Be Cautious About Compost and Manures

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Gardening season will be here before you know it. One of the first tasks of gardening is to prepare the soil, often by tilling in some compost or manure. There are many benefits to this, including better nutrient retention, improved drainage and water holding ability. However, if not chosen carefully, the use of compost and manure can damage plants.

We diagnose many samples of herbicide damage each spring. Most of the time, gardeners bring in young tomato plants with curled, distorted leaves that feel like plastic. These unique characteristics are classic herbicide damage symptoms.

To understand how these products can damage, and even kill plants, 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 the lawn. When this grass is cut, the herbicide remains in the clippings. As the clippings decompose, the herbicide is released. If these clippings have been applied near living broadleaf 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 can be safely applied to pastures to kill poisonous weeds that, if ingested, would kill cattle and horses.)

Of course, not all herbicides are persistent enough to cause this problem. Products of concern are ones that contain picloram, clopyralid, or aminopyralid. These products are registered for use in pastures, grain crop production, residential and commercial lawns, and certain vegetables and fruits.

Eventually, all of these herbicides break down. The rate at which they break down though depends on sunlight, moisture, heat, and microbes. The labels on these herbicides provide information detailing how to safely use grass clippings, compost, or manure that contains these products. The problem comes when gardeners get these grass clippings, compost, or manure without the knowledge of what has been applied to them.

To avoid this problem, gardeners can ask what the animals were fed (if using manure) and 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 the presence of herbicides, 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 symptoms of herbicide damage.

Plants damaged by herbicides can be distorted or stunted and may eventually die. Often, the leaves will be cupped and distorted or strappy. Some plants are also 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, including specific herbicides of concern, visit