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.
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.)
Zucchini leaves are strappy and cupped
Reference: Herbicide Carryover in Hay, Manure, Compost, and Grass Clippings, NC State University, 4/2010.