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This tomato plant has been damaged by herbicide residue.

Compost made with livestock manure is an excellent soil amendment for vegetable gardens. Compost improves soil structure, provides nutrients and helps optimize soil pH. However, gardeners should be aware that cow and horse manure sometimes contains residues that can damage certain plants.

I recently visited the garden of a friend who had noticed discoloration and deformity on tomato and pepper plants after mulching with composted manure she had obtained from a nearby farm. The damage appears to be caused by residual herbicide.

The herbicides of concern are aminopyralid, clopyralid and picloram. These chemicals are used to control broadleaf weeds, including some toxic plants. They are sold under various trade names for use in pastures, commercial turf, residential lawns, grain crops, certain fruits and vegetables, and along roadsides.

Horses and cattle can safely graze treated pastures and feed on treated hay. The chemicals are not digested by the animals, but are excreted in urine and manure. The herbicides eventually break down through exposure to sunlight, soil microbes, heat and moisture, but the timing is variable. Field reports on herbicide breakdown range from 30 days to several years, with the slowest breakdown occurring in piles of manure and compost.

Plants vary in their sensitivity to these herbicides. Lettuce, peas, tomatoes and peppers (among others) are very sensitive, developing cupped leaves, deformed stems, and little or no fruit production. Squashes and mints are more tolerant. Corn, cole crops and tree fruits are not affected.

Before using composted manure, try to find out what herbicides were used on hay or pasture the animals have been grazing. If you are unsure, it’s a good idea to do your own test.

Testing is straightforward. Take several random samples from your compost pile and mix them thoroughly. Prepare three or more small pots with a 1:1 mix of compost and potting soil. Prepare the same number of small pots with just potting soil. Put a separate saucer under each pot, and do not let water flow from one pot to the other. Plant three pea or bean seeds in each pot, water carefully, and let them grow until they have three leaves, about 14 to 21 days.

If they all grow normally, you can probably use your compost with no problem. If the plants in the compost mix are abnormal, you may have a problem with herbicide residue. Washington State University Puyallup has a compost website that gives a complete description of how to do this bioassay at home, and I recommend it as a reference if you want to run a test.

If you think that your manure or compost contains herbicide residue, you can still use it on plants that are not affected. If you have already applied the compost and seen damage to sensitive plants, try tilling the soil and planting a less sensitive crop for a year or two. Then do a pot bioassay before planting any sensitive crop again.

Compost is a valuable soil amendment, but like all other gardening tools it requires your attention.

A WSU Master Gardeners of Chelan County column appears weekly in The Wenatchee World. Connie Mehmel is one of four columnists featured. To learn more, visit wwrld.us/cdmg or call 667-6540.