Herbicide Carryover in Topsoil and Garden Amendments
The Schutter Diagnostic Lab is receiving garden vegetable samples showing distortion symptoms consistent with synthetic auxin, also known as plant growth regulator, herbicide injury. In many of these cases, we suspect carryover of persistent herbicides in newly purchased topsoil or in garden amendments such as compost and manure.
Symptoms
Plant symptoms include extreme cupping and twisting of leaves, twisting and/or cracking of stems, general distortion of plant parts, stunting, and poor germination in extreme cases. Leaves may feel thicker than normal and may have veins that look more parallel than usual. Symptoms are usually most pronounced on newer growth. These symptoms can come from other sources, including herbicide drift, or may look like certain insect or disease issues if symptoms are not widespread.
 |
Above: Aminopyralid symptoms on pole beans.
Photo Credit: Cheryl Moore Gough
Indications that herbicide carryover is the cause of symptoms
Many broadleaf plants are affected; however, sensitivity varies. Typically, the most pronounced injury is to tomatoes and potatoes, with peppers, beans, peas, squash, and other vegetables also showing some symptoms. Another clue is a recent application of manure or compost, a recent load of topsoil being brought in to fill raised beds, or a nearby application of a broadleaf herbicide.
 |
Above: Tomato leaves showing symptoms of growth regulator herbicide carryover.
How does herbicide carryover happen?
Garden soil, manure, and compost contamination with persistent growth regulator herbicides such as clopyralid, picloram or aminopyralid can cause these symptoms. (This represents only a few common herbicide active ingredients which may persist in garden soil, manure, grass-clippings or compost. Discrimination or endorsement is not intended with the listing of active ingredients by MSU Extension.) These pyridine carboxylic herbicides are very persistent in the environment and are commonly used in Montana.
See common herbicide products associated with these active ingredients in Table 1. If residues are present on animal feed such as pasture or hay, the resulting manure will often have active herbicide present. Similarly, if beds are installed in areas where persistent herbicides were used in the past, or if contaminated soil is used to fill garden beds, pots, or greenhouses, then these herbicides can persist for several years.
 |
Is it safe to eat the produce?
Our recommendations are to never ingest produce if you suspect herbicide symptoms are present in your garden and the exact herbicide responsible is unknown. Homeowners should wait until fruits and vegetables no longer show signs of herbicide damage prior to consumption. This may take one to several growing seasons.
What to do
For assistance with diagnosis of herbicide symptoms on plants, contact your local Extension office or the Schutter Diagnostic Lab. For additional information on other potential modes of action contact the Pesticide Education Program. If you think you have purchased topsoil or compost that is contaminated with persistent herbicides you can contact your local Montana Department of Agriculture Field Office to help investigate.
Additional Resources
For more information, including options for remediation of the site, see: Understanding Herbicide Contaminated Soil Amendments for Greenhouses, Nurseries, and Home Gardens or the MontGuide titled: Minimizing Pesticide Contaminated Soil around the Home and Garden.
Contact:
Noelle Orloff, Schutter Diagnostic Lab, 406-994-6297, [email protected]
Cecil Tharp, Pesticide Safety and Education Specialist, 406-994-5067, [email protected]