Identifying & Preventing Herbicide Injury to Landscape Plants & Vegetables

Landscape plants and vegetables can be highly sensitive to herbicides. The following information examines the different causes of injury to desirable vegetation, focusing on herbicide injury, and provides recommendations on preventive measures to avoid such injuries.

Causes of Plant Injury

Plants exhibit injury symptoms when they are subjected to stress. Stress could be caused by biotic or abiotic factors. Biotic factors typically include pests, such as plant pathogens, insects and vertebrates. Abiotic factors include mineral nutrient deficiency or toxicity and extreme weather conditions, such as drought, excessive soil moisture, heat, frost and salt. Apart from environmental factors, pesticides, especially herbicides, may cause plant injury. Injuries are usually characterized by symptoms readily apparent to the trained eye. In certain instances, it may be difficult to discern the actual causal agent without performing additional laboratory analyses. In other instances, abiotic factors or multiple plant stresses may predispose a plant to biotic factors, further complicating diagnosis. Regardless, it is useful to have a thorough knowledge of the conditions to which an injured plant was exposed before making credible deductions.

Herbicide Impact on Non-target Plants

Despite one’s efforts to apply herbicides properly or follow the label as closely as possible, plant injury related to herbicide use happens occasionally. Herbicides may come in contact with vegetables in the form of spray drift or volatile vapors, soil or water runoff from treated areas, or from compost, manure or mulch derived from treated areas.

Drift is the movement of airborne spray particles, usually from nearby areas where herbicides were applied under windy conditions. Spray droplet size and wind velocity are the two primary factors affecting drift, with smaller droplet size and higher wind velocity causing more drift. Volatility is associated with certain formulations of herbicides where vapors may travel longer distances and can occur with herbicides possessing low vapor pressure when applied under hot conditions. Inversion of temperature under certain weather conditions may also cause volatile vapors to move long distances.  Injury under such circumstances would be more widespread and not localized. Runoff includes physical movement of soil particles or water containing low concentrations of herbicide from a treated area. Such a phenomenon is usually associated with significant rains following herbicide application. Leaching, on the other hand, occurs when herbicides move in the solution form to soils containing roots of sensitive plantings to cause injury.

Contaminated mulch, compost or manure are typically the most common causes of herbicide injury in vegetable gardens. Contaminated mulching materials typically include grass clippings from lawns treated with certain classes of herbicides with varying degrees of soil residual activity. Several “weed and feed” fertilizers carry such herbicides in the granular form to control emerged weeds. When such materials are used as mulch, especially during the same growing season, susceptible vegetables express injury symptoms and are unfit for consumption. In some cases, herbicide residues may persist in affected mulch or composts into the following growing season and cause injury to susceptible plants. Similarly, certain herbicides applied in pasture or hayfields may persist in the harvested hay or manure derived from the animal that was fed with this forage.

Herbicides commonly causing injury to vegetables and other plants as a result of carryover are referred to as growth regulator class of herbicides, although herbicides belonging to other classes also can cause injury. Growth regulator herbicides mimic the naturally occurring auxins that regulate plant growth. As a result of exposure to such herbicides, the plants initially undergo unregulated growth and the symptoms manifest as cupping, twisting and curling of stems and leaves (Figures 1a and 1b). The leaves may appear to be strapped or cup-shaped. Such symptoms are collectively termed “epinasty.”

In vegetables, such an injury is irreversible, and the affected plants either die or remain stunted as a result of a single exposure. The active ingredients (trade names within parentheses) of herbicides in the pyridine class that have the potential to cause such carryover effects include clopyralid (Lontrel, Confront, Curtail, Millennium Ultra, Stinger); aminopyralid (Milestone, Grazon Next HL, DuraCor, Chaparral); and picloram (Grazon P+D, Surmount, Tordon RTU). Products containing the active ingredient aminopyralid prohibit the transfer of treated plant materials or manure derived from such materials off the farm in several states, including West Virginia. A waiting period of 18 months is required before affected materials could be moved.

Injury may be exhibited by trees or shrubs subjected to drift or water movement through runoff or leaching to the root zone. Commonly used herbicides in lawns or landscape that contain one or more active ingredients, including 2,4-D, dicamba, mecoprop (MCPP, MCPA), quinchlorac, triclopyr, etc., have the potential to cause injury to sensitive broadleaf plantings nearby through such movement. In some instances, entirely different levels of injury may be expressed by trees belonging to the same species growing side-by-side. This may be due to differences in the root architecture of individual trees, allowing their roots to come in contact with different levels of herbicide concentrations in soil. Proper handling, keeping adequate buffer zones, not spraying when the environmental conditions are not favorable and following other instructions as indicated on the label will reduce the instances of such injury.

Fields where another crop was raised and treated with an herbicide during previous years should follow the rotational restriction guidelines stated on the specific herbicides used. Depending upon the herbicide and the successive crop, rotational restrictions may last up to 36 months in some cases, while there may be no restrictions at all in other cases.

