Weeds are the bane of organic gardens everywhere, sprouting, digging in their roots, and unfurling leaves at just a glimmer of sunlight on bare soil. And a garden full of weeds, though very green, doesn’t look all that good.
Of course, aesthetics aren’t the only reason to keep weeds at bay. All weeds steal nutrients, water, and sunlight from young crops, and a carpet of weeds can seriously stunt the harvest. While provocative new research suggests that organically grown vegetables handle weed pressure better than vegetables that rely on chemical pesticides and synthetic fertilizers, all in all, the fewer weeds, the better off garden plants will be.
One of the most common recommendations for eliminating weeds is ground-covering mulch—straw, newspaper, or black plastic, to name a few. Most mulches suggested for use in the vegetable garden have to be purchased or brought in from somewhere else. Black plastic, and its cousins red and green plastic, are reliable for weed control but lead the pack in environmental unsustainability.
Although plastic barriers suppress weeds and diseases, they are costly, require effort first to lay and then to remove, divert water out of garden beds, and end up in the trash at the end of the season. They warm the soil, which can mean earlier harvests, but they can also kill the good bugs and bacteria that support a healthy organic garden. And plastic mulch is petroleum-based, energy-intensive to create, and unrecyclable.
There is, however, an unexpected hero in the war on weeds, and it’s a familiar face: the cover crop. Also known as green manures, cover crops provide organic matter and nutrients to garden plants as an alternative or in addition to compost. These grow-your-own soil conditioners are usually incorporated into the soil before they flower. But why not take advantage of the aboveground growth and use them as mulches rather than tilling them under? Cover crops can be knocked down or mowed down to create a thick dead-plant mulch into which new plants can be tucked.
Cover crops used as dead-plant mulch are more effective than plastic mulches at suppressing weeds without herbicides. They discourage disease, enrich the soil, and keep rainwater in the garden. Cover crops also boost the microbial life in the soil, fostering fungi and bacteria that are helpful to growing better plants.
At the Rodale Institute, near Kutztown, Pennsylvania (home to the 30-year-old Farming Systems Trial, the longest-running U.S. study comparing organic versus conventional farming techniques), various cover-crop combinations are being studied as alternatives to black-plastic mulch for growing vegetables. In 2010, as part of this study, researchers at the Institute planted 800 tomato plants, including ‘Black Prince’, a delicious dark slicer; ‘Bellstar’, a hearty paste; and ‘Glacier’, a cold-hardy early producer of smaller fruit. They also included a few rows of ‘Kentucky Wonder’ pole beans.
As expected, the cover crops did well reducing weeds, but, unexpectedly, they also boosted yields. “We found that tomato plants in rye and rye-vetch mulches produced just as much fruit as those in black plastic,” says Sandra Wayman, seasonal research technician at the Institute. Tomatoes grown in a mowed rye-vetch mixture produced the most fruit of all the fields, and the beans grown in cut rye gave the largest harvest, outyielding the black-plastic fields.
According to Institute researchers, the best dead-plant mulch for the home gardener comes from fall-planted wheat, triticale, or rye. All are inexpensive and widely available at farm-supply stores. They will be ready to kill in early summer, making a mulch just right for final tomato, pepper, eggplant, vine (squash or pumpkin), or green bean crops.
- Sow the cover-crop seeds in fall as that year’s vegetable crops finish and are removed.
- The next year, cut (hand-scythe or sickle), mow, roll, or trample down the plants as soon as kernels in the grain heads begin to plump. If grains mature and shatter, you’ll reseed the crop.
- Sow seeds or dig in transplants of the summer vegetable crop. Remove cover-crop roots only where you plant; leave most of the roots and topgrowth where they are. This suppresses weeds, holds soil in place, retains moisture, supports soil microbes, and slows water runoff.