Need to Know
- Herbicide damage has a wide range of symptoms. Symptoms include leaf cupping, petiole twisting, epinasty, blotches, color change, severe tissue distortion/deformation (especially on new growth), low vigor, and/or plant death.
- Symptoms may develop within a few days to more than a week after application, depending on the herbicide used, the plant's sensitivity, and environmental conditions.
- Herbicide damage can result from many things, including drift from a nearby application, a misapplication or accidental spraying, and herbicide carryover in contaminated soil, manure, mulch, and compost.
- Damage from herbicides can be easily mistaken for disease caused by pathogens (viruses, bacteria, and fungi).
- There is little that can be done to "cure" a plant that has been unintentionally exposed to herbicide, other than to provide good care to ornamental plants and hope they will recover. Edible plants exposed to herbicides should be removed and replaced.
Not all plant injuries are caused by biotic factors, such as bacteria or fungi; some symptoms are caused by abiotic (non-living or environmental) factors, including herbicides. Herbicide damage can be challenging to diagnose because many of the symptoms can look like those caused by biotic factors.
Learn more about biotic and abiotic symptoms in this article: Biotic vs. Abiotic-Distinguishing Disease Problems.
How Plants are Damaged | Symptoms | Management | Testing & Reporting | More Information
How Plants are Damaged by Off-Target Herbicides
Plants may be unintentionally exposed to herbicides through drift, accidental applications, carryover, or improper use of a product.
Drift
Herbicide drift occurs when spray particles or vapors move off-target due to several environmental factors. Drift is the movement of spray particles or droplets during or after an application, influenced by wind or sprayer setup/equipment.
Drift can also occur when herbicide vapors form and move to other areas. Vaporization or volatilization is influenced by herbicide formulation, temperature, relative humidity, wind, and inversions. Depending on how the drift occurs, it can affect plants only in the immediate area or those over a mile away.
Accidental Applications
Accidental applications happen when sprayers aren’t properly rinsed, contaminating subsequent applications. For example, if a sprayer with herbicide residue is used for a fungicide, unintended plants may be damaged. Careless or distracted applications can also result in herbicides being sprayed where it wasn’t intended. Proper equipment cleaning and careful application techniques help prevent drift and accidental exposure.
Herbicide Carryover
Some herbicides persist in soil, mulch, compost, or manure, causing plant injury. If contaminated materials are added to a garden, plants may show herbicide damage. Grass clippings from treated lawns, wood chips from chemically treated trees, and manure from animals that consumed treated plants can all contain residual herbicides. When added to compost, these contaminants can persist, causing damage to plants when the compost is added to the garden. Depending on the product, the herbicide may be very slow to break down, even when composted.
Herbicide residues can remain in the soil, plant material, manure, and compost for up to a year or longer. There are significant differences among herbicides in how long they will remain active in soil and other materials. The herbicide label provides information on its persistence in the environment.
Improper Use
When herbicides are not used according to label directions, unintentional injury can occur. Always confirm that the desirable plants within or near the herbicide application are not listed on the label as plants that it will kill. Herbicides should not be used on crops, in landscape settings (such as the lawn or vegetable garden), or on plant species not listed on the label.
Damage can also result from applying the herbicide with inappropriate equipment or during unfavorable weather conditions, or from not adhering to reentry, waiting intervals, or use guidelines printed on the label.
Symptoms of Herbicide Injury
Symptoms of herbicide damage vary and depend on the type, formulation, concentration, mode of action, source, and amount of herbicide, as well as the plant species affected, application method, equipment used, and weather conditions during and after application.
Typical Herbicide Injury Symptoms
Symptoms of herbicide injury are varied but, in general, include:
- Distorted, twisted, or irregular stem growth (especially at growing points)
- Cupping or curling of leaves
- Deformed, twisted, and/or irregular leaf growth
- Epinasty (bending or twisting of plant parts, especially stems, downward and outward)
- Leaf veins develop a parallel orientation instead of a typical net venation pattern
- Petiole twisting
- Thickening or callusing of stems
- Blotches or spots in a droplet or spray pattern
- Yellowing of leaves and/or stems
- Die-back of leaves and/or stems
- Reduced vigor or growth rate
- Weak emergence and seedling establishment
- Delayed or uneven flowering and fruiting
- Plant death
Pattern of Symptoms
Herbicide injury typically affects all or most plants in an area, regardless of species. For example, distorted, weak growth with yellowing of the leaves is observed on tomatoes, beans, broccoli, and peppers in the same garden area. Or tomatoes, peppers, and beans at one end of the garden all show symptoms, and the same species at the other end (further from the herbicide application) do not.
Depending on the active ingredient, symptoms may occur only on certain plant types. For example, all the broadleaf plants in the area show symptoms, but none of the grassy plants have issues. While herbicide damage will cause similar symptoms in multiple species in the same area, insect or disease issues rarely affect multiple unrelated plant species in the same way.
Sensitivity
Some plant species are more sensitive to certain types or formulations of herbicides than others. For example, plants such as tomato, grape, pepper, apple, maple, peach, strawberry, elm, redbud, geranium, petunia, and coleus are much more sensitive to off-target applications of 2,4-D or Dicamba than plants such as hosta, daylily, zinnia, marigold, and vinca.
Sensitive species will show more extensive damage than non-sensitive species. More sensitive plant species are also more likely to show damage at greater distances from the herbicide application than less sensitive species.
Time for Symptoms to Appear
Herbicide injury symptoms may develop within a few hours, a few days, or more than a week after the application, depending on the product used.
Herbicide residue or carryover in mulch, manure, compost, and soil can cause injury symptoms in the days or weeks following the application of the soil or amendment.
Knowledge of Nearby Applications
If you know a herbicide application was made in the area prior to the appearance of symptoms, that can further confirm herbicide injury as the cause of the issues. Remember, just because a herbicide was applied nearby does not mean the observed symptoms are due to herbicide drift or misapplication. But when symptoms or patterns indicate herbicide injury and an application was made nearby, it can help confirm the cause of the issue.
Additionally, when the type and formulation of a nearby herbicide application are known, the specific symptoms of that active ingredient can be used to confirm herbicide drift or misapplication.
Herbicide injury symptoms observed on plants are specific to the herbicide chemical class and its mode of action. For example, injury symptoms from an off-target glyphosate application look different from those of dicamba. Below are several classes of herbicides commonly used in home landscapes, along with their potential injury symptoms.
Herbicide Injury Symptoms for Select Classes
| Herbicide chemical class | WSSA Group | Herbicide | Potential symptoms of injury | Mode of Action |
|---|---|---|---|---|
| Growth regulator (synthetic auxins) | 4 | 2,4-D, Dicamba, Triclopyr, MCPA, Picloram, Quinclorac | Epinasty (bending or twisting of plant parts downward and outward), tissue deformation, broadleaf plants affected | causes uncontrolled cell division and growth, destroying plant tissue |
| Carotenoid inhibitor | 13, 27 | Bixlozone, Mesotrione | Chlorosis (yellowing), white, bleached appearance | prevents carotenoid synthesis disrupting photosynthesis |
| ALS inhibitor | 2 | Imazapyr, Penoxsulam, Halosulfuron, Imazaquin, Imazapic, Imazethapyr | Symptoms appear on new growth, chlorosis (yellow), red veins, distorted leaves | prevents the formation of certain amino acids, causing death |
| PPO inhibitor | 14 | Carfentrazone, Sulfentrazone | Necrotic (dark brown or black, dead plant tissue) lesions on leaves contacted by herbicide, broadleaves more sensitive than grasses | destroys cell membranes causing cells to dry out |
| EPSPS inhibitor | 9 | Glyphosate | Symptoms appear on foliage that develops following exposure, chlorosis (yellowing), distorted leaves | prevents protein synthesis needed for growth |
| ACCase inhibitor | 1 | Fenoxaprop, Fluazifop, Clethodim, Sethoxydim | Necrotic (dead) growing point within the stem of grasses, chlorosis (yellowing) | prevents the building of new membranes required for cell growth |
| Photosynthesis Inhibitors (Photosystem II inhibitors) | 5, 6 | Atrazine, Bentazon | Chlorosis (yellowing) between leaf veins and along margins, necrosis (death), symptoms develop from the bottom up | disrupts the process of photosynthesis |
| Photosynthesis Inhibitors (Photosystem I inhibitors) | 22 | Diquat, Paraquat | Necrosis (browning), spotting, water-soaked areas which later turn yellow or brown, symptoms appear within hours of application | forms compounds that destroy cell membranes and chlorophyll |
| Cellulose inhibitor | 29 | Isoxaben, Indaziflam | Mottled or puckered leaves, chlorosis (yellowing), stem girdling, abnormal shoot tips | prevent the formation of cellulose needed to build cell walls |
| Mitosis inhibitors | 3 | Pendimethalin, Prodiamine, Benfluralin, Oryzalin, Trifluralin | Disrupted germination | prevent the separation of chromosomes during mitosis, preventing cell division |
Management of Herbicide-Damaged Plants
General Approach | Edible Plants | Soil, Compost & Other Amendments | Prevention
If plants are damaged by herbicides, they may or may not recover, depending on the severity of the damage. All that can be done is to wait and see what happens while providing good care for the plant.
