Beware of Herbicide Residue in Compost and Manure

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There are many benefits to using manure and compost in gardens and flowerbeds. Both enrich soil, and adding compost to clay soils improves drainage. Certain composts and manures however can damage plants.

To understand how this can happen, you must know a little about herbicides. Herbicides can be divided into two categories: broad spectrum (for example glyphosate, the active ingredient in Round-Up), which can kill all plants, or selective. Selective herbicides only kill certain plants. The most common types of these products are ones that, when applied to lawns, kill broadleaf weeds but not the grass.

In the case of lawns, the grass still absorbs the herbicide, but it does not affect it. When the grass is cut, the herbicide remains in the clippings. As the clippings decompose, the herbicide is released. If these clippings are applied near plants like beans, peas, and tomatoes, the released herbicide can damage or kill the plants.

Although the above example addresses grass clippings, the same can happen with compost and manure. In the case of manure, the herbicide is taken up by hay or forage, which is then consumed by the animal. These products pass through the animal’s digestive system and are excreted in their waste. (Herbicides are safely applied to pastures to kill poisonous weeds that, if ingested, would kill cattle and horses.)

All of these herbicides break down eventually. But how quickly they break down depends on sunlight, moisture, heat, and microbes. The problem comes when gardeners get these grass clippings, compost, or manure that have herbicide residue and apply it to their gardens.

To avoid this problem, gardeners can ask their supplier (if using manure) what herbicides were used on the lawn, hay, or pasture. If that information is not available, a test can be used to determine if the material is safe for gardens.

To test for herbicide residue, mix equal parts of the compost or manure and potting mix and fill a few small pots with it. Plant a few pea or bean seeds or a tomato transplant in each pot. If testing hay or grass clippings, spread them around the plants in the pot. Let the plants grow for 2-3 weeks and look for twisted, cupped, or discolored leaves, which can all be symptoms of herbicide damage.

Plants damaged by herbicide residue can be distorted or stunted and may eventually die. Often, the leaves will be cupped and distorted or strappy. Some plants are more sensitive to herbicide damage than others. Tomatoes are extremely sensitive and are often the plants that show the most severe symptoms and show them the soonest.

For more information on herbicide carryover, click here