April 25, 2018
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Pesticide residues in compost feedstocks are damaging garden plants

By Reeser Manley

Organic gardeners are always on the lookout for compost feedstock: for sources of summer grass clippings, autumn leaves, barnyard manure and stable litter. I’ve been known to rake a neighbor’s lawn for the leaves. You can never have too much.

In the last two weeks of August, Marjorie and I stockpiled two pickup loads of composted goat manure from a local organic farm, covering the pile with tarps to prevent leaching of the valuable nutrients, and two loads of hardwood chips for resurfacing the vegetable garden pathways come spring. Never enough.

Raked leaves are strewn over the grassy drain field and shredded with a mower before being stuffed into plastic bags for winter storage. In May we’ll use them, along with the composted manure, to mulch every garden tree and shrub as well as the perennial bed. We’ll dig composted manure into the vegetable beds and spread it liberally around the high-bush blueberries, strawberries and raspberries. We’ll set some aside to jump-start the compost piles through the summer. Never enough.

Typically, I leave grass clippings on the drain field and other areas of the garden that can loosely pass for lawn. But once in late summer, when the grasses are well beyond setting seed, I catch the clippings for the compost pile, just to heat it up.

Imagine how an organic gardener must feel to see curbside mountains of plastic bags filled with autumn leaves. Or to discover that there are people who bag up their grass clippings as trash. Yet it would be the height of foolishness to beg leaves or grass clippings from a stranger. They might be filled with persistent pesticide residues.

As far back as the mid-1990s, gardeners have been plagued by a group of herbicides that are used on residential and industrial lawns, golf courses, pastures, rights of way and roadsides.

The compost feedstocks that come from these treated areas, including hay and straw, grass clippings, municipal green waste and livestock manures have become vectors for persistent herbicide residues that can damage vegetable garden plants.

These herbicides, used to control dandelions, clover, thistle, coltsfoot and other broad-leaved weeds, are very persistent in compost and manures. One Ohio State University study showed that when grass was treated with one of these herbicides and then composted, the herbicide degraded by about 60 percent over 200 days, leaving plenty of the active ingredient in the soil to damage susceptible crops, including beans, cucumbers and tomatoes.

Symptoms of damage in susceptible crops include cupped leaves and distorted or stunted growth. The most susceptible crops are members of the bean, potato-tomato and sunflower families, with herbicide residues as low as 10 parts per billion causing problems.

According to a recent article in Organic Gardening (Oct.-Nov. 2011), the most recent of these herbicides to attract attention is aminocyclopyrachlor, marketed by DuPont under the brand name Imprelis. Until very recently, licensed pesticide applicators were using Imprelis on both residential and commercial turf grass. And while the Imprelis label carries a warning about the potential for crop damage from composted grass clippings treated with the product, that warning is on page 7 of a 9-page label. You have to wonder how many applicators (or their customers) read this far.

Grass clippings containing Imprelis were composted at municipal composting facilities and the contaminated compost used by homeowners in their gardens. Imprelis also found its way into straw cut from treated fields and rights of way and into the manure of livestock fed hay from Imprelis-treated fields. Some of the contaminated manures ended up in packaged compost.

In early August, it became clear that Imprelis was not just a problem in the vegetable garden. It was killing trees, particularly Norway spruce and white pines, in areas where it was used. DuPont pulled Imprelis off the market and began damage settlements, but it is unclear if they plan to pull their other aminocyclopyrachlor-containing products (Perspective, Plainview, Streamline and Viewpoint) off the market. Meanwhile, DuPont and the Scotts Miracle-Gro Company are working on a new combined lawn herbicide and fertilizer product that contains aminocyclopyrachlor.

The chemical industry has a history of putting products on the market before they can be thoroughly tested. In reality, there is no way to keep these pesticides from affecting nontarget organisms. Other than by not using them.

Stick with people you know when stockpiling straw, grass clippings or manure for the compost pile. Ask suppliers if they know for certain that their straw and composted manure are from Imprelis-free fields and pastures.

While you may never have enough compost, using reliable sources will keep your garden pesticide-free.


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