Monday, July 14, 2014

Herbicide Carryover in Garden Mulch and Manure

During the summer, MU Extension offices get phone calls from homeowners as well as farmers asking what is wrong with their vegetables.  Not always but at times, vegetables show herbicide damage even on vegetables that have not had any sprayed herbicide nearby.

Herbicide carryover has become an increasing problem in gardens and greenhouses. Depending on the active ingredient in the herbicide and weather conditions, herbicide effects can linger in the soil for years.

Tomatoes and other garden plants are especially sensitive to herbicides. Typical signs of herbicide damage include: distorted leaves, plants and fruits, and cupped leaves.

“These are the same signs one would see in a case of spray drift from herbicides, however if there is no possibility of spray drift, herbicide carryover in mulches and manure compost introduced from another location should be considered,” said Jill Scheidt, agronomy specialist with University of Missouri Extension.

According to Tim Baker, horticulture specialist with University of Missouri Extension in Daviess County, there are two instances where he has observed irregular herbicide carryover in mulches and manure compost.

The first situation is that of herbicides surviving the intestinal tract of an animal, in a high enough concentration to cause crop damage.  In this case, a broadleaf herbicide is sprayed on a pasture, creating lush grasses for the animal to feed on.  When the manure is collected, the herbicide is still there. The second situation is the possibility of herbicide being applied to a field, and then manure collected for composting.

In order for most chemicals to speed the process of breaking down, sunlight, air and water must be in the equation. Wet, warm weather promotes the process of chemical breakdown. If there is contamination in a covered greenhouse, consider opening the greenhouse to the outside elements.

If that is not an option, Baker suggests using activated charcoal to absorb the herbicide. In some instances, herbicide can take a number of years to leave the soil, plants may improve, but slight signs of injury can still be seen.
(by David Burton, MU Extension)

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