“I just want to cry,” a distraught gardening friend told me. “Everything I planted this spring is just sitting there—there’s hardly any growth, and some plants look strange.”
With a sinking feeling, I asked about herbicide use, disease, watering issues, or other stresses. There were none – nothing unusual.
And one more question… have any amendments been added to the soil recently?
“Yes, we added compost to the beds in spring.”
My suspicions were confirmed. The only affected beds were those with newly added compost. Her description strongly suggested the compost contained herbicide residue.
And here is the part that hurts: my friend was doing exactly what she was supposed to do, what we all do and recommend—adding compost to improve the soil.
This is a case of good compost gone bad, and it can happen to anyone (including me).
How Herbicide Gets into Compost or Manure
Classic clues of herbicide residue include poor germination, stunted growth, twisted or cupped leaves, distorted growth, misshapen fruit, and reduced yields. These symptoms alone don’t prove contamination; herbicide drift, disease, insects, and other stresses can look similar. However, the case becomes much stronger when affected beds have recently received a topping of compost or manure.
So, how do herbicides get there?
It can be a long, convoluted path from field to garden. Typically, it starts in the pasture, hayfield, wheatfield, or lawn.
Certain persistent broadleaf herbicides are sprayed to control weeds without harming grasses. Livestock eat treated forage, and the herbicide passes through their digestive systems and into the manure.
Treated pasture/hay → livestock → manure → compost → garden → sadness
Other routes include treated lawn clippings and herbicide-treated straw used as animal bedding, which eventually enter the manure or compost pile.
Meet the Troublemakers
If applying compost is on your fall to-do list (and it should be), you’ll want to take note of these three compounds:
- Aminopyralid
- Clopyralid
- Picloram
These are the three most common active ingredients in growth-regulating herbicides that disrupt the development of broadleaf plants. In a hayfield, that’s weeds. In your garden, that’s tomatoes, peppers, squash, beans, and more.
And here’s the difficult part: they can injure sensitive plants at extraordinarily low concentrations—it takes very little. They can survive an animal’s digestive system and composting, including commercial hot composting, and may remain active for months or even years, depending on the herbicide and conditions. Ugh—tears for sure.
Bulk or Bagged: The Issue Remains
You might think herbicide residue only pertains to the mystery manure found in someone’s pony pasture. Unfortunately, it doesn’t.
Compost, manure, and soil blends can contain feedstocks from many sources. That means both bulk materials and bagged commercial products may contain herbicide residue. A bag doesn’t guarantee that its contents are residue-free.
The Canary in the Compost Pile
By now, you’re probably considering swearing off compost altogether; don’t—it’s too important for soil health. Fortunately, you can test it before it becomes a garden disaster.
Before spreading unfamiliar compost or manure, run a simple plant bioassay. It’s not complicated; no lab coat or clipboard required—just pea or bean seeds, clean potting soil, and the compost in question. Beans and peas are highly sensitive to these herbicides, making them useful little canaries in the compost pile.
Collect several samples of the suspect material, mix each with known-clean potting soil, plant several peas or beans, and grow identical control plants in clean soil. Compare the new growth over two to three weeks. If the test plants curl, twist, or become stunted while the controls grow normally, you’ve got a pretty compelling clue.
How to Conduct a Bioassay
To test compost or rotted manure from an unknown source for potential contamination, you’ll need a few supplies:
- 6 clean growing pots or similar containers (yogurt cups, etc.)
- Bean or pea seeds (about 20)
- Plant tags
- Commercial peat-based potting mix without compost as an ingredient
- Material to be tested (compost or soil mix)
Process
Control Pots: Fill three clean pots with the commercial potting mix. Label each pot with a tag marked “control.” Plant three bean or pea seeds in each pot, one inch deep, then water.
Test Pots: Mix compost with clean potting mix in a 2:1 ratio (two parts compost to one part clean potting mix). Fill the test pots and label each with a tag marked “test.” Plant three bean or pea seeds in each container, one inch deep, then water.
Grow the Plants: Ensure both control and test pots have equal temperature, light, and water.
