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Understanding and Using Garden and Home Grounds Herbicides

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R. R. Rothenberger Department of Horticulture, College of Agriculture

Beautiful weed-free gardens and home landscapes provide a feeling of satisfaction. Hand weed control is time consuming and hard work. Proper use of chemicals called herbicides can make weed control easier.

Herbicides are designed to kill plants. Those used among other plants are called selective herbicides because they will kill one type of plant without injury to another. Most home garden uses involve this type of herbicide. Proper use of selective herbicides is essential. Concentration, weather conditions and methods of application may influence the effectiveness of these materials. Too much may damage desirable plants; too little may mean poor weed control. Erratic application may cause damage in some spots and poor weed control in others. Always read herbicide labels carefully before use. Apply only as directed and only to approved plants.

Most selective herbicides are decomposed by microorganisms in the soil. The speed of decomposition is often determined by temperatures and rainfall. Some herbicides applied in early spring may be inactive by midsummer, requiring a second application to keep weed seeds from germinating in late summer.

Terminology

Terminology relating to chemical weed control may be confusing. Following are a few terms relating to herbicides and their use.

Active Ingredient (a.i.) is the percent of herbicidal chemical in a product. In liquids a.i. may be indicated as pounds per gallon. In dry formulations it is usually given as percent of total weight.

Band application is application in a continuous, specific area such as in or along a row rather than over an entire area.

Directed application is application to weed foliage while avoiding foliage contact of desirable plants.

Drift is the movement of spray particles from application area to non-treated area. Extremely fine spray particles, high pressure application and too much wind may cause drift.

Emulsifiable concentrate (E.C.) is a liquid formulation which may be mixed with water for spray application.

Formulation is the mixture of active chemical with inert ingredients to ease application or increase safety.

Granular means a herbicide is formulated in small pellets to be applied dry to the soil. It is usually indicated on the label with "G." Therefore, 10 G would indicate a granular formulation containing 10 percent herbicidal chemical.

Non-directed is a spray application covering all plants without regard to contact on desirable plant foliage.

Non-selective is a herbicide that will kill any plant it contacts.

Preplant is application of a herbicide before the crop is planted.

Pre-emergence herbicide is able to kill weeds only if present while the seeds of the weed are germinating. Label directions indicate proper time for use. Pre-emergence herbicides do not kill growing plants that have developed green color.

Post emergence herbicide kills plants after they are green and growing actively. May be used to kill annual weeds that have emerged from the soil as well as some perennial weeds.

Rate indicates how herbicides are used according to amount per unit of land area (1,000 square feet, acre, etc), while insecticide sprays are recommended at a precise amount per gallon or 100 gallons. Directions usually indicate the best method for spray application of these liquid materials. Fertilizer spreaders may be used for granulars but should be calibrated and set according to directions.

Selective herbicide is more toxic to some plants than others when used at proper concentrations.

Wettable powder is often designated as WP on labels. A 50W indicates a 50 percent chemical in a powdered form that can be mixed with water for spraying.

Mineral soil is sand, clay, silt or other soil with less than 10 percent organic matter.

Organic soil contains more than 10 percent organic material.

Naming Herbicides

Home gardeners often find the names of weed-killing materials confusing. Each material may have names in four separate categories: chemical name, common name, trade name and brand name.

Chemical name is a long, usually difficult name that describes the chemical structure of the material. This name is found in small print somewhere on the container label.

Trade name is a name given the chemical by the major producer or marketer. In many cases the trade name is better known than the common name. For this reason materials have been listed by trade name in Guide 6952. There is no intent to indicate one manufacturer over another, however. Most materials have only one trade name although a few have more. For example, amitrol has several: Amino triazole, Amizol and Cytrol.

Common name is a chemical shorthand which may or may not be given on the label. Example: 2,6-Dichloro benzonitrile (chemical name) becomes dichlobenil (common name).

Brand name is often the name given by the packaging company. Such names as Garden Weed Preventer, Weed-B-Gon, Ornamental Weeder and Garden Weed and Feed are brand names. Materials may be listed under a brand name, although consumers may find some recommended materials in containers giving the trade name in large letters. To determine what chemical is actually contained the label must be read carefully.

This artical does not include brand names since they vary and often are regional.

Label Clearance

For a herbicide to be used among food crops or ornamental plants. it must be tested and receive label registration. Each label will list those plants, crops or conditions in which the material may be used. Labels and the list of plants given on them should be checked carefully before a herbicide is used. Failure to use herbicides as recommended may result in plant damage and is also a violation of the Federal Insecticide, Fungicide and Rodenticide Act.

Application of Herbicides

Sprays. Most herbicides can be applied easily and economically as sprays. Low pressure sprayers should be used among ornamental plants because they produce large coarse droplets less likely to drift than those from fog or mist sprayers. When using sprays for pre-emergent herbicides, use only enough water to thoroughly cover the area. For post emergence herbicides, which must penetrate weed foliage, a larger volume of water is needed. Low volume usually means between 20 and 40 gallons of liquid applied per acre. On the home grounds, 20 to 40 gallons per acre converts to about 1/2 to 1 gallon of liquid per 1,000 square feet. This should be the minimum quantity. It is always better to apply more water but not more herbicide for better coverage. Generally, a low volume application for the home garden using the required amount of herbicide in 1 gallon of water sprayed on 1,000 square feet should be adequate.

As already indicated, the amount of liquid sprayed per area of land is not as critical as the amount of herbicide. If a recommendation calls for 20 pounds of herbicide per acre, this could be reduced to about 1/2 pound herbicide per 1,000 square feet. The 1/2 pound is critical and must be kept constant per 1,000 square feet but might be applied in either 1/2 gallon or 2 gallons of liquid. The amount of liquid used may depend on the equipment available. Sprays should be applied close to the ground. Larger volumes of liquid allow application twice at right angles over the same area for more uniform coverage. Rates may sometimes be recommended in terms of active ingredient. If a rate of 6 pounds per acre of active ingredient is required and a formulation contains 50 percent active ingredient, we need to apply 12 pounds of material from the bag per acre.

Cleaning a sprayer. It is almost impossible to adequately clean a sprayer that has been used for spraying herbicides. Therefore, it is best to designate and mark a sprayer specifically for herbicide use. A sprayer used for herbicide application should not be used for insecticide or fungicide application to desirable crops and plants.

Granular. Granular formulations of many herbicides have become popular because they are easier to use and less subject to drift. Herbicides that act as pre-emergence materials for annual weeds have been most successful in these formulations. However, some post emergence materials have also been formulated for effective use by this means of application.

For large areas fertilizer spreaders may be used at the settings recommended by the herbicide manufacturer. For smaller use, some manufacturers supply shaker cans that help scatter granular materials properly in small areas.

Precautions

Keep all new or unused herbicides in their original containers and locked up where children cannot yet to them. Don't take herbicides internally or allow them to contact the skin or eyes. After using herbicides, wash skin thoroughly. Do not smoke while using herbicides.

Do not pour unused herbicides down the drain or in streams, irrigation channels or drainage ditches.

Trade names are used for better understanding of information presented. No endorsement of named products is intended nor criticism implied for similar products not mentioned.

Note: Detailed information on weed control in turf is given in "Lawn and Turf Weed Control."

Suggested herbicides for use on fruit, vegetable and landscape plants are listed in Guide 6952, "Garden and Home Grounds Weed Control." CAUTION: Always read the herbicide label before use. Never use more herbicide than recommended, or damage to desirable plants may result. Follow directions carefully. The container label will list plants for which that herbicide has been approved. Since registration status of pesticides is reviewed continuously and is subject to change, read the product label before purchasing to make sure it is registered for your need. To use a product in any way that is inconsistent with the label is in violation of the Federal Environmental Pesticide Act of 1972.


TOXICITY OF PESTICIDES

George C. Hamilton, Ph.D Extension Pesticide Coordinator

Toxicity is defined as the extent or degree to which a chemical substance is poisonous to humans or other animals. Therefore by definition, a pesticide must be poisonous to kill or injure the target pest. Toxicity to different organisms varies greatly, however, each pesticide has a unique level of toxicity that varies only to a small degree within specific animal groups. Conversely, this level of toxicity may be drastically different between unrelated groups of animals.

