It was with a sinking feeling that I examined the twisted, cupped and curling leaves and stunted growth of my tomato, potato and pepper plants. Sure that I was witnessing the effects of a malevolent blight or virus, I called the county extension agent to help me diagnosis my problem. After he took a look around, however, he did not rattle off the name of a plant disease. Instead, he asked me if I'd recently added manure to my garden. When I said that I had, he delivered the blow. My garden was not suffering from a plant malady. Herbicide was the culprit.
My immediate question was how that could be, since I'd never once used any sort of chemical on my grounds. And I certainly hadn't used herbicide in my garden beds. Minimizing exposure to such substances is one of the main reasons that I choose to grow my own vegetables. Unfortunately, intentional application is not the only way that herbicides are finding their way into gardens anymore. I learned the hard way that these chemicals are gaining entrance via contaminated manure and compost.
Unfortunately, cases of gardens damaged via this method are becoming increasingly common. Often, the poisonous offender in contaminated manure and compost is found to be from a class of herbicides called pyridine carboxylic acids. Common examples include aminopyralid (Milestone), clopyralid (Stinger), and picloram (Tordon). These substances are used to control a variety of broad-leafed weeds and are approved for use on hayfields and pasture. Animals that graze grass or consume hay that has been treated with these chemicals pass them through their systems largely undigested. After being excreted in the animals' manure, the herbicides can continue to exist at levels high enough to damage gardens for years, even after the manure has been composted. It is also possible for contamination to occur if sprayed plant material is added to a compost pile. As with the manure, the chemicals can continue to persist at damaging levels in the compost for unusually long periods of time.
Symptoms of pyralid contamination typically include poor seed germination, yellowing, cupped and curling leaves, stunted plant growth, misshapen fruit, lowered yields, and a distortion of the plant's growing point that gives it a fern-like appearance. Tomatoes, peppers, potatoes, beans and peas are especially sensitive to this type of herbicide damage, but beets, carrots, lettuces, spinach and other garden crops might also be affected. If you believe your garden has fallen victim to pyralid contamination, Montana State University Extension offers several suggestions via their “Minimizing Pesticide Contaminated Soil Around the Home and Garden” publication. Options include planting non-susceptible crops, such as wheat, corn or berries, moving the location of your garden until its soil passes toxicity tests, incorporating charcoal (one – two pounds/100 sq ft) in the top six inches of contaminated soil, and, in the case of small gardens, removing the contaminated soil and replacing it with soil from a non-contaminated source.
You can also conduct a simple test recommended by Cecil Tharp, the MSU Pesticide Education Specialist. He suggests that gardeners fill five pots with uncontaminated soil and five with soil randomly collected from the garden. Plant the intended vegetables (tomatoes, beans and peas are particularly sensitive to pyridine contamination and will be most likely to exhibit symptoms) in each pot and allow them to grow until they have at least three leaves. Compare the appearances of the plants growing in potentially contaminated soil with that of the plants growing in uncontaminated soil, and examine for the affects of herbicide damage.
To prevent pyralid contamination within your garden, MSU Extension recommends that gardeners question their suppliers of manure, grass clippings and/or compost. If the supplier does not know whether the plant stands or pastures used to produce the manure, clippings or compost were treated with a pyridine herbicide, do not apply it to your garden. MSU Extension also states that you should not use manure from animals that have grazed forage or eaten hay harvested from treated areas within the previous three days. Grass clippings from treated areas should not be used as compost or mulch that will be applied to your garden.
I am happy to report that my own garden contamination occurred a number of years ago, and the test described above has revealed that the soil is finally, once again, herbicide free. Thank goodness. Please learn from my mistake. Manure and compost are very valuable tools within a garden, but herbicide contamination can happen easily. It is incredibly difficult to address after it occurs, so it is worthwhile to be as careful as possible as you consider sources for garden inputs.
If you would like to read more about this issue, you can find useful information within the following articles: