ID-184
Herbicides, when used properly, rarely cause problems on nontarget plants. However, when drift or misapplication occurs, plants that were not intended to receive herbicide may be affected. It is difficult to determine after the fact if nontarget herbicide injury has occurred. Trained individuals may be able to tell if a herbicide should be suspected. However, only analysis of soil or tissue samples can confirm if a herbicide is present. The latter is possible only when the chemical is still present in the plant tissues; extraction and testing procedures are not available for some herbicides.
These herbicides are used to selectively kill broad leaf plants while they are actively growing. This group includes the growth regulators 2,4-D, 2,4-DP, MCPA, MCPP, dicamba, and others.
Herbicides in this group are used to control undesirable grasses. The following herbicides are used preemergence (before the weeds sprout) for control of lawn and garden weeds: Balan (benfluralin), Dacthal (DCPA), and Preen (trifluralin), as well as others. This group is unlikely to cause drift problems. Some herbicides, such as fenoxaprop (Acclaim), are used for postemergence control of weedy grasses and may drift.
This group includes paraquat (Gramoxone), glufosinate (Finale), and soil "sterilants" such as diuron (Karmex, Diuron) and bromacil (Hyvar). Not all of these are available to the homeowner.
The formulation, or form, of active ingredient of an herbicide will determine how it is applied and the likelihood of unintentional spread. For example, a growth regulator such as 2,4-D formulated as a low-volatile ester can vaporize after application and be carried by wind to unintended locations; an amine formulation is less likely to vaporize. In general, granular formulations are rarely carried far from the intended site.
Fine spray droplets have a greater potential for drifting from the site of application than do large droplets.
High temperatures (greater than 85 F) during or immediately after application may cause some herbicides to vaporize and drift to areas outside the site of application. Herbicides that have volatilized may still be capable of causing damage in the vapor state.
Herbicides are not to be applied on windy days; even on seemingly calm days, small gusts can move droplets of spray away from the intended site.
The possibility of root uptake of soil applied herbicides depends on the herbicide applied, the type of soil, and its moisture content. Some herbicides are relatively mobile (e.g. dicamba) and will move readily in sandy or porous soils, especially after a rain or irrigation. Other herbicides may not have little persistence in soil.
Symptoms resembling those caused by herbicide damage may be due to a variety of factors. Mite, insect, or disease damage; adverse weather; soil compaction; drought; root stress; improper soil pH; misapplied fertilizers; genetic mutations; and damage from road salt are just some of the conditions that can mimic herbicide injury. Once mites, insects, and disease have been ruled out, then it is often difficult to determine the cause of the injury.
In order to accurately diagnose injury due to herbicides, one must have knowledge of the symptoms produced by a particular herbicide on a specific plant. This information is NOT readily available for most ornamentals. In addition, the mode of action of the suspected herbicide, the herbicide's behavior and fate in soils, and the dose applied or absorbed are all important in determining if herbicide injury is likely. For example, some herbicides, such as those containing glyphosate (Roundup, Kleenup), become tightly bound to soil particles and are rapidly degraded by microorganisms, minimizing the possibility of root uptake. In contrast, dicamba may be easily absorbed by tree roots extending beneath the treated area, such as a lawn.
Knowledge of other pesticides applied is also important. Previously unsuspected interactions between herbicides and insecticides are becoming known, and improper application of insecticides alone can cause plant damage.
There are several commercial laboratories that can test for herbicide residues in both plant tissue and soil. However, the herbicide in question must be specified and the cost for analysis is high, ranging from $65.00 to $150.00 per sample. A list of laboratories can be obtained by contacting the Plant and Pest Diagnostic Laboratory, Purdue University, 1155 Life Science Plant and Soils, West Lafayette, IN 47907-1155.
Injury from herbicides usually appears within one to two days after exposure, but symptoms may develop up to several weeks after exposure. Injury may show up after longer periods, for example when tree roots grow into sites that were treated up to two years prior with a soil sterilant.
If suspected herbicide damage is observed, it is likely that adjacent plants will show symptoms within the same time frame. Look for injury on two or more different species. Plants that are sensitive to growth regulator herbicides include apple, box elder, dogwood, forsythia, grape, honey locust, horse chestnut, Norway maple, petunia, redbud, rose, tomato, Siberian elm, and sycamore. It is extremely unlikely that herbicide drift injury will occur on just one tree or shrub in a landscape.
Determine where the injured plants are in relation to the area where herbicides were applied. Vaporization of growth regulator herbicides can result in exposure of plants away from the site of application; but again, surrounding plants must be examined. Soil sterilants may move in the soil, but only in the direction of water flow. However, tree roots extend two to three times the length of the longest branches and trees seemingly far removed from the site of application may be affected.
Whether or not a plant will recover from nontarget herbicide injury depends on the overall vigor of the affected plant, the amount of herbicide it received, and the type of herbicide used. Healthy woody plants which receive a low dose of a growth regulator type herbicide will most likely recover. However, if a greater dose was absorbed, the chemical may persist within woody plants and symptoms may appear for the next two or three seasons. Plants which have absorbed a soil sterilant may not recover.
The time when exposure occurred will also affect recovery. Plants that receive an accidental herbicide exposure late in the year when they are preparing to enter dormancy will not be injured as much as plants exposed early in the growing season.
If herbicide injury is suspected, invigoration of the tree or shrub with proper fertilization and watering may help the plant to recover. When edible crops have received herbicide drift, the safety of eating these plants is questionable.
RR 5/93
Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating; H. A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an affirmative action/equal opportunity institution.