Composting With Care

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Soil contaminated with persistent herbicides can retain the herbicides for up to three or four years.
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Ask for a soil sample to test compost before applying it to your garden.
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Grass clippings can make a great addition to your garden compost, but make sure you've confirmed they are herbicide-free before adding them to your compost pile.

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.

Samples of compost can be home tested using a simple procedure. Before purchasing compost, ask the seller for a sample; one shovelful of compost will suffice. Fill a container, such as a plastic pastry clamshell or disposable 9-by-9-inch cake pan with compost to be tested. Sow fresh pea or bean seeds in it, and water thoroughly. Observe over the course of a few weeks. As peas and beans are highly susceptible to persistent herbicides, they will show abnormal growth at extremely low pesticide levels. A second container filled with compost and watered, but not seeded, can be used as a test for the presence or absence of weed seeds in the compost.

If you have already purchased contaminated compost, there are options for its safe usage. Contaminated compost can safely be used to feed grass lawns, but the grass clippings cannot be composted or sent to composting operations. They should be fed back into the lawn with a dedicated mulching mower. Do not use tainted compost in gardens or flowerbeds.

Garden beds accidentally treated with tainted compost may be rehabilitated by several years of cover cropping with grassy crops such as oats or ryegrass, provided all growth is removed and disposed of each season, either composted for lawn fertilizer, incinerated, or sent to a landfill for disposal. Otherwise, contaminated soil must be stripped away and composted for several years to ensure the herbicides have fully degraded, an expensive and impractical solution. An alternate solution would be to plant resistant crops on the contaminated plot. Sweet corn, berries (excluding strawberries), and fruit trees show little damage from persistent herbicides, so contaminated beds and mulches can also be used safely for these crops.

This family of persistent herbicides is no longer available for residential lawn use, and has been banned in several states. However, they are still used municipally for roadside right of ways and other public property applications in the states where they are not banned. They are also still approved for golf courses, sport fields, and nonresidential lawns.

Municipal composting operations relieve some of the pressures of waste streams on our already overburdened waste management programs, and provide residents with a valuable service. Unfortunately, as long as persistent herbicides remain on the market, there is an inherent risk involved with this resource. However, there are ways to protect your garden.

• Avoid compost made with grass clippings.
• Ask the supplier about the sources of their green waste.
• Request a sample and test for residues at home before buying.
• Use already contaminated soil and previously purchased composts on lawns and resistant crops only.
• Return grass clippings to the lawn, instead of neighborhood compost facilities.

Follow these guidelines and you can use green waste compost with confidence.

Related: Turn your compost the easy way with this DIY compost tumbler.

Andrew Weidman lives and writes in Lebanon, Pennsylvania. He is vice president and secretary of Backyard Fruit Growers, a grassroots organization dedicated to promoting growing good fruit in your own backyard.

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