Evaluate the Plants: Note when the seedlings germinate and their size. Allow the plants to grow for about three weeks, noting any differences in size or shape between the control set and the test plants. Look for stunted growth, twisted stems, cupped or curled leaves, or other deformities in the test plants. If the test plants look similar to the control plants, the compost is likely safe. If you see any signs of distorted growth, the material is likely contaminated.To test compost or unknown
When It’s Already in the Garden
But what if, like my friend, you’ve already added the contaminated compost?
First, stop adding contaminated material. Identify the likely source, remove affected plants and roots, and don’t compost them.
Wait it out. Time and microbial activity will eventually break down the herbicide, though recovery can take months or years. Continue bioassay testing until sensitive plants grow normally.
Give degradation some help. Keep the soil reasonably moist and biologically active to encourage the microbes that break down these herbicides. Add clean organic matter and work it in—this is one situation when firing up the rototiller is encouraged. Aeration increases oxygen and microbial contact (just don’t get too crazy with the tiller).
A cover crop may help, too. Tolerant grasses such as barley, oats, corn, or wheat can keep the soil biologically active as contamination declines. Some research suggests that cover crops may also aid remediation, but evidence is limited. Remove the finished crop and roots rather than composting or consuming them.
Remove and replace the soil. It’s expensive and drastic, but it may be practical for raised beds or small gardens with severe contamination. Don’t compost the removed soil; check locally for appropriate disposal options.
What not to do:
Solarize: There isn’t sufficient evidence that solarization reliably eliminates persistent herbicides.
Add fertilizer: Herbicide-damaged plants aren’t suffering from a nutrient deficiency. Feeding them won’t remove the herbicide.
Assume winter will fix it: One season isn’t a guarantee. Test before planting.
Rely on dilution: Clean soil lowers the concentration, but even extremely low residue levels can damage sensitive plants.
Prevention is the Best Option
Before acquiring bulk compost or manure, ask where the material came from, whether herbicides were used, and whether the supplier conducts bioassays. Unfortunately, sellers don’t always know the origin of every compost feedstock.
For manure, ask what the animals were fed and whether herbicides were used on their hay or pasture. Even the animal owner may not know how purchased forage was treated. With bagged products, there may be no one to ask.
When the source is uncertain, test before you spread. Ask for a sample before buying in bulk; with bagged products, test a small sample first.
Is this a pain to do? Absolutely. But sacrificing a couple of weeks and a handful of bean seeds is considerably easier than remediating an entire garden.
Test Now, Grow Later
My friend thought she had failed at gardening. Instead, she’d encountered something she couldn’t see, smell, or identify by looking at the compost she’d added. We assume organic matter is safe.
But soil amendments deserve the same scrutiny we give seeds, fertilizers, and pesticides. If you’re buying compost you’re unfamiliar with, test it. If you suspect contamination, test it. If you’re waiting for the soil to recover, test it.
Know the source of what you’re putting in your garden—and when you don’t, test it.
Related Posts & Resources
How to Prevent Herbicide Carryover in Compost and Mulch
Herbicide-Contaminated Compost and Soil Mix; OSU Extension
Candace,
Wow! Who knew? I have a “micro garden”…a deep 48” cedar planter for 2 Sashas ( which you gave me ) and a smaller 48” for 1 each basil, parsley, cilantro and rosemary. Being so insignificant I almost blew past this article. What an eye opener! Early spring I sprayed 2.4D on the lawn. Several cuttings later after reading your article suggesting clean grass clippings as mulch, I spread a thin layer of said clippings over both planters. Fortunately I had a great summer of tomatoes and herbs…..phew!
My question is: After several cuttings the clippings have never been sprayed. Does the grass plant take up the 2.4D in such a way that subsequent growth is contaminated?
Thank you for a successful summer.
All the best,
Rich
The short answer is yes—2,4-D can remain in grass for 10 to 14 days. It has a much shorter half-life than other persistent herbicides, but there is still a risk of contamination of the clippings (and the soil) for at least the first two weeks after application. Most likely the clippings used later in the season were fine — and the fact that your plants all look good and are productive is proof. But use caution with this product, especially if you plan to use clippings as mulch in the future.
Hope this helps — happy harvesting!
Candace