Herbicides in Compost

Despite the concerns about clopyralid and picloram (see below), most herbicides do not pose problems with compost systems. Homeowners and municipalities have three basic options:

1. Stop or limit use of herbicides. Grasscycling and mulching of bare soil are techniques that reduce problems with weeds.
2. If you use a herbicide on a lawn, grasscycle.
3. Compost herbicide treated clippings in an active, aerated compost pile for at least three months.

Long Lasting Herbicides a Problem for U.S. Composters

For most rules of thumb, there are exceptions. The assumption and experience had been that a properly managed, hot, aerobic compost program would degrade pesticides. That is true in most cases, but recently concerns have arisen about two long lived herbicides: clopyralid and picloram. It took persistence and good lab work to find the culprits because they need only be present in extremely small amounts to cause problems. As yet, this problem has not been identified in Canada because of differences in approved usage, but the story is one that we are well advised to follow.

Three separate incidents, all in the summer of 2000, brought the concern to light:

Penn State University: Compost made at the university was used in a compost response trial with vegetables. Four weeks after transplanting, the plants began to show the leaf cupping typically caused by contact with 2,4 D type herbicides. After several other factors were investigated, the compost was tested for herbicide residues. Only clopyralid was detected, in levels ranging from 10 75 parts per billion. Eventually, it was determined that the herbicide had entered the compost system through grass clippings that had inadvertently been collected with fall leaves. The university has since stopped using any herbicides containing clopyralid on their lawns.

Spokane Regional Compost Facility: This is a facility that composts grass clippings, leaves and other plant materials. The problem was noted when tomatoes planted in containers filled with compost from that facility were damaged. Clopyralid on grass clippings was the source of the problem.

Washington State University: In this instance, the compost was made entirely from manure and bedding. The university sold the finished product to nurseries and private individuals. There were unhappy customers in 2000. The problem was finally traced to picloram which had been sprayed on pasture that had eventually been hayed. The picloram persisted through both the digestive and compost processes. In 2001, they found clopyralid in their compost, again from application to hay.

The chemicals: Clopyralid and picloram are chemically similar, long lived herbicides that control broadleaf plants. Clopyralid is considered the major problem chemical, since it is in much wider use. In the U.S., clopyralid is registered for use on lawns, cereal crops, grass hay, and several horticultural crops. In Canada, clopyralid is not registered for use on lawns, which reduces our problems. It is, however, available for agricultural use under the label Lontrel, and for industrial control as a product called Transline. Picloram is a well known persistent herbicide that is fairly strictly controlled in Canada. It's trade name is Tordon and it is used mostly for industrial weed control.

The Response: This has become a significant problem for large scale composters and those wanting to use their product: further testing found contamination levels at other sites that would affect sensitive plants such as legumes and members of the potato family. Dow, clopyralid's manufacturer, has voluntarily restricted sales of the problematic lawn product Confront in the state of Washington. The American Compost Council released a position paper, calling for expansion of that restriction and a major rethinking of persistent herbicides. This is a major ongoing issue.

For more information, see:

• www.puyallup.wsu.edu/soilmgmt/Clopyralid.htm

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