If you use compost in your garden, you may be setting yourself up for either a bumper crop or a bummer crop.
Gardeners, greenhouse operators, and organic farmers from Washington to California have experienced crop failures on certain plants after using compost to enrich their soil and help their plants grow.
The problem begins when common composting materials such as grass clippings and leaves collected from grounds that have been treated with an herbicide named clopyralid are sent to commercial or municipal composting facilities.
Clopyralid, made by Dow AgroSciences, is used to control dandelions, thistles, and other noxious broadleaf weeds on lawns, golf courses, and many agricultural crops.
The active compound in over 30 brand name products, clopyralid has a reputation for long-lasting weed control, says David Bezdicek, a Washington State University soils professor who is researching the problem. It requires fewer applications than many herbicides, which means fewer chemicals go into the environment, he says. Clopyralid is also considered safe for humans and animals, according to the Washington State Department of Agriculture (WSDA).
Despite its benefits, the persistent weed-control that makes clopyralid products popular with the lawn-care and agricultural industries is what is also causing the problem for gardeners, organic farmers, and the entire composting industry.
Clopyralid not only kills thistles, it can also be very toxic to a variety of plants that include peas, beans, tomatoes, potatoes, peppers, and sunflowers. It remains in the soil for so long that it may take years before the same soil or site can be used again to grow plants susceptible to clopyralid.
The first documented instance of this problem occurred at a Spokane composting facility in June 1999 after tomatoes grown in containers were injured by compost that originated there. The source was traced to lawn clippings that contained toxic amounts of the chemical.
Then in 2000, the composting facility at WSU received complaints of compost contamination that was traced to grass hay and straw that was used in the University’s animal feeding operations.
Since then, several commercial composting facilities in Washington, California, Pennsylvania, and New Zealand have tested positive to containing damaging amounts of clopyralid.
This discovery has set off a flurry of research, testing, and law making, including rules enacted by the WSDA in March that prohibit the use of clopyralid on residential lawns and turf, but allows its use on golf courses if grass clippings are not removed from the site.
WSU researchers are currently involved in testing various compost feed stocks and finished compost for clopyralid residues, as well as conducting analytical tests and bioassays with plants, which determine the effects of clopyralid on living plants.
So far, it is not known how long the chemical will persist in compost, because tests began only two years ago, Bezdicek says.
“It can persist up to 18 months in the soil,” he says, but mentions that in a trial at the WSU Compost Facility, clopyralid concentration increased over a period of 100 days of composting. While the active ingredient breaks down fairly quickly in the soil where microorganisms are available, it seems to have a much longer life in straight compost.
This presents a large problem for composting facilities, because they generally produce compost within a two- to six-month period, a time frame usually long enough for most pesticide residues in compost to break down to a level that is not harmful. Since the break-down period for clopyralid is so long, compost facilities have found it hard to get rid of the stockpiles they now have that contain it. Since March 2002, the Colbert facility in Spokane has stopped accepting any new waste and is currently sitting on 45,000 cubic yards of compost.
The sources of contamination also have Bezdicek and the composting industry worried, because compost is collected from so many places. The main source of contamination comes from grass clippings, but straw and hay are also common contaminants, Bezdicek says. Manure has also turned out to be a problem for compost users, because while food grown with clopyralid is safe to eat, it is so persistent that it will pass through animals with very little breakdown.
“Wholesome and good for the soil—that is how compost was always perceived,” says Mary Fauci, a WSU soil scientist who is working on the clopyralid issue.
But since clopyralid is so pervasive, she says the reality is people need to realize they can no longer use all composts for everything.
Although compost that contains clopyralid may damage certain garden-variety plants, Bezdicek says it can still be useful for other purposes, such as applying it on lawns, certain shrubs, cereal crops, and grass hay, as long as the residues produced from these crops are not composted.
However, if people have an affinity for peppers and tomatoes, Fauci recommends they look into making their own compost.
If composting is not an option, Fauci recommends that people ask questions about the source and history of the compost before they buy it and recommends spreading only a half-inch of compost on a garden per year.