Herbicide damage from contaminated compost or soil mix is caused by growth regulator herbicides. These herbicides more often affect broadleaf plants. Look for distorted or cupped leaves. The damage occurs in new growth, including the ends of the shoots.
Crops prone to damage from this group of herbicides include (grouped by plant family):
Rule out other causes of plant damage. Plant injury from nutrient deficiencies, plant diseases and insect pests can look similar to herbicide damage.
Suspect damage from herbicides when:
Plants show signs of stress when they don’t have the right nutrients at the right time in their growth.
For example, tomatoes with magnesium deficiency have yellow leaves with green veins. Damage affects the older leaves (at the bottom of the plant) (Figure 4).
Symptoms of nutrient deficiencies show in one kind of plant and not another. Herbicide damage often presents in more than one type of plant.
Plant diseases such as viruses also cause stunted growth, distorted leaves and a mosaic pattern. The damage looks similar to herbicide damage (Figure 5).
To tell the difference, virus diseases usually affect several plants of one kind such as tomatoes or grapes. Herbicide injury will affect different kinds of plants.
Also, plant viruses don’t cause distinct cupping, which is a symptom of herbicide damage.
Insects or mites also cause leaf distortion. The photo shows damage from erineum mites on grape leaves (Figure 6).
Insect damage occurs randomly on some leaves and not others.
Herbicide injury often presents on the tips, or growing points, of a plant’s shoots.
Is the herbicide injury a result of contaminated compost or soil mix, or another cause?
If you observe distorted growth or cupped leaves on your plants, determine the cause of the damage.
If you have applied compost or soil mix and ruled out other sources of herbicide damage, you may have herbicide-contaminated compost or planting mix.
Herbicide-contaminated compost affects plants prone to injury from growth regulator herbicides. It causes distorted or cupped leaves and growing points.
Yes. Don’t eat produce from plants showing damage from herbicide contaminated compost or planting mix.
If the herbicide contaminated compost or soil mix has already been applied, don’t plant crops prone to damage.
Take steps to remediate the soil.
If you know the exact herbicide ingredient that caused herbicide injury, call the National Pesticide Information Center at 1-800-858-7378 to determine if you can eat the produce.
To report suspected contamination or file a complaint about an unresponsive vendor, contact the Oregon Department of Agriculture. ODA investigates pesticide complaints in Oregon. ODA doesn’t assist in the pursuit of damage reimbursements.
Submit a pesticide complaint form as soon as possible after the incident. Complaints received after 30 days of when damage was first noticed will be filed, but may not be investigated.
Send your completed incident complaint form via email, postal mail or fax.
Oregon Department of Agriculture
635 Capitol St. N.E.
Salem OR 97301-2532
Perform a bioassay test to determine if a compost or soil mix is contaminated with growth regulator herbicides. Compare plants grown in clean potting soil (the control group) with plants grown in the compost or soil mix in question.
Follow the instructions below to test potentially contaminated compost or soil mix. The procedures were developed by Washington State University Extension, Whatcom County and adapted by Montana State University.
This procedure is also used to test for contaminated manure or animal bedding. Test unknown materials before you make your own compost.
If you observe damage, the compost or soil mix is likely contaminated.
Don’t use it to grow broadleaf plants prone to damage from growth regulator herbicides.
See below for more information about your options if you already have a contaminated product.
If you have a load of contaminated compost or soil mix, apply it to your lawn. Or, put it in planting beds intended for nonbroadleaf plants (monocots).
Herbicide-contaminated compost or soil mix affects broadleaf plants. The herbicides that cause the contamination don’t harm monocots. Turfgrass, ornamental grasses, lilies and irises are all monocots.
If you’ve already applied contaminated compost or soil mix to your vegetable garden area, remove damaged plants. Don’t plant additional broadleaf plants prone to injury from growth regulator herbicides.
Instead, plant monocots such as corn. Corn may accumulate the herbicide residue just like damage prone crops such as tomatoes and beans. But corn doesn’t show symptoms.
