Cover crops are plants sown thickly to from a living mulch in gardens. They help reduce soil splash and erosion and keep weeds in check. When cover crops are turned into the soil to provide organic matter and nutrients, they're called "green manures." Green manures include legumes such as vetch, clover and peas; grasses such as annual ryegrass, winter wheat and winter rye; and buckwheat.

Clover

Some gardeners sow these plants in spring, especially in new garden plots to improve the soil and choke out weeds. In established vegetable or flower gardens, plant a green manure early in the season to improve the soil. After you turn it under, plant warm-season vegetables, bedding plants or container-grown perennials.

If you dig a new garden bed in spring or early summer, grow one or two crops of heat-loving buckwheat or beans. If you start a new garden in late summer, plant ryegrass which grows quickly in cool weather. The following spring, turn in the dead plant material and plant flowers or vegetables in the new, improved bed. The soil will contain more organic matter and more beneficial microorganisms, and there will be fewer weeds than before.

Another way to use green manures is in established vegetable gardens after early-maturing vegetables have been harvested. You can plant a green manure where these vegetables were growing to keep garden weed-free, prevent soil erosion, and add organic matter to the soil.

Sow the seed thickly to create a cover that won't allow weeds to compete. Mow the plants down if they flower, to prevent them from self-seeding and becoming weeds themselves. In spring, turn dead plant material from green manures into the soil before sowing seeds or transplanting seedlings. This is also the time to add fertilizer to the soil. If the green manure is one that doesn't die over winter, wait about two weeks after you turn in the living plant material before seeding or transplanting.

Many plants in the legume family, such as peas, beans, vetch and clover, grow in cooperation with soil-dwelling bacteria. These bacteria live in nodules on the roots of legumes. They take nitrogen gas from the air and convert it to a form plants can use. This process is often referred to as "nitrogen fixation" or "fixing nitrogen." When the legume dies and its roots begin to decompose, residual nitrogen becomes available to other plants.

Most soils in Minnesota have adequate populations of the bacteria needed to form the association with legumes, but you may choose to buy a powdered inoculum containing the bacteria when you buy the legume seed, to ensure that fixation occurs. In order to accumulate enough fixed nitrogen in the soil to nourish future plants, leguminous cover crops would have to be grown for an entire season.

Here are the most commonly grown green manures in Minnesota:

* Annual ryegrass * A thick mat of ryegrass prevents erosion and keeps weeds out. Although it will grow in cool weather, it dies over winter. Chemicals released as it decomposes may keep small seeds, such as those of carrot and lettuce, from germinating.

* Buckwheat * This broadleaf plant grows quickly in warm weather and effectively smothers weeds. Buckwheat flowers are a favorite nectar source for bees, but if you allow the plants to flower and set seed, hundreds of buckwheat plants will come up the next year. Buckwheat dies over winter.

* Clovers * Clovers fix nitrogen. Many are somewhat winter-hardy and may begin growth again in spring. The giant variety of white clover known as 'Ladino' makes a particularly good cover crop.

* Hairy vetch * Like clover, vetch fixes nitrogen. Somewhat cold hardy, it may survive winter and grow again in spring.

* Peas, beans, soybeans * These are legumes and fix nitrogen. They can be grown as both green manures and edible crops: first harvest the pods, then turn the plants under. If sown in fall or late summer, they will die over winter.

* Winter wheat, winter rye * These grains are cold hardy and if planted in late summer will begin growing again in early spring. They're good for areas that will be planted late in spring with warm-season vegetables such as tomatoes, peppers or squash. Turn the green manure into the soil in mid-May, then plant heat-loving vegetables at the end of the month or in early June.

H234G
12/97
Illustrations: Britton, Nathanial L. and Hon. Addison Brown. An Illustrated Flora if the Northern United States and Canada.
Dover Publications, Inc., New York. Vol. I, pages 282, 672, and Vol. II, page 423.