Cover Crops and Green Manure for the Vegetable Garden

Cover crop, green manure crop, and catch crop are three interchangeable terms to describe crops that feed, build, and protect the soil and attract and feed beneficial insects and soil microorganisms.

Cover crops are planted to cover planting beds and enrich the soil. When a cover crop is turned under to decay and to feed the soil it is called a green manure; green manures add humus to the soil. When a cover crop also fills in unplanted spaces in the garden—spaces between the main crop-it is called a catch crop.

Cover crops and green manures are not always feasible in small home gardens. There is simply not enough time and space for dedicated cover crops in a small garden. Compost and organic fertilizers are often more practical and can do a better job of enriching the soil in a home garden.

For a three-season home garden where crops are grown in spring, summer and fall, a good cover crop choice is a winter cover crop such as winter rye. Winter cover crops can be planted in late summer or fall to grow or sit through winter; they then can be turned under in late winter or early spring when the soil is workable—about 4 to 6 weeks before spring crops are planted.

What Cover Crops Do:

• Add nutrients and organic matter to the garden as a green manure. Green manure is similar to compost only it decomposes in the garden rather than in a compost pile.
• Retrieve nutrients deep in the soil and bring them to the surface. The roots of cover crops grow deep to nutrients and bring them to the surface; later when the cover crop is cut down and turned under those nutrients will remain in the topsoil.
• Block weeds from growing by covering planting beds not growing crops. A cover crop can be planted in any unused section of the garden, even around growing crops.
• Encourage beneficial insects—the flowers of many cover crops are food for beneficials.
• Encourage and feed beneficial soil microbes which break down organic matter in the soil; beneficial microbes, in turn, eat pathogen-causing fungi and bacteria.
• Protect the soil from erosion, especially during the rainy season and winter.
• Break up compacted topsoil and break through hardpan and compacted subsoil; many cover crops are deep rooted.

Times to Plant Cover Crops:

Cover crops can be part of a garden crop rotation, growing in a planting bed or field for an entire season. But this is not always doable in the home vegetable garden.

A good home garden cover crop strategy is to use cover crops as catch crops. A catch crop is a filler crop; it grows in a vacant spot before or after a main crop. A catch crop can be interplanted with your main crops.

• Spring Cover Crops: Sow spring cover crops as soon as the soil can be worked in spring. Sow frost-hardy cover crops such as winter rye or oats in beds where warm-season summer crops such as peppers, melons, and tomatoes will be planted later. Spring cover crops can grow for 6 to 10 weeks before main crops are planted. Mow down or chop up and turn under the spring cover crop 1 week after it starts to flower. Turn it under not less than 2 weeks before you sow your main crop.
• Midseason Cover Crops: Grow summer or midseason cover crops in regions where the weather gets too hot for warm-season vegetable crops—where temps rarely dip below 90° Sow summer cover crops after spring leaf and root crops have been harvest. Cut down summer cover crops about 3 weeks before you plan to sow fall vegetable crops. It’s best to wet down the garden and cover crop before turning it under in very hot regions—this will help decomposition. A fast growing cover crop is buckwheat.
• Winter Cover Crops: Winter cover crops can be planted as soon as the summer harvest comes out of the garden. When possible sow the winter cover crop at least 6 weeks ahead of the first killing frost. Some winter cover crops will survive through the winter; others will die during the winter—but can remain in place to stem erosion. Two very quick growing cover crops for winter are oats and winter rye.
• Main-Season Cover Crops: Cover crops can be planted during the main growing season as filler or catch crops. These cover crops are interplanted with your main crops. Main season cover crops serve as a living mulch to keep down weeds and slow soil moisture erosion—while feeding the soil. Main season cover crops also protect the soil and earthworms and microorganisms below from hot temperatures. Sow main season cover crops as an undercrop—that is wait 3 to 4 weeks after sowing your main crop then sow the cover crop as an underplanting. Wait so that the roots of the cover crop are not competing with the main crop.
• Full-Season Cover Crops: Cover crops can be planted as part of a crop rotation for a full season. A full-season cover crop will knock down most perennial weeds and will reduce pest root-knot nematode populations. Full-season cover crops can also feed the soil—legumes will draw nitrogen up from the soil and cover crops turned under will act as a green manure or compost feeding the soil. Put full-season cover crops in your four-year crop rotation—adding an additional year to create a five-year crop rotation (this is doable in very large gardens, but less easy to do in small gardens).

