Cover crops, plants grown specifically to improve the soil, present an exciting alternative not only to weeding, but also to hauling mulch and even fertilizing. Dense growth of cover crops can shade weeds and provide organic materials that you would otherwise have to gather up or purchase, then spread. Some cover crops, such as rye, oats, sorghum-sudangrass hybrids, and subterranean clover, have an “alleopathic” effect; that is, they combat weeds by releasing natural, weed-suppressing chemicals into the soil. Cover crops also help nourish your plants by pulling up nutrients from deep within the soil, by increasing the availability of nutrients locked up in soil minerals, and by clinging to nutrients that rainwater might otherwise wash beyond the reach of roots.
Cover crops can do even more:
After these plants die, their rotting roots leave behind channels for new roots, water, and air, and enrich the soil with humus. Some cover crops can even act as “subsoilers,” breaking up compacted layers within the soil. Some, like buckwheat, attract beneficial insects to decrease pest problems. And finally, the garden simply looks prettier in winter with the ground covered by a dense stand of plants than it does bare.
Where Can They Grow?
Enthusiasm for cover crops could come screeching to a halt with the question of where to put these wonderful plants when you already have a full garden. In a vegetable or annual flower garden, cover crops might grow when the ground would otherwise be bare, such as from late fall to early spring. Or a different part of a vegetable or flower garden might be set aside each year for a whole season’s growth of a cover crop. In perennial borders or mixed borders of perennials and shrubs, cover crops can grow among plants for part of the season, then again during the cool months. The right cover crop might even look decorative among (other) ornamental plants. The show from crimson clover—its blossoms clustered tightly on upright stalks like crimson popsicles—is so spectacular that you’d hardly suspect it was improving the soil.
The Community of Cover Crops
There is no “best” cover-crop plant. Which one to grow depends on when you are going to plant, what your climate is like, and what, specifically, you want from the cover crop.
Most plants used for cover crops are cither grasses or legumes. “Grasses” here means the whole grass family, from lawn grasses to grains such as rye, barley, and wheat. As cover crops, grasses are valued for their extensive roots (a total root length of 385 miles has been measured beneath a single rye plant!). These roots plow through the soil to keep it loose, and upon dying, contribute to long-term fertility.
Legumes include familiar garden plants like peas and beans, as well as various types of clovers, vetches, and medics. Roots of leguminous cover crops harbor beneficial bacteria that extract nitrogen from the air and put it in a form usable by plants. Legume roots, however, are not as dense as those of grasses and they generally do not provide as much long-term soil improvement.
Planting grasses and legumes together can tap the benefits of each, and the right combinations can bring more specific benefits. A planting of sudangrass, alfalfa, or yellow mustard, for example, is good at breaking up compacted soil. Where the soil needs loosening in its lower depths, you might try a crop of sweet clover or some sorghum-sudangrass hybrid, Where fertility is low, a crop of buckwheat or sweet clover helps release nutrients such as phosphorus and potassium from soil minerals. Annual ryegrass, sorghum-sudangrass hybrids, rye, and subterranean clover are excellent soil builders for Eastern soils; subterranean clover, medics, and barley are especially good for this purpose in the West.
If weeds are threatening, smother them with a thick stand of annual ryegrass, rye, sorghum-sudangrass, buckwheat, barley, oats, cowpeas, subterranean clover, or woollypod vetch. Cut and allowed to lie on the surface of the ground, dead rye suppresses weeds first by smothering them and then by releasing natural, weed-suppressing chemicals into the soil for 30 to 60 days. In one study, rye effectively reduced pigweed by 95 percent, ragweed by 43 percent, and purslane by 100 percent. The effect is only on small seeds, so it’s possible to plant large seeds such as corn, cucumber, or lupine right into the downed rye.
Note that the aforementioned yellow mustard and buckwheat are neither legumes nor grasses, but are useful cover crops nonetheless. (The mustard also decreases problems from soil-borne pests called nematodes.) And certain plants such as sudangrass, buckwheat, and cowpeas thrive only when grown in warm summer months. (See the Selected Cover Crops chart, here, for characteristics and uses.)
All’s Well That Ends Well
How do you deal with cover crops once they serve their purpose? Annual plants eventually die, flopping down on the ground, but not necessarily on schedule. For instance, rye (the grain) is a cover crop usually planted in early fall. It grows in cool weather, goes dormant in winter, then starts growing again at the first hint of spring. The plant matures by summer, ripening grain, then dies. If you use rye in a vegetable garden, the problem is that you won’t want to wait until summer to plant vegetables.
