Cover Crops for Improving
Vegetable Garden Soil

Cover crops are an old idea that's time has come again. They are receiving a lot of attention on production agriculture and horticulture, but also have a place in the intensively managed vegetable garden. Cover crops have the following properties.

They add nutrients to the soil. Plant roots go down onto the soil and take up the nutrients necessary for plant growth and development. After plants die and the residue decomposes, these nutrients already in "plant-available" form are easily taken up by the next plant that grows in that spot. Legume cover crops are known for fixing large amounts of atmospheric nitrogen. All plant roots bring up phosphorous, potassium and micronutrients from the soil.

Cover crops reduce soil erosion. Water running off bare ground causes soil erosion and this problem is the worst from November to May. Sediment in streams, rivers and lakes is a big water quality problem. During the winter, cover crop residue decreases the rate of water flow across the soil surface, which allows better water infiltration and less runoff. In the summer, living and killed cover crops soften the impact of raindrops and irrigation water and slows down runoff.

Cover crops reduce weeding time. A vigorously growing cover crop can act as a "smother" crop and outcompete many weeds and suppress the germination of weed seeds. However, cover crops are rarely vigorous enough to outcompete noxious weeds like quackgrass or Canada thistle.

Cover crops capture leftover fertilizer. Grass cover crops in particular will take up unused nitrogen, preventing it from moving into ground water.

They improve soil structure. The addition of organic matter enhances water infiltration, root growth and nutrient uptake by crops.

Cover crops suppress disease. Many diseases, such as anthracnose in tomatoes, begin when the disease-causing fungus splashes up from the ground to the foliage during rain or irrigation. Cover crops cut down on this splashing much more than plastic mulch.

They decrease evaporation from the soil surface. There are drawbacks to cover crops, however, which make them something only an energetic or experienced gardener should try. They can keep the soil wet and cool in a late spring, which can make it hard to get into the garden on time and also cause poor seed germination. Some of them, like hairy vetch, can be hard to kill. In a dry spring, any of the covers may deplete too much soil moisture, causing poor crop seed germination and growth. Also in a dry spring, the clovers can be hard to kill because of their strong tap roots. The grasses may tie up nitrogen for 4 to 8 weeks after incorporating them into the soil and require extra nitrogen fertilizer to prevent nitrogen deficiencies in crops. Small seeded crops, such as lettuce, have poor germination in heavy cover crop residue.

Cover crops are managed in many different ways. Some are planted in August or September, allowed to grow in the fall and early spring and then killed by tilling in late spring. Other fall planted covers may naturally winterkill, providing a residue early the following spring. These less hardy species may also be planted in the spring and killed in the summer for a late planting of green beans. Cover crops can also be used as living mulches, such as subterranean clover, planted between beds of raspberries. The clover keeps down the weeds and fixes nitrogen for the crop. Another method combines killed and living mulch by seeding or transplanting crops into strips or spots of killed cover crop. Some cover crops can be killed by just mowing and others by rototilling.

Before buying the most available cover crop seeds, try to determine your goals. Do you need quick nitrogen or lots of organic matter? Cover for a slope? Something to hold down the soil over the winter and get a jump on spring weeds? Deep cover crop root growth to bring up plant nutrients and improve water infiltration? Different cover crops will do different things for you. (Seeding rates are given in parentheses.)

Grain rye will add more organic matter to your soil than any other crop. If left to grow to its maximum height of over six feet in mid-June, the roots will go equally deep down into the soil, forming channels for better water infiltration. Rye can be planted later than any other cover crop, often into October. Its lush growth, when cut by early June, will form a thick mat into which cabbage, tomato, squash or melons can be transplanted. However, rye is notorious for getting away from gardeners. It can grow so fast in the spring that it is difficult to cut and after cutting, the heavy residue may keep the soil too cool and wet. Rye is also allelopathic, which means that its roots release compounds that inhibit the germination of seed, both weeds and vegetables. It also can tie up nitrogen for longer than any other cover crop (1.4 to 2.8 lb/1000 sq.ft.).

Oats can be used as a fall cover crop. They are inexpensive, grow quickly in a cool, wet fall and are completely winterkilled, providing a nice residue over the winter (1.2 to 2.3 lb/1000 sq. ft.).

Annual ryegrass is one of the least expensive and most hardy of all the covers. It has quick and competitive growth and is very shade and traffic tolerant. Annual ryegrass forms a thick sod with a fibrous root system, making it good at capturing nitrogen and preventing erosion. It reseeds easily and quickly and may turn into a problem weed. It is best suited as a living mulch in systems where it will be incorporated the following spring.

Perennial ryegrass works better in a system that requires a living mulch for at least two seasons. It is slower to establish but more persistent than annual ryegrass (0.5 to 0.6 lb/1000 sq. ft.).

Hairy vetch is a legume. It fixes more nitrogen and produces more biomass than most legumes. Vetch is often planted in August or September and killed the following spring. Later planted crops like squash, melons and tomatoes do very well growing over the vetch residue. This decrease in fruit contact with the soil can reduce some of the fruit rots. Fall-planted hairy vetch can be killed by just mowing after it blooms the following June. It may also winterkill in a severe winter. Vetch also can be planted in the spring, mowed in the fall and early crops can be planted into the residue the following spring (0.8 to 0.9 lb/1000 sq. ft.).

A combination of fall-planted grain rye and hairy vetch gives the benefits of both. The vetch can vine up the stiffer stalks of rye. The vetch fixes atmospheric nitrogen, while the rye uses leftover nitrogen. Nitrogen from the decomposing vetch will cause the rye to decompose more quickly and not tie up nitrogen as long. Pumpkins and tomatoes transplanted into this residue in early June perform extremely well (0.7 to 1.4 lb rye/1000 sq. ft. and 0.4 to 0.5 lb vetch/1000 sq. ft.).

Medium red clover is inexpensive, readily available and tolerant of many conditions including shade. It is easily frost-seeded at any time during the winter, but only makes moderate spring growth. Medium red clover can also be interseeded into squash and melons before runnering or into sweet corn at tasseling and allowed to grow until the following spring (0.2 to 0.4 lb/1000 sq. ft.).

These are only a few of your options. Be creative, but keep an eye on cover crops. In a spring like we just had, some people got a little more cover crop than they had planned on!

Source: Ohio Gardening, August 1995, Vol. VI, No. 6

SPECIAL NOTE:
Cover crops and green manure can be purchased from:

Prairie State Commodities
P.O.Box 6
Main Street
Trilla, IL 62469
(217) 235-4322
Catalog $1.00

Necessary Trading Co.
One Nature's Way
New Castle, VA 24127-0305
(703) 864-5103
Catalog free

Harmony Farm Supply
P. O. Box 460
3244 Highway 116 North (Sebsatopol)
Graton, CA 95444
(707) 823-9125
Catalog $2.00

Bountiful Gardens
18001 Shafer Ranch Road
Willits, CA 95490 (707) 459-6410
Catalog free

Filaree Farm
Route 2, Box 162
Okanogan, WA 98840-9774

Late Summer 1996
Cover Crops | Tomato Problems | Annual White Grub Homeowner Control Options | Phalaenopsis: "No Better Orchid for Beginners" | Bug Bites | Lawn Care Calendar | To A Violet | Hort Shorts | Hort Tips | OTC Hearburn Drug | Food Safety Checklist |Healthy Cooking: Cabbage | Nutrition Facts: Meal Planning 101 (Part 3 of 3) | Health and Household Tips | Did You Know?

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