Grow Your Own Green Manure Cover Crop

Need to save time and money in your garden? You can fertilize, mulch and prevent weeds and add organic matter all in one easy step. Just grow your own green manure cover crop.
By Kris Wetherbee
April/May 2000
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Learn about growing your own green manure cover crop for your homestead. Improve your soil and increase vegetable yields with easy-to-grow cover crops.

Whether you garden in raised beds, a small backyard or on acres of land, cover crops can provide a wealth of benefits to the garden. For one, a green manure cover crop often grow fast and develop in thick stands. Plus, they provide nutrients to turn a cover crop hack into the garden while the plant is still green and it's called "green manure."

If you have a weed problem, fast-growing cover crops like buckwheat, red clover or Austrian peas can outcompete and smother unwanted invaders. Even closely planted sunflowers, with their allelopathic tendencies, are a good weed-suppressing crop. While a sunflower grows, it temporarily inhibits nitrogen-fixing bacteria in the soil . . . bad news for the germination and growth of any plant needing nitrogen. And few plants need nitrogen more to germinate and grow than do the weed grasses.

Is your garden crawling with pest insects? Cover crops are a boon to beneficial life — providing food and shelter for both soil-dwelling creatures and aboveground insects that pollinate and help control garden pests. Countless beneficial insects thrive on the pollen or nectar of cover crops in bloom. (Pest insects, conversely, feast not ran pollen but on plants, choosing their targets by the gaseous odors they emit. In my experience, cover crops have far less trouble with pest insects than do vegetable crops.)

If you're looking to lure welcome guests to your garden, look no further: alfalfa attracts a host of parasitic wasps, lady beetles, damsel bugs, big-eyed bugs and assassin bugs. White clover attracts tachinid flies, ground beetles and parasitic wasps that prey on aphids, scales, caterpillars and whiteflies. Most grains attract lady beetles. Clovers and vetches attract minute pirate bugs. Fava beans attract predatory and parasitic wasps, as does buckwheat, which has the added benefit of luring syrphid flies (also called hover flies) and bumblebees.

Cover crops are good for the environment, too. Established legumes or fall sown, grains and grasses that quickly cover the ground protect the most fertile part of your soil from erosion caused by assailing winds and pounding storms. Likewise, grasses, grains and brassicas that grow quickly in the fall help capture easily leached nutrients like nitrogen and calcium before they wash away with winter rains. When you turn these crops under in spring, the needed nutrients are released back into the soil.

Cover crops also regulate soil temperature, keeping the earth cooler in summer and warmer in winter. By providing an insulating blanket, they allow important soil life to thrive, especially earthworms. The more worms in your garden, the more effective they'll be at creating channels to break up and aerate the soil. And, once that blanket of green is turned under, it returns to the earth an abundance of organic matter essential to earthworms and other soil creatures, which in turn are at the very heart of healthy soil.

Finally, if you want to fertilize your garden naturally, grow cover crops. Legumes such as vetches, alfalfas and clovers can actually transport nitrogen right out of the air we breathe and into the soil, where bacteria can then convert it into a plant-friendly form. Grains and other cover crops also work to replenish the soil, by recycling nutrients normally beyond the reach of other plants. Their roots reach deeply into the subsoil, drawing up valuable minerals and nutrients that are then stored in the plants' tissues. Once the cover crop is turned back into the soil and allowed to decompose, those nutrients gradually become available to future plants.

Choosing Your Green Manure Cover Crop

Which are the best cover crops to grow? Your choices are limited only by your needs. Do you want cover in the spring, summer, fall or winter? Is your goal to add the most abundant organic matter, tremendous amounts of nitrogen, bring up valuable nutrients from the subsoil or grow a killer crop that suppresses the nastiest of weeds? Where you live, along with your soil and climate conditions, can also play a factor. Ultimately, you'll want a crop that works for you — not one that's going to become a nuisance.

Suggestions for which crops to grow are listed in this article, "At A Glance: Best Crops for Special Conditions" (see end of article). There are, however, some generalities that can help you decide which covers to grow. First, decide whether you want an annual or perennial. Annuals usually grow fast, are quick to establish, last a season and won't resprout from their roots. Perennials tend to be deep-rooting (though some annuals are as well), slow to establish and are great for areas, like orchards and vineyards, where you want a long-term cover.

Second, consider which of the two basic groups of covers you'd like to grow: legumes or grasses and grains. All add a certain amount of organic matter (though grasses and grains tend to add more). Legumes decompose rapidly when turned under and are unsurpassed as nitrogen-catchers, securing more of this important element in the soil than any other cover crop.

Grasses and grains, on the other hand, often germinate and grow quicker than legumes, making them the first-choice for fast, effective ground covers. They are also especially valued for the bulky organic matter they add to soil, as well as for reducing nematodes in the garden.

