Everybody loves tomatoes, but they’re not the easiest vegetable to grow. They get diseases, leaves drop, fruits rot, and gardeners will always listen to a new tip for growing healthier ones with better yields.
Tomatoes (and other crops as well) are often grown using black plastic as a mulch to suppress weeds and warm the soil in spring, but black plastic is hard to love. It’s expensive, it’s hard to dispose of ecologically, and it’s just plain ugly. Organic mulches such as hay or chopped leaves, on the other hand, can not only curb weeds and prevent soil erosion but also provide nutrition for the plants. As these materials decompose, worms, bacteria and other organisms incorporate them into the soil, improving its fertility, soil structure and water-holding capacity. You can even get the mulch for free, or at the minimal cost of seed, if you grow your own.
This was the subject of a study by plant scientists at the Agricultural Research Service in Beltsville, Md. Now 25 years old, the study’s results still offer rewards for professional tomato growers and home gardeners alike.
Months before planting tomatoes, the scientists raised a winter cover crop in the tomato beds. The system they used requires more foresight and planning than laying black plastic at tomato planting time, but the benefits are much greater. They chose hairy vetch, a legume that, like its cousins peas and beans, develops root nodules. Through bacterial action, the nodules convert atmospheric nitrogen into a form the plants can use. So just by growing a legume and leaving its roots in the ground, the gardener is improving the soil for the crop to follow. Tilling the vetch under, letting it decompose and then planting your crop will go one step further by adding organic matter to the soil.
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Hairy vetch, as a winter annual, will sprout in the fall, overwinter, regrow in spring, go to seed and die.
The scientists sowed the vetch in mid-September. In spring, when the vetch plants were tall, bushy and in bloom, they were mowed down to an inch above the ground, and the residue piled above the stubble to form a thick, smothering mulch.
And here’s the cool part: In early May, when the soil was warm enough to plant tomatoes, holes were dug in the soil, right through the mulch and the stubble, and tomato transplants set in without any tilling.
With all the benefits the vetch conveyed, those tomato plants were both healthier and higher-yielding than two other plantings done for comparison, one with black plastic as a mulch and the other with no mulch, just bare soil. This happened in all three years of the study. A further study, using greenhouse tomatoes, yielded similar results. And in an intriguing follow-up, a researcher at the same lab, Autar Mattoo, did a groundbreaking molecular analysis of the results, pointing to genes for longevity and disease resistance being activated by the presence of the vetch.
Do try this at home. Now, with empty beds opening up in the garden, it’s the perfect time to provide extra fertility for any crop you’ll be setting out as transplants next year. To cut the vetch in spring, a home gardener might use a string trimmer or hedge clippers, depending on the size of the plot. If you are concerned about vetch regrowth, you might cover the residue with a paper mulch to prevent re-sprouting. Hairy vetch seeds are available at feed stores and online. And while you’re Googling that, take a look at images of the plant; its name might suggest a scary Halloween costume, but, in fact, it sports beautiful lavender-blue flowers for you to - briefly - enjoy.
Damrosch is the author of “The Four Season Farm Gardener’s Cookbook”; her website is www.fourseasonfarm.com.