I am frequently asked to recommend where to get organic matter to enrich garden soils. I support municipal composting to reduce the rate of filling up available land fill sites and would like to refer gardeners to this material for garden use. Unfortunately compost produced by municipal composting programs is one material to avoid for vegetable and ornamental gardens. You may be importing someone else’s discarded plant diseases or herbicide-laced turf clippings.

My concern for what is in municipal compost was sparked when I placed diseased peony vegetation at curbside one garbage pickup day. Some peonies in our display gardens had become afflicted with botrytis brought on by last season’s unusually long, cool and wet spring. Peony botrytis blight is a fungal disease that, if left untended and untreated, can quickly spread to other peonies with devastating results. Knowing this, diseased plant parts were quickly gathered, stuffed in garbage bags and placed at roadside for garbage pickup.

The reason we do not place diseased vegetation on our own compost pile is that we are never sure whether the entire contents of our pile have heated to the needed 160 degrees F. to kill fungal and other disease organisms. Outer layers of compost piles seldom reach disease-killing temperatures and successive pile turnings do not necessarily guarantee that all disease organisms have been heat-sterilized. We therefore routinely rid our property of diseased vegetation by sending it to the dump for burial. At least we thought it was going to the burial section of the dump, not creating a problem for other unsuspecting gardeners, until our diseased peony vegetation was left at curbside on the day in question.

A phone call resulted in a prompt visit by our area waste pick-up supervisor. If I hadn’t been near the roadside when he arrived, I would still be none-the-wiser about how municipal waste is ‘streamed’. I learned in an amicable tailgate chat that, in our municipality’s waste diversion program, any visible plant material, diseased or not, is directed to the compostables stream rather than the burial stream. A separate truck picks up compostables on specified dates. Since plant material was visible in my bags, and since that day of collection did not include compostables pick up, my diseased plant matter had been left at the roadside signalling that I was to put out this material again at the next compostables pick up date. But since the supervisor now understood that diseased materials were in the bags, he chucked them into his pick-up truck and I assume they eventually did get buried.

But I have a lingering concern for the quality of municipally-generated compost. Does their compost handling process guarantee uniform heat sterilization of the entire content of each batch of organic matter? Unfortunately municipal composting is fraught with the same uncertainty of turning a pile’s outer layers into the core of subsequent rebuilt piles. There is no guarantee that municipal compost is disease-free.

I have similar disease concerns about organic matter extracted from small plastic home composters. These composters in no way ‘compost’- they simply reduce waste volume by rotting. Conditions in these ‘rotters’ are typically cool and lacking sufficient oxygen and other conditions for bacterial decomposition to produce the high temperatures of a true composting process. Diseased materials should not be placed in these ‘rotters’ as their end product will not be disease-free.

Scouring the internet for information on municipal composting problems while preparing this article brought to light another compost quality issue. Toxic herbicide residues of picloram and clopyralid have been found in Washington State University, Seattle and California compost produced using lawn clippings and manure from cattle fed herbicide-treated feed. These two pyridine-derived herbicides, used to control broadleaf weeds in hay fields and turf, have been found to negatively affect vegetable garden crops. Both picloram and clopyralid are approved for use in Ontario and thus may similarly be expected to be present in Ontario municipalities’ compost.

Choose compost sources carefully. If you are considering using municipally-produced compost, obtain written guarantees that the compost is herbicide-free before applying it to food-producing or ornamental gardens. You can however help municipalities reduce their compost supplies by choosing it for long-term nutrition of lawns.

References:

Web sites for information about picloram and clopyralid:

Clopyralid in compost. http://css.wsu.edu/compost/compost.htm

One Year later: Persistent Herbicides in Compost. http://www.jgpress.com/BCArticles/2001/070125.html

Dow Seeks to Ban Its Own Weedkiller. http://www.grrn.org/dow/la_times_7-27-2002.html