But why not grow a mix so you can have the best of both worlds? Your fast-growing grains will act as a nurse crop to help sustain and provide support for your slower-growing vining legumes. Plus, the legume-grass/grain combination holds nitrogen in the soil better than either grown alone.

When to Plant Cover Crops

Depending on where you live, you can plant cover crops anytime from early spring through late fall. Spring or summer planted covers can be used as part of your rotation, grown between rows or underneath established vegetables. Frost-tolerant covers like bell bean, Austrian peas and oats can be planted as soon as your soil can be worked. For others, like cowpeas, buckwheat and soybeans, wait until all danger of frost has passed.

Fall and winter covers are sown as early as August in cold climates and as late as November in the deep South.

Since I sow a variety of cover crops, I'm usually planting throughout the year. In the spring I might sow nitro-alfalfa and spring oats, which are turned under in midsummer, before I plant my fall crops. Buckwheat is an excellent green manure for summer (I plant in May or June, tilling under six to ten weeks later). Fava beans, Austrian peas, crimson clover and vetch are my favorites for planting in the fall (September or early October here in our mild zone 8 Oregon climate) and then tilling under in spring.

Together, these crops provide erosion protection against our considerable winter rains, organic matter to loosen our heavy clay soil, plus nutrients — specifically nitrogen, but also phosphorus (supplied by buckwheat) — in which our soil tends to be lacking.

Every year I also plant my previously weediest bed with sunflowers — the lazy gardener's way to control wild growth.

How to Plant Cover Crops

Planting your cover crop is easy you don't have to worry about tilling the soil first. Just clear the bed of weeds and spent vegetables, then rake the soil free of clumps. You'll need an average of one and a half cups of seed to sow a 100-square-foot area.

Use a broadcaster or throw your seeds on by hand, then work in with a rake or cover with additional soil. A good rule of thumb is to cover the seed to a depth three times its diameter. Water thoroughly and be sure to keep watering in summer or during dry weather.

It's important to remember to use an inoculant on any legume seed before you plant. These commercially available powdered substances contain live rhizobia bacteria. While different legume crops are naturally host to different types of rhizobia bacteria, inoculants ensure that sufficient populations of the right rhizobia are present at the right time (the inoculant's packaging should tell you which crop it's intended for). Rhizobia bacteria colonize the roots of legumes and feed from their carbohydrates. In return, the bacteria take gaseous nitrogen from the air and change it to a form that plants (both legumes and their neighbors) can use. This symbiotic process, commonly known as nitrogen-fixation, is one you'll want to encourage to ensure that your legumes absorb the maximum amount of nitrogen.

Turning The Cover Crop Back In

Cover crops can be worked back into the soil at anytime before they set seed. For legumes, the prebloom stage is ideal. Grasses and grains should be turned under when they start to form grain heads (though most can be turned under sooner, this is when they are at their nutritional peak).

Exceptions to these rules are legume or grass/grain crops planted in fall, which are then turned under in spring (usually May or June), and, in my experience, cereal rye and ryegrass, both of which can get quite vigorous and should be cut, then turned under before grain heads form.

If a crop is tall, you should first mow it, weed-whack it or use a scythe. Compost the tops or till, hoe or dig them in along with the remaining stubble. After digging your cover in, wait at least two to three weeks before planting again. This allows the soil microbes and organisms to begin breaking down the plant material into valuable nutrients, bringing rich rewards to your soil - and your harvest.

Related info:
Choosing Cover Crops for the Homestead

At a Glance: Best Crops for Special Conditions

Tolerates Acidic Soil: bell beans, most clovers, most vetches, buckwheat
Tolerates Alkaline Soil:
alfalfa, barley, ryegrass, Sudan grass
Tolerates Drought Conditions: alfalfa, hairy vetch, barley, cereal rye, ryegrass, Sudan grass
Tolerates Wet Conditions: bell beans, subterranean clover, Austrian peas, mustard, oats, ryegrass
Tolerates Shade: most clovers, hairy vetch, cereal rye, ryegrass
Tolerates or Enjoys Heat: cowpeas, soybeans, buckwheat, Sudan grass
Breaks Up Compacted Soil/Deep Roots: alfalfa, bell beans, most clovers, barley, buckwheat, cereal rye, kale, mustard, ryegrass
Suppresses Weeds: most clovers, Austrian peas, field peas, soybeans, vetches, barley, buckwheat, cereal rye, oats, ryegrass, Sudan grass

Seed Sources

Fedco Seeds
Waterville, ME

Peaceful Valley Farm Supply
Grass Valley, CA

Johnny's Selected Seeds
Albion, ME

Territorial Seed Company
Cottage Grove, OR

Read more gardening tips at www.motherearthnews.com


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