If you know the exact herbicide ingredient that caused herbicide injury, call the National Pesticide Information Center for assistance at 1-800-858-7378 to determine if you can eat the produce.
Perform the bioassay test one to two times per year until there are no signs of damage. When a bioassay test shows no damage, it is likely you can grow broadleaf crops such as tomatoes and beans without damage from the contaminated soil.
If you’ve already applied herbicide contaminated compost or soil mix to your garden area, it will eventually break down. It can take from two to five years for the herbicide to completely degrade.
Perform bioassay tests before planting crops prone to damage from growth regulator herbicides. Keep performing the bioassay test one to two times per year until there are no signs of damage. When a bioassay test shows no damage, it is likely safe to plant broadleaf crops.
Naturally occurring microbes in the soil eventually break down persistent growth regulator herbicides. You can speed this process by increasing the soil’s biological activity.
Soil microbes including bacteria and fungi feed on the herbicides in the soil. The microbes need air, water and energy to live. You can increase these factors with the following methods:
Additionally, products designed to promote microbial growth, such as Soil Diva and Algae AquaCulture Technologies, might help. These products may or may not be available in your state. Scientific evidence on the effectiveness of these products isn’t available. Anecdotal evidence suggests that they could hasten the decomposition of growth regulator herbicides in soil.
Plant a cover crop to bioaccumulate the herbicide. Use broadleaf plants such as sunflowers, peas or radishes. Grow the crops. Remove the stems and roots.
Put the material in the trash or take it to a landfill. Don’t put it in your yard debris bin or take it to a compost facility.
If you don’t want to remove the contaminated cover crop residue from your site, you could compost it. Then apply it to your lawn. Or, use contaminated cover crop residue as a mulch for ornamental grasses, lilies, irises and similar plants.
Don’t use the cover crop residue or its compost for broadleaf plants prone to damage from growth regulator herbicides.
Another option is to plant a monocot cover crop such as sudangrass. Sudangrass and other grasses aren’t damaged by the herbicide-contaminated soil.
A Sudangrass cover crop produces a lot of biomass. In this case, incorporate the cover crop into the soil. The organic matter will increase soil biological activity.
Activated carbon and biochar bind to herbicides in the soil, making them inactive.
To apply activated carbon, mix 1 pound activated charcoal in 1 gallon of water. Sprinkle the mixture over 150 square feet of soil surface.
Follow the application rate on the label instructions for biochar products. Biochar may contain pollutants that are hazardous to human health. Further research is needed about using biochar in garden soils and soil remediation.
Both additives can increase soil pH. Increasing the soil pH could be an issue for areas that already have alkaline soils.
Also, both additives could reduce the efficacy of future pesticide applications. The carbon from the products could bind with soil-applied products.
Some people may want to remove herbicide contaminated soil.
For a small volume of contaminated soil, put the material in the trash or take it to a landfill.
For a large volume of contaminated soil, rent a dumpster from a waste management company. Notify the drop box company that it contains herbicide-contaminated soil for the landfill.
Call the landfill ahead of time to get disposal instructions and costs.
The cost to remove contaminated soil and properly dispose of it is very high. Consider other options to remediate the soil.
Do some research before you purchase a load of material from a vendor. You can avoid problems with contaminated compost or planting mix.
Dana, M.N., S. Weller, G. Ruhl, F. Whitford. 1991. Diagnosing Herbicide Injury on Garden and Landscape Plants, ID-184-W, Layfayette, Indiana: Perdue University.
Derr, J., M. Flessner, E. Bush, M Hanson. 2016. Plant Injury from Herbicide Residue. PPWS-77P, Blacksburg, Virginia: Virginia Cooperative Extension.
Tharp, C. 2019. Herbicide Contaminated Soil and Amendments, Bozeman, Montana. Montana State University.
Washington State University Extension, Whatcom County. 2011. Herbicide Contamination of Organic Matter, Bellingham, Washington.