SANTA ROSA — Gardeners and compost producers worry that an herbicide used in landscaping and farming is finding its way into the compost, and could wind up hurting plants instead of helping them.
The weedkiller, clopyralid, is produced by Dow AgroSciences and has been found in at least three Sonoma County compost operations. It also may be in commercially bagged soils.
The chemical is not considered a threat to animals or humans. It already has been found in animal waste, and may already have found its way into the water.
Sonoma County Agricultural Commissioner John Westoby said his office has asked commercial landscapers and gardeners to stop using the herbicide, and said most have.
Dow said in a release that the chemical is an important tool against “hard-to-control invasive or noxious weeds such as yellow starthistle, which currently infests about 15 percent of California.”
Most herbicides and pesticides become inert in the composting process, but clopyralid tends to concentrate and is usually at a higher density when found in manure or compost.
The company doesn’t know why that happens.
“It’s a puzzle that still has us stumped,” said Dow spokesman Garry Hamlin. “We think there’s something in the composting process that changes the compound.”
Last week, state regulators began tightening restrictions on some products containing clopyralids, which can kill broadleaf plants such as tomatoes, peas, lettuce, peppers and daisies. The new regulations address only those herbicides designed for lawns.
And state officials say they have no documented cases of the compost that has clopyralid in it adversely affecting plants.
The state is waiting for detailed lab analysis of the composts.
“We certainly believe there is a potential hazard here,” said Glenn Brank, spokesman for the state Department of Pesticide Regulation, adding that the agency will look into the matter. “We want to act not only as quickly as possible, but as effectively as possible.”
Mike Reynolds, who runs Santa Rosa’s Laguna composting operation, said he doesn’t think a total ban would come for at least three years. By then it would take even longer to get the chemical out of the composting cycle, he said.
“Ultimately, I think a national ban is the only way to handle this,” Reynolds said.