and influence cell division of germinating seeds will not have as it’s application made “postemergence”. Labels will point out the necessary precautions associated with those herbicides which have the potential to adversely affect desirable vegetation.

“Non-potential injury” residual herbicides: Benefin; Bensulide; Dithiopyr; lsoxaben; Oryzalin; DCPA; Siduron; Prodiamine; Metolachlor; Trifluralin; Napropamide; Pendimethalin.

“Potential injury” residual herbicides:  Prometon; Bromaci; Diuron.

“Nonresidual” these herbicide offer no residual herbicidal activity once they come in contact with the soil.  Examples include:  Glyphosate; Glufosinate-ammonium; Paraquat; MSMA; Sethoxydim; Fluzaifop-P-butyl; Bentazon; DSMA

The following table is prepared to show how some of the more common herbicides fit into the different classifications. It is also intended to point out that the different classifications are just that, different classifications and it is not possible to make generalities between the different groupings. For example, just because an herbicide is a “preemergence” it will not necessarily control both broadleaf and grassy weeds. Also, not all residual herbicides are preemergent, because some have both pre and post activity.

*   The values here represent:

• “Yes - B&G” means yes selective, controls broadleaves and grasses “

• "Yes - B” means yes selective, control only broadleaves

• “Yes - G” means yes selective, controls only grasses

• “No - B&Y” means not selective, controls broadleaves and grasses

 Herbicides and the Environment - Plant Interactions

In order for an herbicide to do it’s thing in the plant there are a couple of requirements which have to be met:  The herbicide has to come in contact with the plant. This is not as critical with postemergence herbicides as with the preemergence herbicides. Preemergence herbicides are applied with the understanding that they will be moved into the soil prior to the germination of the weed species. Often times the material is applied and left alone on the top of the soil. Though the label may indicate that it will remain in it’s herbicidally active form for a couple of weeks, the weeds may not wait and may germinate before the active ingredient is moved into the zone of herbicidal activity.

Once inside the plant, the herbicide has to make it to it’s site of herbicidal activity within the plant. It is this principle that is involved with some of the selectivity that is observed.  There are several factors which are going to influence whether the herbicide comes in contact with the plant or makes it to it’s site of herbicidal activity within the plant. Some of the more common ones include:

Age of the plant - it is well understood that younger more actively growing weeds are more susceptible to control than are the established more mature plants. Associated with this growth factor is the fact that older leaves exhibit a reduced translocation rate which will decrease the movement from the site of herbicidal uptake to the site of herbicidal activity.

Shape of the leaf - broader leaves will affectively retain more herbicide than narrow leaves.

Surface makeup of the leaf - here we are concerned with the amount of cuticular waxes deposited on the leaf surface. It is this layer of waxes that keeps water in the plant and keeps the herbicide in the water carrier on the outside of the leaf, just wanting to get in.  Varying level of these waxes will then influence the movement of the herbicide molecule through the leaf, with the thicker layer posing a greater barrier than the thinner layer.  Also associated with the surface of the leaf is the presence, or absence, of hairs or trichomes. It is these structures, often, that keep the herbicide spray droplet from coming into direct contact with the leaf surface. As a result, there is a reduction in the amount of actual active ingredient which makes it into the plant.

Environmental Condition - the influence of the environment is obvious and considerable. It is the principle of stress placed on the plant, as a result of its environment, that is of interest here. Stress due to lack of moisture, or heat, or even physical stress results in a reduction in translocation or movement in the plant. The formation of a thicker cuticle, which makes it more difficult for the herbicide to get from the outside in, is also the result of stress placed on the plant.  By understanding some of the factor involved with the interactions of herbicides and plants helps one to appreciate what it says on the label, “do not apply to stressed plants”.

Herbicides and the Environment - Soil Interactions

As we look to the interaction between herbicides and the soil, we first ask ourselves what are the major players. It becomes quite obvious there are two, the herbicide and the soil.  With the herbicide there are four major factors involved with it’s behavior in the soil, solubility, adsorption, degradation or breakdown, and residual. It is the first two which are the most critical in relationship to the movement of herbicides through the soil profile.

