Need to Know
- Herbicide damage has a wide range of symptoms. Symptoms include leaf cupping, petiole twisting, epinasty, blotches, change of color, 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 the application, depending on the herbicide used, sensitivity of the plant, 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 confused with diseases caused by pathogens (viruses, bacteria, and fungi).
- There is little that can be done to "cure" a plant with unintentional herbicide exposure 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 a biotic factor, like 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 can be the movement of spray particles or droplets during or after an application, influenced by wind or sprayer set-up/equipment.
Drift can also happen from vapors of the herbicide forming and moving to other areas. Vaporization or volatilization is influenced by herbicide formulation, temperature, relative humidity, wind, and inversions. Depending on how the drift happens, it can affect plants only within the immediate area or plants 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 herbicide 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 also persist, which can cause 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, as well as not adhering to reentry, waiting intervals, or use guidelines printed on the label.
Symptoms of Herbicide Injury
Symptoms of herbicide damage are varied and depend on the type, formulation, concentration, mode of action, source, and amount of herbicide, as well as the species of the plant 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 or irregular 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)
- 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
- Plant death
Pattern of Symptoms
Herbicide injury typically affects all or most plants in an area, regardless of species. For example, distorted and weak growth with yellowing in 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 only be on certain types of plants. 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 like tomato, grape, pepper, apple, maple, peach, strawberry, elm, redbud, geranium, petunia, and coleus show much more sensitivity to off-target applications of 2,4-D or Dicamba than plants like hosta, daylily, zinnia, marigold, and vinca.
Sensitive species will show more extensive damage than non-sensitive species. Those plant species that are more sensitive are also more likely to show damage at distances further from the herbicide application than those that are less sensitive.
Time for Symptoms to Appear
Herbicide injury symptoms may develop within a few hours, a few days, to 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 symptoms observed are caused by herbicide drift or misapplication. But when symptoms or patterns indicate herbicide injury, and it's known an application was made nearby, it can help further 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 dicamba. Below are several classes of herbicides commonly used in home landscapes with their potential symptoms of injury.
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 (Photosytem II inhibitors) | 5, 6 | Atrazine, Bentazon | Chlorosis (yellowing) between leaf viens and along margins, necrosis (death), symptoms develop from the bottom up | disrupts the process of photosynthesis |
| Photosynthesis Inhibitors (Photosytem 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
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, active ingredient(s), concentration, mode of action, and many others. The safest course of action is not to consume any part of a plant 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 gone from the environment, the length of 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 time the product is persistent in the soil to know how long you must wait before harvest.
Even if you know the product used, the label will not typically provide safety information for fruits and vegetables since they are not intended for use on these plants.
Replanting
Depending on the time of year and contamination source, replanting the crop may be possible. Some vegetables will not be able to 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.
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. Only apply herbicides when the 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. More information on reading herbicide labels can be found in these resources:
- What You Need to Know about Reading a Pesticide Label from Penn State Extension
- The Environmental Protection Agency (EPA) pesticide information page
Be Aware of Nearby Applications
Scout often and be aware of spraying that occurs in your landscape or in 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 can have herbicide residue that can injure ornamental plants. Manure from animals grazing on herbicide-treated plants and compost made from herbicide-treated plant materials can contain herbicide residue even after composting, depending on the amount of time and type of herbicide used. Carryover in mulch and soil amendments can be avoided by reading the label for information, 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 samples from the pile or the top 4 to 6 inches of soil in different areas (the more, the better) and mix them thoroughly. Combine the sample in a 1:1 ratio with commercial potting soil 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 the development of the plants for at least four weeks.
If symptoms occur, they will be visible by the time the plants set fruit, often five to seven weeks later. Beans, peas, and tomatoes are particularly sensitive to many commonly used herbicides. 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
An analysis of plant material, such as leaves, stems, or roots, for herbicide residue can be done, but they do 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 found in the leaves or other parts of the plant, it still does not give any 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. IDALS Pesticide Labs List
Information on costs and how to collect the sample are found on the websites of each respective laboratory.
Reporting and Enforcing Herbicide Drift
The IDALS Pesticide Bureau provides enforcement of the Iowa Pesticide Act, which governs pesticide use in Iowa. This would include herbicide injury situations that involve misapplication, drift, or negligence on the part of the applicator. More information on pesticide use investigations and enforcement can be found on their website: IDALS Pesticide Bureau.
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
- American Phytopathological Society (chemical injury section)
- 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
- Effect of Metsulfuron-Methyl-Containing Herbicides on Ornamentals
- Chemical Injury to Garden Plants
- Home and Garden Pesticide Guidelines (pdf)
- Toxicity of Common Lawn, Garden, and Ornamental Pesticides in Iowa (pdf)
- Understanding Pesticide Labels (pdf)
- Small Sprayer Calibration (pdf)
- Factors Affecting Pesticide Drift (pdf)
- Integrated Pest Management for Home Gardens and Landscapes (pdf)
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.