Herbicide Damage on Tomatoes

Friday, July 5th, 2013 by Jenny Watts
    • Dress up for the Fourth! Red, white and blue petunias, verbena or combinations of these with lobelia, geraniums, impatiens and salvia will make a nice display for the Fourth of July.
    • There’s still time to plant summer vegetables: tomatoes, peppers, zucchini, cucumbers and corn will bear for you if you plant them now.
    • Attract birds to your garden with a concrete bird bath. They come in many attractive styles and make good gifts.
    • Pepper plants should be fertilized when the first blossoms open.
    • It’s time to set out Brussels sprouts for fall harvest.

Herbicide Damage on Tomatoes

There is a new generation of herbicides being used to control broadleaf weeds and thistles on golf courses, pastures and non-crop areas. They are the plant growth regulators picloram, clopyralid and aminopyralid. They go by names like Tordon, Forefront, Curtail, and many others. Two things make these herbicides a popular choice. First, they are persistent, so they do not need to be applied often. Second, they appear to have little to no effect on the health of animals and people.

But they may affect your garden. In fact, ironically, the people most likely to wind up with these materials in their gardens are organic gardeners.

These herbicides can be introduced to your soils through compost, manures, lawn waste, and straw mulches. As little as 3 parts per billion will wreak havoc with tomatoes, potatoes, spinach, lettuce and beans. Even composting will not affect the herbicide, which takes as much as a year in the soil to break down.

These herbicides do not affect grasses and so are used on pastures to control thistles and other unwanted weeds. Contaminated manure and bedding can come from livestock fed crops that were treated with these herbicides. Be sure to ask whether animals have been fed hay harvested from treated pasture.

Dow Chemical, a manufacturer of these herbicides, claims that only a few plants are affected. But we are seeing damage among local gardeners on tomatoes, peppers, eggplants and beans, as well as other plants.

When sensitive plants are exposed to these herbicides, they develop cupped or fern-like leaves and twisted or flattened stems. They do not produce well, though in theory the crop is “safe” for you to eat. Some crops such as the squash and mint families are less sensitive.

If you are experiencing herbicide damage in your garden, do not despair. It may take a long time, 1-3 years, but eventually the herbicides will degrade. In the meantime you can grow grass-family crops including corn, wheat and barley!

Actually, it is important to keep something growing on the affected area. A warm, well-aerated, fertile soil with a near-neutral pH is most favorable for microbial growth and, hence, for herbicide breakdown.

You can grow a lawn, or, if you have contaminated material left, use it on your lawn and then grasscycle. Do not use these grass clippings for compost or mulch, just mow the lawn and leave them to compost on the lawn area.

You can test for contamination in the spring by mixing equal parts of the manure, or compost, with some good potting soil. Fill 3 or 4 pots with the mixture and then plant 3 pea seeds in each pot. Fill some other pots with only potting soil and plant the same seeds in those. Water them in separate saucers and watch their growth for 4 weeks. If you see cupped leaves, fern-like growth on new shoots, or twisted stems, there is still herbicide residue in the compost. Keep it moist and aerated for another year.

If your tomatoes are looking strange, check the source of your “organic” compost. Tomato diseases are one thing, but this is a much bigger problem.