The toxicity of a pesticide is determined by running laboratory tests on animals. The animals tested include bobwhite quail, mice, rats, rabbits, and rainbow trout. Testing is conducted by subjecting the animals to a series of different types of exposure to the pesticide's active ingredient or that portion which causes toxicity and includes dermal, oral, and inhalation testing. Dermal testing is conducted by exposing the skin of test animals to the chemical. Oral tests are run by feeding the chemical to the animals. Inhalation tests are done by introducing the chemical into the air that the animals breathe. Skin and eye irritation tests are also conducted.

The toxicity measured by these tests is expressed as LD50 and LC50 values and are the amount (LD - lethal dose) or concentration (LC - lethal concentration) of the pesticide's active ingredient that is required to kill 50 percent of the tested animals under standardized test conditions. These values are based on single doses and are expressed in milligrams per kilogram of test animal body weight (mg/kg) or in parts per million (ppm). Both are useful in making toxicity comparisons for different active ingredients and for determining the degree of toxicity to a specific group of animals. Pesticides with very low LD50 and LC50 values are highly toxic. Those with high values are least toxic when used according to label directions.

By law, pesticides are categorized into four classes based on LD50 and LC50 values. A pesticide's toxicity class must be stated on the product label and be designated by one of these signal words:
DANGER-POISON, WARNING, CAUTION, and no signal word.

The class with the highest toxicity is designated by the DANGER-POISON signal word. This signal word must be in red letters and a skull and crossbones must be displayed on the product label. Ingestion of a few drops of material in this class could result in death to a 150-pound person. Moderately toxic pesticides are designated by the signal word WARNING on the label. From one teaspoon to one ounce of material in this class could be toxic to a 150-pound person. The third category carries a CAUTION signal word on the pesticide label and denotes either slightly or relatively non-toxic products. An ounce to more than one pint of a chemical in this category could kill an average-size adult. The final category carries no signal word. The specific toxicity ranges for each category are listed in Table 1.

Despite the fact that some pesticides are relatively non-toxic, all pesticides can be hazardous if label instructions are not followed. Remember to use the pesticide only as recommended on the label provided by the manufacturer. As the applicator, you are liable if the pesticide is misused in any manner.

For information regarding the toxicity for a specific pesticide read the product label or contact the New Jersey Poison Information and Education System by calling 1-800-962-1253. Should a poisoning situation occur, follow the procedures listed on the product label, then call a physician. Remember to bring the product label with you when you see the physician.

Table 1. Categories of Pesticide Toxicity
Route of Administration
Category Signal Word Oral a Dermal a Inhalation b
I Danger-Poison 0 to 50 0 to 200 0 to 2000
II Warning >50 to 500 >200 to 2000 >2000 to 20000
III Caution >500 to 5000 >2000 to 20000  
IV none >5000 >20000  
a Ranges represent LD50 values in mg/kg, single dose.
Minimum 14 days observation.
b Ranges represent LC50 values in ug/l, 1 hour exposure.
Minimum 14 days observation.

HERBICIDE INJURY TO TREES

John A. Meade

An herbicide is a chemical used to kill unwanted plants. Often these plants grow in association with desirable plants. To kill one plant growing in close association with another is difficult and relies on careful selectivity. This selectivity depends on several things: the herbicide, temperature, rainfall, and species of plant involved. When selectivity fails, there is tree injury. Herbicides generally involved fall into two classes: 1. compounds used to kill broadleaf weeds in turf; and 2. total vegetation control agents. The first group causes injury in four ways:

1.Direct contact
- if sprayed on leaves these herbicides will cause severe injury all through the trees. Bark absorption is usually minimal except on young and thin-barked trees. Application to bark, however, is to be avoided.

2.Drift
- movement of spray particles through the air. These particles contact the leaves of desirable plants and injury results. This, of course, shows up only during the growing season. Conifers are more resistant than deciduous plants.

3.Volatility
- these compounds are sometimes used when air temperatures are high (85 F.) At elevated temperatures, some formulations evaporate into a gas. This gas then moves through the air and contacts desirable plants. The herbicide is absorbed and injury occurs.