Solubility: the solubility of an herbicide is based upon the amount of that particular material that will dissolve in water or the liquid phase of the soil. These will range from being practically insolubility, such as is the case with trifluralin, pendimethalin, or oryzalin, to those which readily dissolve in water, glyphosate, dicamba, and 2,4-D amine salts.

As water moves downward through soil, it carries with it water-soluble compounds. This movement is referred to as “leaching.”  It is understood that the greater the water solubility, the greater the potential for movement through the soil with the liquid phase.  In looking at the solubility values of these two examples, it is easily to predict which of the compounds will dissolve in water more readily.  Glyphosate. 1,000,000 mg/I sol., Oryzalin   2.5 mg/I sol.

Absorption also referred to as “magnetic charge” is the next factor to consider in relationship to the herbicide and it’s relationship to soil movement. Along with its characteristic solubility, each herbicide has a characteristic charge associated with it. This charge can range from little or no charge to strongly magnetic.  Herbicides or compounds with an overall positive charge will be tightly bound to the soil fraction, while those with an overall negative charge will be repelled from the soil fraction to varying degrees.  Many soil factors influence pesticide adsorption. Soil texture is one of the major factors. Soils high in organic matter or clay are going to be more adsorptive than coarse, sandy soils. Other factors include pH, temperature, and water content of the soil.

In reporting this characteristic, the term “Koc” is used to measure the degree to which a particular herbicide is bound either through chemical or physical bonds.  When looking at the following examples, note that “Koc” values above 1,000 indicate that the herbicide is held quite tightly to the soil fraction.  If an herbicide has a strong “Koc” value, what will that tell you about it’s movement in the soil?  In looking at one examples we now have two pieces of information to report:  Glyphosate:  Solubility   1,000,000 mg/liter, Koc Value 10,000

Using these two pieces of information what can be concluded regarding this particular herbicide ability to leach. From the solubility value, one would conclude it should leach, yet on the other hand, the Koc value is in the range which indicates a strong binding to the soil phase. What we see here is opposing factors, and one has to over ride the other. In this case it is the adsorption or binding that over rides the solubility. This results in the fact that glyphosate will not leach through the soil, why, because it is held to the soil very tightly, so tightly that it can not be moved through the soil with the soil water. This is understood, because when this herbicide is used, we find there is not soil activity. This is a postemergence systemic herbicide which offers no residual activity.

As important as solubility potential of a particular herbicide may be, it is found that the magnetic charge, as it relates to the soil binding of the herbicide, is a more important factor to consider when discussing the movement of herbicides in the soil.

Degradation or Breakdown is the process whereby reactions take place to render the active ingredient inactive. The following ways are some of the different ways herbicides can be degraded:

Microbial degradation of herbicides is the breakdown by fungi, bacteria, and other microorganisms which use the herbicide as a food source.  Soil conditions such as, moisture, pH, aeration, temperature, and the amount of organic matter affect the rate of breakdown under the microbial method.

Chemical decomposition is the breakdown of the herbicide by non-living soil processes.  This includes hydrolysis, dealkylation, substitution, and others.  Temperature, moisture, pH, chemical and physical properties of the herbicide, and adsorption are all involved in the chemical degradation process.

With these two breakdown systems there is a tight relation to the soil conditions. Those conditions which favor microbial decomposition, also favor chemical degradation. These include:  High organic matter;  Warm soil temperatures; Adequate moisture.

Under conditions of low organic matter, hot and dry soils, which is the situation for a lot of our state, what does this tell us about the breakdown of herbicides under those conditions -is it slower.

Photodecomposition certain herbicides can be broken down by sunlight. This is done in a way in which the bonds between certain elements are excited to the point of breaking or shifting. When this occurs the molecule changes shape and is rendered inactive as an herbicide.

