Composting With Care

Safe composting practices can keep your garden free of harmful herbicides.


| March/April 2018


Municipal composting is a hot topic. Across the country, townships and municipalities increasingly divert green waste — leaves, yard waste, grass clippings, and more — from overburdened landfills to composting facilities, offering the resulting compost to their residents at a reduced cost, or even free of charge. While this can provide a great resource for gardeners, there are risks involved, thanks to a group of highly potent persistent herbicides.

In 2012, Green Mountain Compost, operating in Vermont, sold compost consisting of composted waste hay and manure from horse stables in the area. By June, gardeners who had purchased the compost began complaining of abnormal plant growth and crop failures in gardens where it had been applied. Green Mountain Compost suspended sales, and tested samples taken from their compost stocks as well as from affected gardens. (Theirs is a success story now, as Green Mountain Compost rectified and corrected the oversight, and remains a strong business today.)

At the time, test results detected trace amounts (less than 16 parts per billion) of clopyralid or picloram, members of the picolinic acid family of persistent herbicides, which are highly effective, highly specific broadleaf herbicides. These herbicides are readily taken up by grasses and are not toxic to humans or animals when used at labeled rates. They do, however, affect many broadleaf plants. Because of these characteristics, this herbicide family is a favorite for ridding lawns, golf courses, and hayfields of broadleaf weeds, such as creeping thistle.

The hazard is a result of three other characteristics of the group: they are systemic, persistent, and potent. Grasses readily take the compounds up, storing them in their tissues. The compounds do not readily break down, often taking several years to decay into ineffective simpler materials, and pass through animal digestive systems unaffected. Compost containing trace levels (as low as one part per billion) of these herbicides has been shown to be damaging to non-target plantings.



Compost contaminated with persistent herbicides can be difficult to identify. Laboratory testing of finished compost is typically a prohibitive expense for cash-strapped local governments, and residents are often unwilling to wait for test results. However, because the herbicides do not break down readily, and will pass through most animals’ digestive tracts largely unaffected, composts made from grass clippings, waste hay, and even manure from animals pastured on treated land or fed treated hay contain at least some level of these herbicides.

Garden crops, shrubs, and other broadleaf plants mulched with tainted compost can quickly show abnormal, weak growth, eventually failing and dying. Germination is also poor in contaminated soils. Affected new growth is thin, stretched, and exhibits curled, cupped growth reminiscent of curled fern fronds. Sensitive plants rarely survive to produce a crop. Beans, peas, tomatoes, sunflowers, and cucumbers are highly susceptible to persistent herbicide residues. Soil treated with tainted compost can retain these compounds for several years.







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