Herbicide Persistence & Bioassays

Composting the herbicide-treated hay, grass clippings and manure from animals fed with treated hay for extensive periods (more than 200 days) may break down most active ingredients by more than 50%. However, it would be better not to use any of these contaminated organic materials to produce finished compost for horticultural or vegetable production. Other susceptible plants include those in the legume family (peas, beans, clover, etc.); composite family (sunflower, lettuce, etc.). Among landscape plants, ash, spruce, pines and other coniferous plants are highly sensitive to such herbicides.

Simple bioassays may be performed to detect the presence of herbicide residues in suspected materials. To perform a simple bioassay, suspected material may be placed in containers and planted with sensitive species, such as beans or tomatoes, and compared to that from untreated material (control). Symptoms indicative of the herbicide in question shown by the seedlings should reveal its presence. Detailed methodology for bioassays can be obtained from Washington State University’s website.

Other Causes of Plant Injury

Many biotic and abiotic diseases or stresses can cause symptoms similar to herbicide injury. Careful observation and diagnosis are warranted on a case-by-case basis. Conifer diseases, such as Diplodia tip blight and Cytopsora canker, can be misdiagnosed if not examined carefully. Close inspection of affected trees will reveal a resinous substance being exuded from infected new shoots if the symptom is caused by any of these diseases. Other foliar diseases, such as needle casts caused by a few species of fungi, will always cause yellow to brown spots or girdling on the needles. Needle tips usually die, starting from the girdled areas. Eventually, small black fruiting structures develop on dead needles. A hand lens or magnifying glass can help you see these structures. In herbicide damage, no such spots or black structures appear on the affected areas. Other signs of diseases to evaluate include, but are not limited to, mushrooms around the trunk and tree base, hyphal growth/mat underneath the bark, brown discoloration of the cambium or even insect boring on the trunk.

Other important observations, such as the distribution of affected plants/trees in the garden or landscape, will help you make a proper diagnosis. For example, needles on one side (roadside) of trees exhibiting browning symptoms while needles away from the road are fine, could be an indication of salt injury. Usually, diseases occur randomly or in patches, whereas damage resulting from abiotic causes or herbicides occurs in a pattern around the application site or from the side of exposure. Also, progression of symptoms due to herbicide injury is typically faster than disease.

Commonly used herbicides, such as glyphosate (Roundup), may also cause injury to exposed plants. When applying such herbicides in the proximity of desirable plants, it is a good idea to leave a sufficient buffer from the treated area. Limited contact from systemic herbicides, such as glyphosate can cause plant death. It may take two to three weeks for the affected plants to exhibit injury and die.

Preventing Herbicide Injury

It is imperative to keep records of fields or lawns treated with herbicides. If there is potential for treated plant material to be transferred as grass clippings, hay, manure or compost, proper lines of communication should be in place from the source to the end user. Often, lawns or pastures may be treated by a commercial pesticide applicator and managed subsequently by the owner or someone else. Transfer of herbicide records among the parties involved is a good practice. Gardeners who use manure from a local farm may need to ask the owner about the source of all hay and bedding material. If confirmation is not available, one may need to exercise caution (e.g., performing a bioassay). To avoid injury to sensitive plantings, maintaining a dedicated sprayer to apply herbicides is highly recommended.

The herbicide label includes a precautionary statement warning the user about any potential crop injury if the treated crop is used as hay or manure. Some labels recommend that a sensitive bioassay be performed to determine if there are any harmful traces in crop residues or compost derived from such residues or manure. Failure to follow the label, inadvertent transfer or gaps in communication between the applicator and the end user may result in crop damage or crop failure.

Plant Diagnostic Clinic at West Virginia University Extension

You may contact your county WVU Extension agent to assist you with troubleshooting. If conclusions cannot be derived on-site, send samples to the WVU Plant Diagnostic Clinic for an in-depth analysis. Relevant information, including the first date of symptom appearance, the part of the plant showing injury, a brief history of herbicide application in the vicinity and prevailing weather conditions, any materials like mulch or compost used, any fertilizer or other chemical application, age of foliage, etc., is always helpful in getting the correct diagnosis.

Figure 1a. Herbicide injury symptoms demonstrated by potato exposed to growth regulator herbicides.

Figure 1b. Herbicide injury symptoms demonstrated by green pepper exposed to growth regulator herbicides.

Rakesh Chandran, WVU Extension Specialist – Weed Science
Mahfuz Rahman, WVU Extension Specialist – Plant Pathology
Thomas Basden, Retired WVU Extension Specialist – Nutrient Management
John Murray, WVU Extension Agent – Taylor County

Bioassay Test for Auxinic Herbicide Residues in Compost: Protocol for Gardeners in Washington State, March 8, 2011 Web document, http://whatcom.wsu.edu/ag/aminopyralid/Bioassay2011.pdf

The Compost Gardener, Residues of Picloram, Clopyralid or Aminopyralid Herbicide Create Web Document, http://whatcom.wsu.edu/ag/aminopyralid/Bioassay2011.pdf

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