Persistence is the ability of an herbicide to resist breakdown. The rate of breakdown, which is persistence, is measured in terms of half-life of the active ingredient. The chemical half-life is the time required for half of the original herbicide application to be broken down into its metabolites.

Some of the factors influencing persistence include:  Herbicide Solubility; Herbicide Adsorptivity; Mechanism of Breakdown; Environmental Conditions; Soil Type; Organic Matter Content.  In dealing with the soil aspects of this interaction between herbicides and the soil, we readily identify a couple of critical factors.

Soil Texture with this we are concerned with the different soil particles, sand, silt, and clay. The soil texture affects the movement of herbicides in several ways:  Movement of water through the soil, with coarser soils having a more rapid downward movement.  Binding or absorption coarser soils have fewer binding sites and can be saturated with the herbicide at a lower application rate than can the heavier soils.  Degradation with the heavier soils having more organic matter, moisture, and ideal temperature range for the degradation to occur microbially and chemically.  Organic matter content is the amount of organic matter in the soil.  This value will greatly influence how well the soil can retain the herbicide. Increasing the soil’s organic matter content, through practices such as application of manure or plowing under cover crops, increases the soil’s ability to hold both water and dissolved herbicides in the zone of herbicidal update and degradation.

We have covered the two major players involved in the process of pesticide movement. We have identified the different factors associated with the different processes. The status of herbicides in the soil is a complex situation. There are three major fates associated with the reaction of an herbicide and the soil, these include, adsorption, movement, and degradation. These are all interrelated and difficult to separate on their own, but it is necessary to have a general understanding of them in order to have a better feeling for what is going on in the soil once an herbicide comes in contact with the soil.  By taking the time to discuss these different principles and factors influencing the activity of herbicides in the soil, it becomes apparent there are no simple statements which can be made regarding the status of herbicides in the soil without a good foundational background.

The following books are good reference books and provide a good background into this complex subject:

“Applied Weed Science” -  Author(s): Merrill A. Ross and Carole A. Lembi;  Publisher: Burgess Press
This is a basic weed science textbook which does provide a good general background in the science of weeds and their management.

“Weed Science: Principles” -  Author: W. Powell Anderson - retired faculty member at New Mexico State University;  Publisher: West|
This textbook is being revised, yet the older edition still has a lot of excellent background information dealing with the science of weeds and their management.

“Interactions Between Herbicides and The Soil” -  Edited by: R.J. Hence;  Publisher: Academic Press
This textbook deals specifically with the topic of herbicides and the soil interaction.

“Fundamentals of Weed Science”- Author: Robert L. Zimdahl;  Publisher: Academic Press
This is a new weed science publication and in it Dr. Zimdahl brings up some good points regarding the science of weeds and their management. 

 “The Standard Pesticide Users Guide”-  Author: Bert Bohmont;  Publisher: Prentice Hall
This is an excellent reference book for anyone involved with the science of pesticides. The author has done an exceptionally thorough job of addressing the topic.

Remember, “Weeds Are Forever”.

 Listing of Commonly Used Herbicides and Their Turf Situation

 SELECTED WEEDS IN NORTHERN NEW MEXICO URBAN SITUATIONS

Annual Weeds:  Complete life cycle in one year

Winter Annual:  Fall germination, flower and set seed in the spring.

Downy Brome:  Soft leaf texture, hairy; Drooping, open seed heads

Mustards:  Flowers in the shape of cross.

Flixweed:  Finely divided leaves.  Flowers are yellow.  Seedpod with single seeds, one on top of another.   Nontoxic species.

Tansymustard:  Similar to flixweed.  Flowers are yellow.  Short, stubby seed pod, seeds in pairs.  Toxic plant.

Shepherdspurse:  White flowers.  Seedpods are flat, heart shaped, narrowing to a point at the base.

Summer Annual:  Spring germination, flower and set seed in the summer - fall.

Sandbur:  Characteristic spiny seed pod.

