University of Maryland Extension

Gardener Alert! Beware of Herbicide- Contaminated Compost and Manure

Jon Traunfeld, Extension Specialist, Fruits and Vegetables, and State Master Gardener Coordinator

As gardeners know- “it all starts with the soil”. Improving soil quality with organic matter is essential for growing healthy plants. Many of us rely on manure and compost to improve soil structure and add slow-release nutrients. Unfortunately, even these natural materials can become contaminated by human-made products. Clopyralid and aminopyralid are widely used herbicides that kill many species of broadleaf plants growing in golf course turf, grain fields, and roadways. They have made the headlines in recent years when unsuspecting farmers and gardeners have applied compost, manure, and grass clippings contaminated with these herbicides, to soils growing vegetable crops. These herbicides mimic natural plant growth hormones, disrupting cell division and other growth processes. They injure plants at concentrations as low as 3 parts per million and can remain active in the environment for more than two years (which makes them attractive to farmers and land managers). Symptoms include reduced seed germination, distorted and twisted leaves and stems, stunting, low yields, and death. Clopyralid and aminopyralid will damage most vegetable crops, except for those in the cabbage family. Grasses (including sweet corn), tree fruits, berries, and most woody and herbaceous ornamental plants do not seem to be affected.

Manure becomes contaminated when it passes through a farm animal that ingested sprayed plants. Compost becomes contaminated when it’s made with grass and leaves that have been sprayed or when it’s made with contaminated manure. Heat, moisture, air, and microorganisms all help to breakdown most pesticides in the environment. These particular herbicides are simply more resistant to these natural processes. Problems with contaminated compost began to surface in 1999 and 2000 in Washington, California, and Pennsylvania. These herbicides are still widely used in Maryland by farmers and commercial turf and landscape companies and are applied to crop fields, pasture fields, commercial turf, and roads and right-of-ways. They can only be purchased and applied by certified pesticide applicator, and they cannot be applied to residential turf.

Some Maryland cases

I have encountered this issue in backyard and community gardens on several occasions. In one case, well-decomposed horse manure from a very old and large pile was incorporated into a Master Gardener demonstration garden and damage symptoms (stunting, reduced germination) were observed for the next two years on vegetable crops. In another case, customers of a garden center purchased contaminated compost which led to much frustration and disappointing gardens.

Distortion of tomato leaves caused by clopyralid

In the summer of 2004 I was called out to the Friends House Retirement Community in Sandy Spring, MD. Their community garden received a large supply of free grass clippings dumped next to their garden plots last spring for use as mulch. The clippings looked perfectly fine and were spread thickly by the gardeners throughout much of the garden. Within 2 weeks time, gardeners began noticing severe stunting, twisting, and distortion of foliage in tomato, pepper, squash, and bean crops. The garden leader determined that the clippings had come from a golf course sprayed with an herbicide containing clopyralid. Symptoms did lessen after the mulch was removed but the affected plants did not resume normal growth and produced few fruits. (Washington State University experts claim that the compound does not travel into the fruits of vegetable plants grown in contaminated soils, making them safe to eat.)

What’s a gardener to do?

  • Grass clippings can make a wonderful organic mulch and addition to the compost pile. Just be certain that the clippings you use were not sprayed with any herbicides. Don’t use neighborhood yard waste unless you’re sure it’s free of herbicides.
  • Herbicide-contaminated compost and manure do not look or smell unusual. Most farmers who sell or give manure away may or may not know if their animals grazed on grasses or ate hay sprayed with aminopyralid or clopyralid. Ask commercial compost suppliers if their products are free of herbicide contaminants. Maryland Environmental Service (MES) is the producer of Leafgro, a very popular yardwaste compost available at garden centers throughout Maryland. MES has Leafgro tested regularly by an independent lab and have not detected aminopyralid or clopyralid.
  • Bioassay test- this is the best way to test for possible contamination. You just mix some of the suspect material (hay, grass clippings, manure, compost) with a soil-less growing mix, dump it in a nursery pot, plant pea or bean seeds and observe what happens. Contamination is indicated if the seeds don’t germinate or seedlings emerge that are twisted and deformed.

Zucchini leaves are strappy and cupped

General tips on using manure in home and community gardens

  • Composted manure (that has reached 130 º F. for 3 consecutive days) can be incorporated into the soil in spring or fall.
  • Un-composted manure should only be incorporated in the fall.
  • No top-dressing or side-dressing of vegetable crops with un-composted manure.
    Wash all produce thoroughly after harvest.
  • Never use dog or cat manure in your vegetable garden. They can be high in human pathogens.
  • Do not make compost teas from animal manures. Only use plant-based composts for making compost tea.
  • Horse manure is notorious for spreading weed seeds into gardens. Composting kills most of the weed seeds.

Reference: Herbicide Carryover in Hay, Manure, Compost, and Grass Clippings, NC State University, 4/2010.

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