Growing Concerns 564: Compost- black gold, things to consider


Gardeners know compost is black gold, stay informed to avoid risks


Dear Janet  & Steven,

When I use the compost from my city I wonder about chemicals in it. It is composed of lawn clippings, etc., that are picked up for recycling from residents. Should I be concerned that I do not know what went into this compost, as far as lawn chemicals and fertilizer? I assume it is fine for the perennial beds, but what about using it on a vegetable garden? Could this be a problem if someone were to have overs prayed their lawn or is this such a small chance that it is fine to use it? - J.C. -

Dear J.C.,

We use compost from yard waste facilities as we do any compost, because it improves soil's nutrient- and water holding capacity, reduces the need for fertilizer and suppresses disease. Researchers have studied the risks as well as the benefits of using compost with results so far that make us feel the benefits outweigh the risks. You should decide for yourself. Here are some things to consider.

There can be heavy metals in municipal compost from run-off on streets where bulk leaves were picked up. There may be traces of every chemical used by contributors of yard waste, from pesticides to swimming pool treatments and cleaning agents. Yet my alternative to compost may be to use soil stripped from building sites, which may have its own contaminants.

Compost may contain pesticide residues. For instance, researchers at Michigan State University studied three lawn products through the composting process. They found that these three, diazinon, 2,4-D, and pendimethalin (Prowl), changed form but were still detectable. In those new forms in the end product, however, they were so much less available to plants and animals as to be no longer "bio-active."

A Rutgers University researcher analyzed compost from municipal sites for traces of PCB's and 27 pesticides in the 1990's, and found only chlordane. Chlordane was once widely used but was banned in 1988. Some people may have applied this chemical after the ban, but it's so persistent in the soil that it could have been old residue in the tested compost. That Rutgers study concluded "no risk" since plants do not take up chlordane from the soil.

A California Department of Health study added hazardous chemicals such as carbofuran insecticide and simazine herbicide to compost piles.  After 50 days of composting, 100% of the carbofuran and 98.6% of the simazine was degraded.

Recently there have been problems with plants being stunted or killed by clopyralid residue in compost. Clopyralid, an active ingredient in several agricultural and lawn herbicides, seems to persist through composting. Although the product label states "Do not use compost containing grass clippings from turf treated with Confront in the growing season of application," that's pretty hard to control unless it's your own home compost pile. So we're keeping an eye on this issue. It was first identified as a problem in Washington State, where the use of clopyralid has been banned as a result.

Still, municipal composting has great advantages over most home methods. Where yard waste arrives in huge amounts, it must be composted quickly. So the process is handled scientifically, with piles turned and mixed regularly to keep them as hot as they can be. The end product is more likely to provide a benefit that Cornell University has studied, the elimination of disease organisms.

At Cornell it was determined that hot composting killed fungi including gray mold (Botrytis), damping off (Rhizoctonia), late blight (Phytophthora), white rot (Sclerotinia), stem rot, bacterial blights, various nematodes, wilt (Fusarium) and tobacco mosaic virus.  This has been verified by other studies such as at the National Arboretum. The key is that composting must be done at 122 degrees and above. That's hot composting, as done at large municipal sites and regularly turned, well made home piles.

Perhaps you'll accept the risks in exchange for the luxury of having compost in bulk. Then follow our lead in staying abreast and using caution, too. We learn of new developments in compost testing by reading news, magazine and Extension service reports. We discuss it with others on this website. We wear gloves to handle compost, since skin can absorb things that even a plant cannot.

Short reports

Avoid transplanting plants in active growth.

We've  transplanted at all times of year but know the riskiest times are now when leaves are forming and in autumn when they're falling. Give anything you move right now extra special after care.


Green thumbs up

to pinching starting in May. If you want a mum, aster or other plant to be bushier, clip its tips several times at three week intervals before those shoots begin rapid upward expansion. Once summer comes, pinching does little good.


Green thumbs down

to a gardener's fickleness. Several weeks ago we were in love with our daffodils. Before that we couldn't get enough of the early tulips and crocus. Now we find ourselves impatient for their foliage to make way for the next round of color. Shame on us!

 Originally published 5/1/04