Recommendations for Accelerated Remediation of Persistent Herbicides
Green Mountain Compost/CSWD has scoured a variety of resources to compile the best information and advice available on how persistent herbicides break down and disappear in soils, and how you can best hasten that breakdown if herbicides are in your soils. We are not offering soil removal as part of our remediation plan because extensive research into the effects of these compounds supports our belief that you will not see herbicide effects in any plants in the next growing season.
How to know if herbicides are present
Plants such as tomatoes, potatoes, eggplant, beans, peas, peppers, and sunflowers are sensitive to persistent herbicides. If you have any of these plants in your garden and they are not showing any of the distinctive, characteristic symptoms of distorted stems and leaves, then you do not have soil issues related to herbicides that need to be addressed.
How these herbicides interact with plants
These herbicides are initially applied to fields and are taken up by the plants in those fields. Though all the plants incorporate the herbicides into their cells, clopyralid and picloram are formulations that specifically target the growth hormones that only certain plants possess. That’s why crops in the grass family such as corn and grains are not affected—they don’t have the targeted hormone.
How these herbicides break down in soils
As soon as any herbicide has been applied, it begins to break down in the soil. As stated in a publication by the Washington State University Cooperative Extension, “You might expect manufactured chemicals, such as herbicides, to be antagonistic to microorganisms. Fortunately, that is not true, as these chemicals are composed of natural elements. Naturally occurring soil microbes actually feed on many herbicides, digesting and changing them to forms having no herbicidal properties. If they are unable to use the herbicide as a food source, microbes may produce enzymes that break down herbicides.”2
These microbes are happiest at temperatures ranging from approximately 80˚F to 100˚F, so warm summer temperatures can be a boon in that regard. The microbes will continue feeding on these herbicides—and breaking them down into harmless carbon dioxide and water, all the way down to about 50˚F as long as the soil also stays moist.1,2
According to the Weed Science Society of America Herbicide Handbook, clopyralid has a soil half-life of approximately 40 days (range 12 to 70 days) and picloram has a soil half-life of approximately 90 days (range 20 to 300 days) with the upper limit of the range observed in cold dry soils such as those typically found in the western high plains of states like Montana. Both of these herbicides dissipate much more rapidly under warm, humid conditions such as those typical to a Vermont growing season, and in the presence of plant roots and at higher soil organic matter concentrations.1 Chemical reactions such as those that occur when a compound is exposed to water and the ultraviolet rays of sunlight also contribute to herbicide break down.2,4,5
The rate at which a chemical degrades is expressed as the half-life. The half-life is the amount of time it takes for half of the pesticide to be converted into something else, or its concentration is half of its initial level.5 Finally, like you, these microbes need oxygen to thrive, so aerating soils through tilling also helps speed the breakdown process.1,2
As mentioned above, many plants grow just fine in soils containing herbicides, taking the compounds up through their roots and incorporating them into their cells without any adverse effect. The more actively the plants are growing, the more herbicide they will absorb. Plants growing in an environment with too little or too much water will take up less herbicide. Planting a grass cover crop such as winter rye or oats (see recommendation below to take advantage of our Persistent Herbicide Absorption Kit) will speed the uptake and metabolization of any herbicide present. Note, however, that any such crop would have to be removed and not tilled into the soil the following season.
Summary of Suggested Steps for Home Gardeners
- Keep the soil moist. It should clump together if compressed in your fist, but no moisture should run out. Overwatering will reduce the available oxygen in the soil, in turn reducing the microbial activity.
- Till the compost into the soil to mix native soil microorganisms into the compost as thoroughly as possible.
- Keep the soil aerated by turning it over regularly, if physically possible to do so. This can be done with a rototiller, a shovel, or even a pitchfork.
- Take advantage of our Persistent Herbicide Absorption Kit. Upon request, CSWD and Green Mountain Compost will provide you with an appropriate amount of cover crop seed (Oats) according to the acreage of affected gardens. Beginning August 15th, it will be ready for you to pick it up within two business days of your request, at the Green Mountain Compost office between 8-4 M-F. Included will be specific soil preparation, seeding, and harvest instructions. Oats will grow best if seeded between mid-August and the first week in September, though can still be seeded after that day with a lesser amount of growth. The cover crop should be harvested by mid-November and brought to Green Mountain Compost in the specially-printed paper bags we will provide. Alternatively, you can take part in our harvested cover crop collection program, during which we will collected specially printed paper bags left on the curb during a specified week (the exact timing of this collection week is to be determined and announced on our website).
If you are interested in receiving the Persistent Herbicide Absorption Kit, send an email with “Cover Crop” in the subject line to email@example.com, or call in your request to GMC at 660-4949. Be sure to include square footage of your garden in all communication. We’ll let you know when it’s ready for pickup.
Notes on human toxicology of picloram
Rats fed the equivalent of a 150-pound person eating more than ¼ pound of pure Picloram every day for 13 weeks experienced no clinical or blood changes. Dogs, sheep and beef cattle fed low levels of picloram for a month experienced no toxic effects.3
In a study of the fate of picloram in humans, six volunteers were fed picloram or absorbed it through contact with their skin at 0.5 or 5.0 mg/kg of body weight [equals 34mg or 340mg for 150-pound person]. Study results found that picloram was absorbed rapidly through the gastrointestinal tract when ingested but passed through skin slowly with dermal exposure (Stevens & Sumner 1991). After 72 hours over 90% of the ingested picloram was passed through unchanged in the urine. The volunteers reported no adverse effects (Stevens & Sumner 1991).4
- John J. Jachetta, Ph.D., Regulatory Sciences and Government Affairs Leader, DOW AgroSciences
- Robert Parker, Ph.D., Washington State University Cooperative Extension weed scientist, WSU Prosser Irrigated Agriculture Research and Extension Center. College of Agriculture and Home Economics, Pullman, Washington. Revised March 2003. http://cru.cahe.wsu.edu/cepublications/em4858/em4858.pdf
- EXTOXNET, A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University, and University of California at Davis. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program. http://pmep.cce.cornell.edu/profiles/extoxnet/metiram-propoxur/picloram-ext.html
- The Nature Conservancy Handbook on Invasive Species Control
- Soil Quality Information Sheet, USDA Natural Resources Conservation Service January 1998, Prepared by the National Soil Survey Center in cooperation with the Soil Quality Institute, NRCS, USDA, and the National Soil Tilth Laboratory, Agricultural Research Service, USDA.
CSWD, August 6, 2012