Composting Caution: Herbicide Carryover Can Affect Tomatoes

Even small amounts of carry-over herbicides in compost can curl and stunt vegetable crops in the fields or in gardens.

Published on: May 21, 2013

Compost is a beneficial soil amendment for improving soil organic matter and biological activity. But as Ann Hazelrigg, University of Vermont Extension plant pathologist reports, compost also can harbor things that aren't so beneficial.

Last June, for instance, Vermont gardeners began noticing that their tomatoes and other broadleaf plants were showing severe foliar curling and stunting. In some cases, seed germination was affected. The culprits turned out to be two persistent herbicides, clopyralid and aminopyralid, present in minute amounts in the bulk compost used by all the gardeners.

Clopyralid, which will break down during the normal compost curing process, is an endemic problem and appears to be present in many compost products, notes Hazelrigg. Amounts found in the tested Vermont composts were below 10 parts-per-billion threshold, generally too low to harm or cause symptoms in plants and to cause human health concerns.

A LEAF CURLER: Sensitive crops such as tomatoes, can experience leaf-curl or stunting as a result of trace contamination of herbicides still active in compost.
A LEAF CURLER: Sensitive crops such as tomatoes, can experience leaf-curl or stunting as a result of trace contamination of herbicides still active in compost.

Aminopyralid is more problematic, she points out, because it's active at lower concentrations, causing plant damage at levels above 0.2 ppb. It's the herbicide linked to last year's plant damage. It's believed to be associated with horse manure, a common component of many commercial compost products.

It can remain in compost for up to a year, but will breakdown more readily when mixed with soil, adds Hazelrigg.  Last year, many affected plants began growing out of the damage

How it happens
A number of restricted-use herbicides, including picloram, can be present in food crop wastes, grass clippings, livestock feed crops (hay, molasses, sugar beets and oats) and straw. They also can pass through animals into manure, urine or bedding.

All three products mentioned can withstand the heat and moisture of composting. And, they can significant impact on sensitive plants. While such problems can occur, they're rare, adds Hazelrigg. She still thinks using compost is still a good idea.