January 31, 2002

Composters Need to be Careful of Herbicide

Gardeners planning on composting need to be aware of a herbicide that can leave damaging residue in grass clippings, according to a North Dakota State University turf grass specialist.

"Composting is an art and science that has been practiced by gardeners for longer than we can count. Properly used, compost results in healthier plants. If the compost is contaminated with herbicide residue the result can be dead plants," says Ron Smith of the NDSU Extension Service.

Clopyralid is a herbicide used in both agricultural operations and by professional lawn care operators to control broadleaf weeds. It is a component in a product called Confront.

It is an effective herbicide needing a low level of active ingredient to control weeds. The downside, says Smith, is that it is very persistent, lasting up to a year in the environment after application. When grass clippings are collected after an application containing clopyralid are used in composting, there is a plant-damaging residue at levels as low as 3 to 5 parts per billion. The lingering effect of this herbicide also has an impact on grass clippings used as mulch.

With older side-discharge mowers, the operator needs to be careful where clippings are expelled. Flower and vegetable gardens are most vulnerable to herbicide damage, so discharge of clippings should be away from those areas, Smith says.

Recycling the clippings onto the surface of the lawn is the best option, as it keeps the herbicide where it belongs. The newer recycling mowers do this very effectively, he says, but any mower that leaves the clippings where they drop will do the job.

"Check with your lawn care provider to find out if clopyralid is being used on your lawn," says Smith. "If it is, either request something else be used, or don’t recycle clippings into a compost bin or use them as mulch."

From an environmental standpoint, clopyralid is not toxic to warm-blooded animals, including humans, when used as directed.

"Under just about all other conditions, composting accelerates the breakdown of pesticides into harmless metabolites. Clopyralid happens to be an unusual exception," Smith says.

###

Source: Ron Smith, (701) 231-8161, ronsmith@ndsuext.nodak.edu
Editor: Gary Moran, (701) 231-7865, gmoran@ndsuext.nodak.edu