Crabgrass:  Broad, light green leaves.  Roots at the nodes.  Seed head is “fingerlike”.

Russian Thistle:  Bushy, round plant.  Seedling is succulent.  Also called “Tumbleweed”.  Blows and rolls across the streets to spread seed.

Kochia:  Small fuzzy seedling.  Introduced, erect growing 6 feet tall.  Multistem, often turning red as matures.  Inconspicuous flowers are born upper leaf axils.

Puncturevine:  Prostrate growth habit.  Yellow flowers, leaflets are fuzzy to the touch.  Seed pod is five nutlets, each with 2 spines/nutlet.

Prostrate Spotted or Prostrate Spurge:  Prostrate growth habit.  Off-center leaf attachment.  Milky juice in the stem.

Common Purslane:  Succulent, prostrate plant.  Round shaped leaves.  Self-rooting when cut off or pull up. if left on soil.

Pigweed, Careless weed, or Amaranth:  Erect plant, often reaching 6 feet in height.  Central stem, often turning red at maturity.  Terminal seed head is more than twice as long as the lateral seed head.  Alternate leaves are lance-shaped.

Lambsquarters:  Erect plant.  Leaves have a mealy texture mainly on the underside of leaves, but can be on top.  Lower leaves lobed, upper leaves are strap—like.

Knotweed:  Prostrate plant.  White papery sheath at leaf axil.  Leaves are lance-shaped.  Flowers are green-white and born in the leaf axils.

Common Chickweed:  Prostrate growth habit.  Leaves are round and entire.  Flowers are star-shaped, sepals are green and hairy.  Petals are white and deeply lobed, giving appearance of 10 petals but there are only five.

Speedwells:  Prostrate growth habit.  Flower is four united white petals.  Several different species.  Seedpod is heart shaped.

Biennials:  Weeds which require two years to complete life cycle. Rosette produced the first year and bolts the second year, sending up seed stalk.

Musk Thistle:  Rosette the first year.  Can grow up to 7 feet tall.  Solitary flower, pink—purple, on the end of flowering stalks.  There are many biennial thistles around, many of these are native and are not related to the musk.

Mallow:  Bushy annual or biennial.  Short thick taproot.  Leaves are almost circular with 5-7 lobes.

Perennial Weeds:  Along with seed production, these weeds have some form of vegetative reproductive structure, such as, roots, rhizomes, tubers, crowns, etc.

Simple Perennial:  Nonbranching, vegetative structure.

Dandelion:  Yellow, showy flower.   Plant is edible.

Creeping Perennial:   Branching vegetative structure which allows the plant to move through the soil.

Johnsongrass:  Erect, with open panicle.  Vegetative reproduction is by a rhizome.

Bermuda grass:  Prostrate growth habit.  Narrow strap-like leaves.  Reproduces by seed, stolons, and rhizomes.

Nutsedge:  Triangle shaped stems.  Leaves are three-ranked.  Tubers at the end of the rhizome.

Field Bindweed:  Prostrate, vining, creeping. Trumpet-shaped flowers.   Deep penetrating roots. Looks like morning glory, but is not an annual like the morning glories.

Silverleaf Nightshade:  Purple flowers with yellow centers.  Spines are over plant.  Seed born in berries which are green-yellow.  Branching root system.

Creeping Woodsorrel:  Prostrate perennial.  Creeping stems, 3—8” long.  Leaves  3 heart shaped leaflets at tip of long stalks.  Leaflets are greenish, purplish, or bronze. Flowers are yellow.

Other Weeds Not Listed:

Annual Bluegrass:  Low spreading annual.  Narrow, green, boat-tip leaf blades are 3” long.  Active growth is in the fall, winter and early spring, when temperatures are cool.

Black Medic:  A low growing annual, biennial, or perennial which reproduces only by seed production.  Leaves are divided into 3 leaflets with rounded tips and finely toothed.  Central leaflet is on a short stalk.

For additional help on identifying and recognizing weeds click on this link
 Western Weeds or

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02/01/2009