How to Plant Cover Crops:

Planting a cover crop is easy. Here are the steps:

1. Remove weeds and crop debris from the planting beds; break up clumps of soil and rake the seedbed even.
2. Determine the square feet you intend to plant; you will need about 1 cup of cover crop seed for every 50 square feet you want to plant.
3. Broadcast the very small cover crop seed (such as annual rye or clover) across the planting bed. Large cover crop seed such as beans and peas can be sown in furrows ½ inch to 1 inch deep 6 inches apart. After broadcasting or seed sowing rake the bed lightly to ensure the seed is covered. If you are sowing legumes such as soybeans, clovers, and vetch as a cover crop, add an inoculant that contains the bacteria needed to work with the cover crop’s roots to fix nitrogen.
4. Keep the seedbed just moist until the cover crop germinates and becomes established. If the weather is very hot keep the seedbed from drying out by sprinkling straw over the seedbed. If birds start digging up the seed, lay a loose spun poly row cover over the planting bed.

How to Kill and Turn Under Cover Crops:

When the cover crop flowers, it is time to turn it under and let it begin decomposing. There are a couple of ways to turn the cover crop:

1. Chop or mow the top growth (use a lawn mower, string trimmer, hedge trimmer, or hand sickle or scythe) let it dry in place and then hand dig or turn it under. If the top growth is higher than 2 feet, you may want to remove all but about 2 inches of stubble from the garden as it will be very slow to decompose; leave about 2 inches of top growth stubble and roots in place to turn under; the tall top growth can be used as a mulch in other beds or put in the compost pile.
2. Rototill the cover crop under. Set the tiller at a shallow setting; do not till the crop residue too deeply as it will be slow to decompose. Rototilling is likely to disturb or kill earth worms and soil microbes so it is not recommended unless the garden is very large and slow decomposition is not possible.

Spring planted cover crops will likely not decompose fast enough to follow with a summer main crop, so it is almost imperative that you remove spring planted cover crop top growth before turning the stubble and roots under. Do this several weeks before planting the main crop. Be sure to turn the stubble and roots under fully.

Five Cover Crops for the Home Garden:

Peas: garden peas can be grown as a cool-weather green manure crop that is edible; broadcast pea seeds in unused spaces in the garden; plant early spring or early fall—grows best in cool weather; will fix nitrogen in the soil.

Beans: green and yellow snap beans can be grown as a warm-weather green manure, cover crop; sow in spring 3 weeks after the last frost and continue to sow until 8 weeks before the first fall frost; beans sprout quickly; plant beans 5 to 6 inches from other main crop plants; beans fix nitrogen in the soil; beans are an edible cover crop.

Buckwheat: sow spring to mid-summer; warm-weather cover crop; ready to turn under in 6 weeks; do not let it go to seed; frost tender; fast growing; smothers weeds; releases potassium as it decomposes; roots grow deep and improve soil texture; attracts pollinating bees and predatory wasps.

Annual Ryegrass: you want the grass, not the grain; you want annual ryegrass—not perennial ryegrass; sow spring through fall; easy to work into the soil; winter cold will kill; can serve as winter protection of soil after it dies back; turn under before ryegrass drops seed; loosens compacted soil; attracts beneficial insects.

Winter Rye: grain crop, not a grass; is fast growing spring and fall; plant up to 3 weeks before first fall frost; winter hardy; good winter cover crops; improves soil structure; don’t let it grow taller than 10 inches or remove top growth before planting main crop; requires some time to decompose.

Other Cover Crops: Alsike clover, Austrian winter pea, Berseem clover, cowpea—southern pea; crimson clover, fava bean, hairy vetch, kale, oats, oilseed radish, rape, red clover, soybean, Sudangrass, sweet clover, white clover.

Adding Phosphorus and Potassium to the Garden: Many cover crops—especially legume family crops– add nitrogen to the soil, but at the same time they draw up phosphorus and potassium. When setting out fruiting crop transplants in spring such as tomatoes, peppers, and melons, add a phosphorus and potassium organic fertilizer to planting holes to make up for any loss of those nutrients to cover crops.

More tips at Making Compost, and also Soil Nutrients.