Organic Answers Column – July 20, 2022

Home Testing for Herbicide Contamination

If your vegetables are failing to thrive or are showing unusual signs such as misshapen leaves or odd curling, wilting or distorted new growth, your plants might be exposed to herbicide contamination. Tomatoes in particular will show this damage that might be misdiagnosed as spider mites. Though it seems counter-intuitive, your compost could be the culprit if it contains composted horse manure or was made with mulched hay that was contaminated with a persistent herbicide.

University of Maryland Extension photo of herbicide damaged tomato

Dow’s clopyralid (still sold as Confront) was found to be the contaminant in compost that killed home garden and nursery plants in Washington, Pennsylvania and New Zealand. Aminopyralid, the active ingredient in common herbicides Milestone and Forefront, belongs to the same class of chemicals that includes clopyralid. Picloram is a similarly damaging product.

Unfortunately these and other similar products are commonly recommended and used in agriculture where the compost, manure and mulch products come from. If you suspect the problems in your garden are from the presence of one of these chemicals, there’s an easy home test to find out if your organic products are contaminated with persistent herbicides. The home screening method is a proven Scientific Method.

Compost, manure, and even mulch can be contaminated with herbicides

For compost testing – set up seedling pots or flats and fill half of them with a sterile peat moss-based potting soil (this is the only time I recommend a peat moss potting soil). Fill the other half of the containers with a mixture of two parts of the compost you want to test and one part potting soil. Be sure to label for clear identification. Plant the containers with any legume, but peanuts are best. If certain persistent herbicides are present, germination will be poor and seedlings that do grow will show curled leaf edges. If peanuts are used, the leaves will almost instantly fold up.

Plants grown from raw peanuts are best for testing for possible contamination of materials

For manure testing – plant seedling pots or flats with peas, beans or peanuts (the best choice) and let them grow for a couple of weeks. Mix a slurry of equal parts manure and water in a 5-gallon bucket or other container and let it sit for about an hour. Water half of the seedlings with the manure water and use just water on the other seedlings. If the manure is contaminated, the leaf curling symptoms will appear within a few days.

For mulch or hay testing – use the same procedure as with manure testing, but soak the mulch, hay or other materials overnight before draining off the water for applying it to the seedlings.

In summer, you can use tomato seedlings as test subjects. Materials that turn out to be contaminated should be reported to your local extension service, to the manufacturer of the product, and to the EPA’s Special Review and Re-registration Division.

There is a detox solution if you discover contamination in your garden or the products you’re using. Visit the Dirt Doctor Soil Detox topic.

Here’s a good website to check out for additional information: http://www.the-compost-gardener.com/picloram.html and here is another external source for contamination testing information: https://extension.umd.edu/resource/herbicide-damage-vegetables