The conventional way to kill a rye cover crop before its natural time is by tilling it into the soil as soon as possible in spring. But tilling would disrupt the soil, bring up weed seeds, burn humus, and bury the rye stalks and stems.
In Weedless Gardening we find other ways to kill cover crops, ways that need not disrupt the soil. Simplest of course is to grow something that dies by itself at the right moment. In my vegetable garden, I plant oats in any beds that become free of vegetables before the middle of September. Oats enjoy the cool weather, grow on into winter, and eventually are done in by temperatures dipping near 0°F. The plants then flop down on the ground dead, their leaves and stems still protecting the surface. Come spring it takes no more than my bare hands or a grass rake to “roll up” the dead leaves and stems (like a carpet) before planting. Woollypod vetch and barley are other cool-season plants that can be used similarly where winter lows drop near 0°F. Seeds of either of these plants might also be scattered within a perennial flower or shrub border (if not too shady) toward the end of the season to further enrich the soil and smother any late-season weeds trying to creep in.
With other annual cover crops, or where winter temperatures are not sufficiently low to kill cover crops, planting schedules in a flower or vegetable garden can be adjusted to accommodate the cover crop’s natural death Any sacrifice of space can be minimized by devoting only part of a garden to such a cover crop.
Another way to bring a premature death to many kinds of cover crop plants is by mowing. (See the Selected Cover Crops chart, here, for cover crops especially willing to succumb to mowing.) The timing for effective kill is critical, usually just before the plant is getting ready to flower. With some cover crops, two or three mowing’s would be most effective.
In a small garden, grass shears can be used for mow killing. A sickle, scythe, weed whacker, or power mower dues the job in larger areas. Around woody plants or in areas slated for large seeds of transplants, the clipped stems and leaves might not interfere with planting. In this case, just leave them in place, letting the plants dead tops protect the soil as their roots improved it. One advantage of leaving the mowings in place is that they’ll release their nitrogen (which is present mostly in their above-ground portions) right back into the soil within a month or two in warm weather. Where mowings would interfere with planting, push them aside to leave just enough space for planting, or rake them off entirely.
The simplest way to deal with a cover crop is to plant it, then kill it at a later date. Innovative strategies for using cover crops more effectively and with less fuss are on the horizon. Consider the possibility of a cover crop permanently protecting and improving the soil, a living mulch, An obvious pitfall to this strategy is the living mulch acting like a weed and starving garden plants for food, water, or light. Sidestep this problem by choosing a cover crop plant that is not very competitive, or by periodically weakening it. Both white clover and perennial ryegrass have been used with varying degrees of success as living mulches between vegetable plants, with the growth of the cover crops weakened by periodic mowing.
An even more elegant possibility is a cover crop that plants itself to grow only when it would not interfere with cultivated plants. Subterranean clover and crimson clover are two such cover crops for regions with mild, moist winters. Planted in late summer, cither of these clovers will sprout, then thrive in the cool, moist weather of fall and early winter. The plants go dormant in midwinter, reawaken when the weather warms, then set seed and die in spring. “Subterranean” highlights the subterranean clover’s knack for sowing its own seeds right on or below the ground’s surface. Hard-seeded varieties of either clover are slow to sprout, and wait until late summer or fall to initiate the cycle again.
Another unique use of cover crops is in the strawberry bed. Recent research has shown that after fruiting, June-bearing strawberry plants are very tolerant of shade. A cover crop—-of oats, for example—sown right in the strawberry bed after the berries have been gathered can shade out weeds through the growing season, then eventually flop down dead to provide the mulch in which strawberry plants thrive.
As you have no doubt fathomed by now, there is no single recipe for best utilizing a cover crop in your own “back forty.” All sorts of possibilities exist if you take the time to study specific cover crop plants and then experiment.
The question remains whether cover crops can totally replace wood chips, straw, compost, and other organic materials that would otherwise blanket the soil Time will tell. For now, I integrate cover crops with imported materials (such as neighbors leaves or manure from a horse farm), sprinkling oats or crimson clover in shrub borders, and having some vegetable beds go into winter with a sprinkling of soybean meal and a dense stand of oats growing up through a blanket of compost.
