Growing winter cover crops
Fall is a time of transition in the garden. You worked hard all spring and summer to build healthy soil and habitat. Now winter rains take over, eroding soil, leaching valuable nutrients, depositing pollutants into waterways and compacting the tiny particles of your soil, which makes it harder for water to carry nutrients to plant roots.
Fortunately, cover crops can nourish and protect your soil from the harshness of winter! Plant them from seed in the fall after removing spent tomatoes, squash and other warm-weather crops. Cover crops will slow erosion and runoff, feed the soil with nutrients and organic matter, and help squelch weeds by giving them fewer places to land and grow. Once you have harvested fall vegetables, continue feeding and protecting the soil by sowing cover crops.
Sow, Grow & Turn It In
Grasses, cereal grains and legumes are freeze-hardy plants commonly used as cover crops. Sow them in late summer to early fall and then turn them into the soil or cut them down and use them as a mulch the following spring. They are easy to plant, so there’s no need to worry about sowing too much seed or thinning them since blanketing the soil in sprouts leaves little room for weeds. There’s no need to dig in compost before sowing or to add fertilizer, either; you are laying the groundwork by sowing cover crops now, which will amend the soil next spring when the warm weather means plants are more ready to take in fertilizer.
If you still have productive plants in September that you are not quite ready to get rid of, like tomatoes, start your cover crop by undersowing. This method works well around taller crops that will be removed by the end of the month. Lightly cultivate the soil between the existing crops, then sow the cover crop seeds. When you are done harvesting the vegetable crops at the end of September, remove them to let in more light and give the cover crops more room to grow. You can also sow more cover crop seeds in any bare patches at this time if desired.
The real work comes in the spring when it is time to turn the crops into the soil. Wait until about a third of legumes are flowering before you take them down. This point in their growth represents peak nitrogen possibility. If a cover crop is allowed to continue flowering, some of the seed may mature and drop, becoming a weed problem.
Turn in grains and grasses when they grow to about a foot tall. If they grow larger, they will break down more slowly and be difficult to manage. Arm yourself with a spade or shovel to chop the cover crop into small (2-3 inch) pieces and work them into the top few inches of soil. Small pieces will break down more quickly. If a cover crop is too tall to manage, cut it to soil level and use shears or hand pruners to cut up the vegetation before turning it in. Cover the bed with burlap to retain moisture and keep weeds from sprouting, waiting two to three weeks before planting to allow the crop to decompose.
If you prefer “no-till” methods, chop up the vegetation into similar-sized pieces and leave it on top of the soil to decompose or set aside to use later as mulch for your edible garden. You will still need to cover the bed with burlap to let the roots decompose, this time for six to eight weeks. You can also cut down the crop, chop it up and use it to make compost. Dig in finished compost before planting if you do this.
Loving Legumes
Legume cover crops are part of a family of plants, including peas and beans, that has the added benefit of returning nitrogen to the soil. Nitrogen is often deficient in Pacific Northwest soils thanks to the rain. Legumes are able to “fix” or create nitrogen due to their unique relationship with certain soil bacteria called rhizobia, which infect legume roots and form little nodules where atmospheric nitrogen is transformed into a form of nitrogen usable for plants. The nitrogen is released for future use when the crop is turned into the soil the following spring. Many seed companies sell cover crop seeds that are “inoculated” or coated with the correct rhizobia bacteria to promote better nitrogen fixation. Even peas and beans that are grown as an edible crop can fix and add nitrogen to the soil as long as the roots are left in the ground when it is time to take down the plants. Don’t pull them out! Chop them down and turn the vines into compost or mulch and let the roots decompose in the soil.
Legumes need soil that is not too wet and drains well. Some legumes, such as fava beans, grow big taproots that help break up clay or compacted soils. A few popular nitrogen-fixing legume cover crops for the Pacific Northwest:
Crimson clover (Trifolium incarnatum). This charming member of the pea family has pretty red flowers and is easier to turn into the soil than some other cover crops. It does best when grown alone rather than in a mix. Plant during the first half of September so that it has time to get well established.
Vetch, hairy (Vicia villosa) and common (Vicia sativa). Hairy vetch takes off in the spring and climbs other plants with tendrils. Common vetch is less cold hardy, but usually does fine in our milder winters. Common vetch has fewer tendrils than hairy and may be easier to turn back into the soil.
Austrian winter peas (Pisum sativum). Grow winter peas alone or in a mix. If grown alone and not well established, they might be choked out by other winter weeds. Some varieties may produce tasty edible side shoots in early spring.
Fava (Faba) or bell beans (Vicia faba) These large legumes grow more quickly in cool weather than other legumes so they can be planted in October; however, they might die or be damaged if a frost hits in November or early December before hardening off. Keep some extra seed on hand to replant when the ground is not frozen. Bell beans are favas with small seeds, often grown as a cover crop rather than as food. They may be included in mixes; favas are usually grown as a single crop.
Cereal Grains and Grasses
Grains and grasses grown for cover crop are usually annuals that grow quickly in cool weather. They do not fix nitrogen, but they grow a large and deep network of roots that captures nitrogen present in the soil and holds it so that it won’t be leached out by rain. At the same time, the leaves actively photosynthesize to create plenty of new nitrogen and organic matter to return to the soil in spring and feed young crops. A few popular grain and grass cover crops for the Pacific Northwest:
Cereal rye (Secale cereale). Very cold hardy and vigorous, so it may be difficult to turn under in spring. It’s most productive in capturing nitrogen if it’s planted by early September, but will still provide benefit if sown later.
Oats (Avena sativa). Tolerates wet and clay soils better than other cover crops. Grows quickly when the soil is still warm, so plant in early September to get a good crop established.
Annual ryegrass (Lolium multiflorum). The extensive root system reduces soil erosion and can withstand short-term flooding, but may be more difficult than cereal grasses to turn back into the soil.
Mix It Up
When the weather is cool, legumes grow more slowly. They speed up again as it warms up in late winter. A cover crop mix of only legumes may not provide good coverage if it was started late and did not have a chance to become well established before winter sets in. A mix of grains or grasses and legumes can be a good strategy. Legumes can be floppy and therefore benefit from a tall grass companion to provide structure, keeping them off the ground. A mix also builds healthy soil by introducing a diversity of plants that attract and feed different types of soil organisms. Many seed companies sell mixes that include a variety of legumes and cereal grains in order to take advantage of the soil building properties of both types of crops. Find cover crop seed at Seattle Farm Co-op, Walt’s Organic Fertilizer Company and local nurseries.
To learn more about growing cover crops, visit the Garden Hotline at www.gardenhotline.org or call us at (206) 633-0224. You can also find us on Facebook, Twitter, Pinterest and YouTube. Or take a class from Seattle Tilth on ways to improve your soil at www.seattletilth.org/learn/adultclasses.