4.Root Absorption
- while the life of these compounds in soil is short (2-3 weeks), and they do not penetrate deeply into the soil (1 to 4 inches), tree roots can intercept some of these compounds and move them up into the tree where injury symptoms show up. Excessively high rates maximize this effect.

The second group (total vegetation control) usually causes injury by root absorption. These herbicides are designed to be long-lasting in the soil and some persist for 1 year or longer. Since they remain longer, they move deeper into the soil with rainfall. Hence, they have a greater chance of coming in contact with tree roots.

APPEARANCE OF INJURY SYMPTOMS:

Herbicides in the first group - 2,4-D*, MCPP (mecoprop), 2,4-DP (dichlorprop), Banvel (dicamba), and triclopyr all affect plants in similar fashion. They generally move from the point of contact to the growing plants. Hence, injury symptoms most often appear in young leaves first. These compounds cause malformation of the leaves and petioles. The edge of the leaf may curl up to form a cup or may curl downward. If the leaf has points, these points become elongated and often look like strings. The petiole will often curl also. With some species, the leaves assume a "strap-like" shape, long and narrow. The herbicide bromoxynil (BROMINAL/BUCTRIL) used to control small weeds in newly seeded or established lawns has no effect on trees outside of a localized burn if sprayed on the leaves. The total vegetation control materials generally enter trees through the root system, either because the tree roots were growing into the treated area or because the herbicide had washed across the surface of the soil and then penetrated the root zone. Incidentally, once herbicides have moved downward into the soil, they do not move sideways to any significant degree. One problem in using herbicides is trying to determine how far tree and shrub roots grow away from the trunk. This varies significantly with the species and soils.

Materials, such as HYVAR, PRAMITOL (prometon), UROX, SPIKE, TELVAR and various other trade names, enter the root system and move up into the tree. The injury symptoms are manifested first at the edge or tip of the leaf and then generally move back to the base. The leaf first turns yellow and then brown. HYVAR sometimes blackens the leaf. Depending on the dosage, the tree defoliates during the year of treatment and then refoliates. In the second year, the tree will put out weak foliage and then death occurs.

TREATMENT AND PROGNOSIS:

Trees often recover from exposure to the first group (compounds used to kill broadleaf weeds). The leaves will become distorted and will often drop, but depending on species and dosage, the trees will appear normal in about 2 years.

However, death can occur if the dosage is high. There is no specific treatment to alleviate the injury symptoms. Pruning the branches will often help the appearance of the tree. It is probably best not to fertilize the trees, but additional water would be helpful. Don't be in too much of a hurry to remove injured trees during the first year because they can make surprising comebacks from the action of these herbicides.

Trees reacting to exposure to the second group of herbicides (total vegetation controls), however, have shown a smaller rate of recovery. Again, after the material is in the tree, little can be done to prevent injury. Pruning is of little value since the materials have been widely distributed throughout the tree. It would be best to wait 1 to 2 years to see if the tree will recover. If it does not produce foliage the year after exposure, it can be assumed that the tree is dead and should be removed. If you recognize a misapplication, activated charcoal can be used to neutralize the herbicide. Trademark names include GRO-SAFE, AQUA NUCHAR, AND FLOWABLE CHARCOAL.

OTHER HERBICIDES:

Preemergence crabgrass preventers and postemergence crabgrass killers pose no threat to trees or shrubs. Also, herbicides used in flower beds and vegetable gardens will not injure trees.

OTHER CAUSES OF INJURY:

Trees can do only so many things and factors other than herbicides can cause twisting, curling, yellowing, and defoliating. Stress from physical injury (mowers, etc.), drought, and heat can all cause the same symptoms that herbicides cause.

Notice: The information contained on the following web pages is derived from industry sources which are considered reliable. Information is subject to change and withdrawal without notice; therefore, it is the responsibility of the consumer to verify reliability on an individual basis based on specific consumer needs. We assume no responsibility, and extend no guarantees for information provided. Trademarked names are used in an editorial context with no intent of trademark infringement.

Copyright ©1995 - 1999 Barrington Multi Media., all rights reserved.


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