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How do weed killers damage plants?

Herbicides are classified according to their usage in controlling weeds. Pre-emergent herbicides, such as those used to control crabgrass, are applied to the soil BEFORE the unwanted crabgrass seeds germinate. Post-emergent herbicides, on the other hand, are used to destroy unwanted vegetation once the foliage has leafed-out. Some herbicides are selective, designed to eliminate specific plants without harming surrounding vegetation. Other herbicides are non-selective, and will kill all vegetation in the treated area. Herbicide damage to desirable plants  will depend on the chemicals in the herbicide, the amount of herbicide applied, and the sensitivity of the desirable plant to the specific herbicide.

Symptoms of herbicide damage may show up in a day or so, or sometimes not for several weeks, after herbicide use. Therefore, herbicide damage can often be hard to diagnose. A trained individual can generally tell when damage from an herbicide is a likely cause, but validation requires a lab test of the plant tissue and/or the soil while the chemical is still present. Such tests can cost between $65 and $150 or more. individual tests need to be performed for each suspected herbicide, and testing may not even be available for some herbicides. Often though, a trained person can offer a pretty reliable diagnosis after examining the damaged plant and surrounding plants, the damage pattern, and the likelihood of a certain applied herbicide having caused that plant damage.

Symptoms of herbicide injury may include: twisting and curling of stems and leaves, leaves taking on a strap-like appearance, abnormally large or fan-shaped leaves, increased distance between leaves, stunting and cupping of leaves, or distortion of flowering and fruiting parts.

Applying excess rates of selective herbicides may injure all the plants in the applied area., Mulch, such as lawn clippings treated with herbicides, may cause damage to plants with which the mulch comes in contact. Additionally, some herbicides react chemically with fertiliizers and insecticides in a way that will cause damage to plants.

Herbicide vapors can also severely affect various plants, including grapes, tomatoes and many ornamental shrubs, flowers and trees. Herbicides vaporize more readily on days when the temperatures are above 80 degrees, and vapors can be carried by air currents for considerable distances even on calm days. Especially true for broadleaf weed killers, spraying in cool temperatures and calm days will minimize vapor drift. To prevent accidental herbicide problems, and to assure the safe and effective use of herbicides, consult knowledgeable personnel, read the product literature carefully, and follow the directions and precautions stated on product labels.

Recovery of herbicide-damaged plants depends on the herbicide, plus the amount of plant damage. Trees and shrubs that receive minor damage from a broadleaf herbicide are likely to recover with only minor damage. At the other extreme, plants damaged by soil sterilant herbicides will probably never recover. The survival of damaged plants can be increased by reducing other stressors. Water during dry periods, fertilize according to a soil test report to increase plant vigor, and watch for and control any insect or disease problems.

Herbicide Testing Labs

Following are some laboratories that could be contacted to get more information on laboratory testing for herbicides and or other chemicals or pesticide residues on plants or in soil:

A & L Great Lakes Laboratories, Inc., 3505 Conestoga Dr., Fort Wayne, Indiana 46808, (260) 483-4759,
Four herbicide screenings range in price from $125 to $225 for testing of plant tissue. One screen will test for Bentzon; Dicamba; 2,4-D; 2,4,5-T; 2,4,5-TP; Triclopyr, for a fee of $225.

Animal Disease, 9732 Shattuc Road , Centralia , Illinois 62801-5858 , (618) 532-6701,
Individual herbicide tests on soil cost between $25-60, herbicide screens cost $60. These fees are doubled for out of state residents. Call to inquire about the cost of testing plant material because plant analysis fees vary by the type of plant.

Columbia Food Laboratories, Inc., 36740 E. Historic Columbia River Highway, P.O. Box 353, Corbett, Oregon 97019, (503) 695-2287,

Columbia Pesticide Profile includes a few herbicides (e.g. atrazine, simazine, metribuzin, linuron, bromacil, terbacil) in the list of 216 chemicals tested for a charge of $225. Testing for just glyphosate (Round-Up, Kleen-Up) costs $300 for a soil or plant tissue sample.