Preventing other stresses from harming the plant is the best way to minimize herbicide injury. This includes watering during hot and dry periods, fertilizing if the soil is deficient, and scouting for other diseases and insect pests. When herbicide injury is observed, no chemical sprays or nutrient/fertilizer applications can reverse the damage.
New growth may be unaffected, and long-term damage may not occur. This depends on the plant species, its overall health, and the product it was exposed to.
Management of Edible Plants with Herbicide Injury
Whether a crop is safe to eat after accidental exposure to a chemical depends on many factors, including the product used, its active ingredient(s), concentration, mode of action, and other factors. The safest course of action is not to consume any part of a plant that has been exposed to a known or unknown herbicide, and to remove and replace the plants.
For perennial edible plants, like fruit trees, fruit should not be consumed as long as the herbicide is present in the soil or the surrounding environment. Once the herbicide is no longer present in the environment, the time before you can consume the crop varies depending on the product and growing conditions. It is typically at least one growing season. Check the product label or website for the pre-harvest interval and/or the persistence time in the soil to determine how long you must wait before harvest.
Even if you know the product used, the label will typically not provide safety information for fruits and vegetables, since these products are not intended for use on these plants.
Replanting
Depending on the time of year and the source of contamination, replanting the crop may be possible. Some vegetables cannot be replaced and have enough time to mature and be harvested. However, there may still be some alternative crops you can replant. This article can help you determine if and what you can replant: Vegetable Planting and Harvesting Times.
Count the number of days from the anticipated replanting date to your average first frost date. Locate the days-to-maturity printed on the label or seed packet. As long as the days to maturity are fewer than the days left until frost, it’s worth attempting to replant.
Additionally, many garden centers have large, established vegetables in containers that could be purchased to replace damaged plants, and farmer's markets can be a good source of locally grown vegetables and fruits.
Management of Contaminated Soil, Compost, and Other Amendments
If soil testing confirms herbicide carryover in soil, compost, or other amendments, several practical options are available.
Relocation
One approach is to relocate the garden altogether. Establish a new bed in an area that has not received contaminated soil, compost, or other amendments, ideally at least several yards away from the affected site, to reduce the risk of inadvertent mixing. The new location should have similar light and soil conditions so the plants can grow successfully. This is often the simplest and most dependable solution, especially for vegetable gardens. Learn more in this article: How to Kill Grass to Create a New Garden Bed.
Remove and Replace
Alternatively, you can remove and replace the contaminated soil. This typically involves excavating 12 to 18 inches of soil from the entire bed and refilling with clean topsoil and compost from a reliable source. While labor-intensive, this allows for immediate replanting.
Allow the Garden to Sit Fallow
A third option is to allow time for natural breakdown. Herbicides breakdown in the soil through microbial action. Some herbicide formulations break down more readily than others. Leave the area unplanted or, more ideally, seed a tolerant grass cover crop such as oats, sorghum-sudangrass, annual rye, or teff. Avoid broadleaf species, including brassicas, as they are more sensitive to many residual herbicides. While there is no research that definitively shows that increased soil enzyme activity from using cover crops reduces the persistence of residual herbicides in the soil, there are many other good reasons to plant cover crops in this area while it sits fallow, including weed suppression, erosion control, and fertility building.
Plan on at least one full growing season, and often longer, before replanting. Reevaluate the soil the following spring using the same bioassay method, and only resume planting vegetables once results indicate no injury.
Prevention is Important
When herbicide damage occurs, nothing can be done other than wait and see and continue normal practices that maintain plant health. Because of this, preventing herbicide damage is essential.
Follow Label Directions
Always apply pesticides as directed on the label. Pay close attention to the instructions on the label and follow them precisely. Apply herbicides only when environmental factors, such as wind direction, wind speed, and temperature, are appropriate. Align and follow all other factors outlined on the label, such as personal protective equipment (PPE), re-entry interval (REI), and pre-harvest interval (PHI). Equipment used for mixing and spraying must be cleaned thoroughly after applying a chemical to prevent contamination. Learn more in this article: Understanding Herbicide Labels.
Be Aware of Nearby Applications
Scout often and be aware of spraying in your landscape and surrounding fields, yards, and gardens. If an application is made, watch your plants for symptoms of damage in the following days. Knowledge of the herbicides applied in the area can help identify the potential cause of the symptoms.
