Gardening Articles: Care :: Soil, Water, & Fertilizer
Fall Garden Cover Crops
by Charlie Nardozzi
Fall Garden Cover Crops
Buckwheat is a fast-growing, warm season cover crop.
When Is a Cover Crop a Green Manure?
A cover crop is a general term for any plant grown to prevent erosion, to improve soil structure, and to maintain or build soil fertility. Sometimes, you'll see the term "green manure" used in gardening literature. These are crops used primarily to add nutrients to the soil and are tilled into soil when they're still green. The distinction between the terms isn't terribly important as long as you choose the best plant for the purpose you have in mind. In warm-winter climates, green manures overwinter; in other areas, they are tilled under in summer or fall.
In September, most gardeners can plant overwintering cover crops. If you live where winters are mild, such as USDA Hardiness Zones 8 and south, you can plant and till under a green manure crop before planting a winter cover crop. Wherever you live, sow seeds of your winter cover crop at least 30 days before the last frost date in your region. In this article, I'll discuss cover crops you can plant in the fall and either till under in the late fall or overwinter and till under in spring.
Cover-Crop Benefits. Most gardeners know about the benefits of adding organic matter to the soil. Many, as I do, build the soil with annual applications of compost, manure, leaves, and grass clippings. However, in some areas, those forms of organic matter may be difficult to find or transport. Cover crops not only add large amounts of organic matter, but they provide other benefits as well. Let's look at some of them.
Adding Organic Matter. A primary reason to grow a cover crop is to increase the amount of organic matter in the soil. Additional organic matter improves the soil's structure, increases its water retention and drainage, and improves its aeration. It also provides necessary food for earthworms and microorganisms that increase biological activity in the soil. Increased biological activity in turn helps keep the soil healthy by enhancing decomposition; well-nourished beneficial microorganisms also compete better against disease-causing organisms.
Erosion Control. Traditionally, an important reason to use a cover crop was to "cover" the soil during the winter. Hardy crops such as winter rye are particularly important in preventing erosion and topsoil loss, especially in areas with high winds and inconsistent snow cover.
Loosening Compacted Soil. Certain cover crops, such as bell beans and oilseed radishes, have aggressive taproots, sometimes reaching 3 feet deep, that help break up compacted soils. The taproots also "mine" nutrients such as calcium from deeper soil, and when the plant dies, the nutrients are released in the root zone for the next crop.
Nutrient Balancing. Legume cover crops such as hairy vetch and crimson clover-through a symbiotic relationship with the rhizobia bacteria on their roots-convert atmospheric nitrogen into a form they can use to grow. When the cover crop is tilled under, the nitrogen is released for the next crop. In contrast, cover crops such as phacelia absorb and accumulate excess levels of nitrogen and other nutrients already in the soil that may otherwise leach out and pollute ground water or nearby streams.
Weed Control. Some cover crops are good weed blockers. Broad-leaved cover crops such as buckwheat and sunn hemp shade and smother weeds with their vigorous growth, and others like winter rye grain release s that prevent weeds from germinating.
Attracting Insects. Cover crops such as crimson clover and mustard, if allowed to flower, attract bees and beneficial insects that help with pollination and insect control in the garden.