By Ann Hazelrigg, Extension Plant Pathologist at UVM
Composting can reduce the amount of waste material going into our landfills and can serve as a beneficial soil amendment used to improve organic matter and biological activity in gardens. Unfortunately, as was found out last spring, compost can also harbor some not-so-beneficial things.
In June 2012 gardeners began noticing that their tomatoes and other broadleaf plants were showing severe foliar curling and stunting. In some cases, seed germination was affected. After confirmation of the damage by the Vermont Agency of Agriculture, Food and Markets, followed by several weeks and thousands of dollars of laboratory testing, two persistent herbicides, clopyralid and aminopyralid, were found to be present in very minute amounts in the bulk compost used by all the gardeners.
More than 500 gardens, mainly in Chittenden County, were affected. The Chittenden Solid Waste District (CSWD), where the compost was purchased, and the Vermont Agency of Agriculture have worked diligently, testing dozens of samples to understand this complicated problem and propose solutions to address the contamination.
Clopyralid, which will break down during the normal compost curing process, is an endemic problem and appears to be present in most compost and likely has been for many years. The amounts found in CSWD compost and other Vermont composts that were tested were below the 10 parts-per-billion (ppb) threshold, a level generally regarded as necessary to harm or cause symptoms in plants.
The amounts found were also too low to cause human health concerns. The presence of aminopyralid is more problematic because it is active at a lower concentration causing plant damage at levels above .2 ppb. This is the persistent herbicide linked to the plant damage seen in gardens last year. It is believed to be associated with horse manure, a common component of many commercial compost products.
Aminopyralid has the ability to remain in compost for up to a year but will break down more readily when mixed with soil. Last year many of the affected plants began to grow out of the damage. Soil, microorganisms, sunlight, and precipitation all contribute to the breakdown of the herbicides.
Unfortunately, checking each batch of compost for possible persistent herbicides is costly and unfeasible. Bioassays—plant growth tests—are the best means to assure the quality of compost. Most commercial composters incorporate these tests into their production protocols to help ensure that their products do not contain levels of the persistent herbicides that would cause damage to plants.
However, no test can be 100 percent representative of an entire batch of compost. Diluting compost is a good strategy to protect sensitive plants. To apply compost at a minimum 80 percent dilution rate, mix one bucket of compost with four buckets of soil.
Gardeners also can do a simple bioassay before using compost or planting in gardens that may contain contaminated compost. To do this bioassay, collect samples from garden soil where the compost was applied or from the actual compost pile. Mix the compost 50:50 with potting mix. Put in clean pots or flats and plant indicator seeds like tomatoes, peas, or beans. Grow the plants under lights or in full sun for four weeks to see if any twisting or curling symptoms emerge. Before purchasing compost, ask your compost suppliers if they have done their own bioassay.
Although problems such as gardeners experienced last year are rare, they do occur. But don’t let that keep you from buying Vermont-made compost. Using compost in your garden is still a good idea.
What are herbicides?
Herbicides are a class of pesticides that kill unwanted plants. Some herbicides are designed to remain active and effective in the field for up to one full growing season. A group of these herbicides are used to control broadleaf weeds and can be present in human food like vegetables and grains; food waste and grass clippings; livestock feed crops like hay, molasses, sugar beets, and oats; and livestock bedding.
These herbicides can pass through animals into manure, urine, or bedding and can remain unaltered after the grasses have been eaten. These herbicides are used legally in other states and the products containing the herbicides may be shipped to Vermont in animal feed.
The wide use of these herbicides, as well as the lack of a tracking system or chain of custody for the products, makes it difficult to determine how these herbicides enter a particular composting system. Therefore, many farmers do not know which herbicides have been used on purchased feed or bedding materials.
A smaller group of these herbicides can withstand the heat and moisture in the composting process. These herbicides remain intact and are called persistent herbicides. They have a significant impact on sensitive garden plants at a very low parts-per-billion concentration range in finished composts. The four primary persistent herbicides that can impact compost operations are clopyralid, aminopyralid, picloram, and aminocyclopyrachlor. All are registered for legal use in Vermont but have been classified as “restricted use,” making them only available to licensed applicators.
The Vermont Agency of Agriculture now requires a special permit to apply these herbicides. Applicators must follow a list of guidelines developed by the state to ensure the proper management of the crops that have been sprayed with the materials including any manure generated from ingestion of the sprayed crops. Fortunately, all four of these persistent herbicides have very low toxicity to mammals, fish, amphibians, and fowl.