Grass crops such as oats and rye have an additional benefit, explains Vern Grubinger, vegetable and berry specialist with University of Vermont Extension.
"Late in the growing season, they take up available nitrogen in the soil that would otherwise be lost to leaching since the garden crops are no longer growing and taking up nutrients. And they help prevent soil erosion caused by winter winds and water from rain and snow melt."
Related: Soil Nitrogen Content
What's more, "cover crops add variety to your rotation," says Tom Kaspar of the USDA Agricultural Research Service's National Soil Tilth Lab in Ames, Iowa. A grass cover crop is good for breaking up pest and disease cycles. And grasses germinate easily and consistently.
Oats: Soil Saver + Mulch
Fall-planted oats don't survive the winter in colder locations, but they'll absorb and store nutrients into the fall and then protect bare ground with the dead mulch they leave behind. "For northern gardeners, in Zones 6 and below, oats are the easiest winter cover crop to use, since they reliably die back, leaving a residue the next spring that does not require a lot of tillage to incorporate," advises Grubinger. "In fact, in a 'no-till' or 'low-till' garden, oats are ideal, because the residue can just be raked aside or spaded in before planting crops."
For the gardener who hopes to avoid spring tillage or who doesn't want to wait for a just-killed cover to decompose before planting the next crop, oats are the answer. Sow them early enough--6 to 8 weeks before hard frost is expected--so plants can put on ample biomass before the mercury drops. Sow seed at a rate of about 1 ½ pounds per 1,000 square feet.
Winter Rye: Hardy Weed Warrior
"Rye and buckwheat provide weed control by outcompeting weeds for resources such as sunlight and water," says Molly Hamilton, extension assistant at North Carolina State University. "And they produce toxic chemicals that inhibit germinating weed seeds and seedlings."
Winter rye is the answer for gardeners who want a hardy cover crop to start late in fall and grow over winter, even when temperatures are barely above freezing. Don't sow it too late in the season, since extreme cold may prevent seeds from germinating or kill young seedlings before they've had time to toughen up. Broadcast rye seed at about 2 pounds per 1,000 square feet 2 to 4 weeks before the first frost. Rye's thick tangle of roots and tough stalks adds fibrous organic matter to soil, too. Work it in and let the bed sit for about 3 weeks before planting a spring crop.
Annual Ryegrass: Nitrogen Booster
A less hardy cousin of rye, annual ryegrass forms a dense mat of nitrogen-scavenging roots that can reach 3 to 5 feet underground, helping to break up compacted soil. Seed annual ryegrass at least 40 days before the first killing frost. If ryegrass sets seed, it can become a problem; turn it under in late spring before it goes to seed.
Legumes: Plant Food Aplenty
Winter legumes give your garden the nutrient recharge it craves in the off-season, providing an essential dose of nitrogen. For southern gardens, crimson clover and winter peas are favorites.
Vetch--particularly hairy vetch--is often bred for winter hardiness, so it's a common choice in the North. What's more, a 2008 study by Cornell University found hairy vetch to be a prolific nitrogen producer, with some studies finding a "Nitrogen Fertilizer Replacement Value" of around 80 pounds of nitrogen per acre. Using hairy vetch means you'll need less fertilizer, which saves money.
Winter legumes can be mowed or incorporated after flowering. Sow about 50 days before the first killing frost. Seed hairy vetch at a rate of about ½ pound per 1,000 square feet; smaller-seeded crimson clover should be seeded at ¼ pound per 1,000 square feet.
Mix + Match
Many gardeners like to mix grass and legumes for biodiversity and biomass. Grains grow more quickly and protect slower-growing legumes. Oats, which die back over the winter and yield a sheltering mulch, work well combined with peas. Rye and hairy vetch team well together, but sow the two early enough to allow the vetch to establish.
Beds nourished by cover crops give spring plantings a strong start.
Fall-planted oats pull nutrients from the soil for crops to use in spring.
If your last fall crop isn't ready to be harvested in time to plant cover crops so that they're safe from frost, simply sprinkle clover seed in between the rows of the growing vegetables. The clover will fill out and nourish the ground after harvest.