Avoid Using Amendments Suspected of Herbicide Contamination
If you suspect mulch or manure could be contaminated with a herbicide, avoid using it. For example, do not use grass clippings from recently treated lawns as mulch in a vegetable garden. Mulch or wood chips from trees, stumps, or brush treated with herbicide may contain residual herbicide that can injure ornamental plants. Manure from animals grazing on herbicide-treated plants and compost made from herbicide-treated plant materials can contain herbicide residues even after composting, depending on the type of herbicide used. Carryover in mulch and soil amendments can be avoided by reading the label, following reentry and waiting intervals, and applying the appropriate labeled rate.
Testing and Reporting for Herbicide Contamination
Testing Soil and Amendments at Home
Suspected herbicide-contaminated compost, soil, and other amendments can be tested at home. In early spring, take a representative sample of the compost or soil by collecting from the pile or the top 4 to 6 inches of soil in different areas (the more, the better), then mix them thoroughly. Combine the sample with commercial potting soil at a 1:1 ratio and fill a small container (4-inch, 5-inch, or 6-inch containers are all good options). Prepare a "control" in the same size pot with only potting soil. Plant 3 pea or bean seeds in each container and let them grow for two to three weeks. Plants should develop at least three sets of true leaves before evaluation. Alternatively, you can transplant healthy tomato seedlings into each container and evaluate plant development for at least 4 weeks. Beans, peas, and tomatoes are particularly sensitive to many commonly used herbicides.
If symptoms occur, they may be visible after a couple of weeks and certainly by the time the plants set fruit, often five to seven weeks later. If plants in all the containers remain healthy, the soil, amendment, or location is safe to use. If the control plants are normal and the plants growing in the sample mix show herbicide injury symptoms, the soil or amendment is likely contaminated and should not be used in your garden.
Testing Plant Material (leaves, stems, etc.) for Herbicide Residue
Analysis of plant material, such as leaves, stems, or roots, for herbicide residues can be done, but it does not always provide conclusive results. Additionally, little is known about acceptable thresholds for many herbicides on most vegetables, trees, shrubs, and other garden plants. So even if you conduct a herbicide residue test and now know the concentration of herbicide in the leaves or other parts of the plant, it still does not provide guidance on what to do with the plant, since the acceptable threshold of damage is unknown.
If you are still interested in testing the plant material for residue, Iowa State University does not provide this service. The Iowa Department of Agriculture and Land Stewardship (IDALS) provides a list of private labs that can perform a pesticide residue test. Information on costs and how to collect the sample is available on each laboratory's website.
Reporting and Enforcing Herbicide Drift
The IDALS Pesticide Bureau enforces the Iowa Pesticide Act, which governs pesticide use in Iowa. This would include herbicide injury situations involving misapplication, drift, or negligence by the applicator. More information on pesticide use investigations and enforcement can be found on the IDALS Pesticide Bureau website.
Information on how to file an "Incident Report" with the IDALS Pesticide Bureau to report any pesticide complaints (drift, misapplication, etc.) can be found here: Pesticide Investigation & Enforcement.
More Information
- Understanding Herbicide Active Ingredients for Home Garden Weed Management
- Using Pesticides Safely in the Home and Garden
- Understanding Herbicide Labels
- Managing Weeds in Landscapes, Gardens, and Lawns
- Chemical Injury to Garden Plants
- Home and Garden Pesticide Guidelines (PDF)
- Toxicity of Common Lawn, Garden, and Ornamental Pesticides in Iowa (PDF)
- Factors Affecting Pesticide Drift (PDF)
- Diagnosing Herbicide Injury on Garden and Landscape Plants from Purdue
- Introduction to Weeds and Herbicides from Penn State University
- Herbicide Damage to Plants from the Missouri Botanical Gardens
- Herbicide Mode of Action Key for Injury Symptoms from the University of Wisconsin
Photo credits: 1: Jenny Pollard; 2: Dow AgroSciences; forage herbicide label; 3: Aaron Steil; 4: Iowa State University Plant and Insect Diagnostic Clinic; 5: Ajay Nair; 6: Jenny Pollard; 7: Iowa State University Plant and Insect Diagnostic Clinic; 8: Iowa State University Plant and Insect Diagnostic Clinic; 9: Iowa State University Plant and Insect Diagnostic Clinic; 10: Iowa State University Plant and Insect Diagnostic Clinic
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 21, 2022. The information contained within may not be the most current and accurate depending on when it is accessed.