R. R. Rothenberger
Department of Horticulture, College of Agriculture
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.
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.
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.
relating to chemical weed control may be confusing. Following are
a few terms relating to herbicides and their use.
(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.
is application in a continuous, specific area such as in or along
a row rather than over an entire area.
is application to weed foliage while avoiding foliage contact of
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.
concentrate (E.C.) is a liquid formulation which may be mixed with
water for spray application.
is the mixture of active chemical with inert ingredients to ease
application or increase safety.
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
is a spray application covering all plants without regard to contact
on desirable plant foliage.
is a herbicide that will kill any plant it contacts.
application of a herbicide before the crop is planted.
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.
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.
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.
is more toxic to some plants than others when used at proper concentrations.
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.
is sand, clay, silt or other soil with less than 10 percent organic
contains more than 10 percent organic material.
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.
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
is a chemical shorthand which may or may not be given on the label.
Example: 2,6-Dichloro benzonitrile (chemical name) becomes dichlobenil
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.
does not include brand names since they vary and often are regional.
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.
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.
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.
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.
are used for better understanding of information presented. No endorsement
of named products is intended nor criticism implied for similar
products not mentioned.
information on weed control in turf is given in "Lawn and Turf Weed
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.
Hamilton, Ph.D Extension Pesticide Coordinator
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.
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.
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.
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.
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.
1. Categories of Pesticide Toxicity
||0 to 50
||0 to 200
||0 to 2000
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.
John A. Meade
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:
- 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.
- 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.
- 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.
- 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.
OF INJURY SYMPTOMS:
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.
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.
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.
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.
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.
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.
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.
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.
- 1999 Barrington Multi Media., all rights reserved.