Cover Crops for Home Gardens West of the Cascades (Home Garden Series)

What Is a Cover Crop?

Cover crops are plants grown to both cover and improve the soil. They may be used as a living or dead mulch on the soil surface, or they can be tilled into the soil as a “green manure.” Gardeners usually plant cover crops in the fall for winter cover, but some gardeners also use cover crops as part of a summer rotation. Cover crops can be any type of plant but are generally grasses (including cereal grains), legumes, or grass/legume mixtures. Some non-legume broadleaf plants can also be used.

Why Grow a Cover Crop?

Cover crops serve the gardener in many ways, typically by protecting and improving the soil, suppressing weeds, and attracting beneficial insects (Table 1). However, no single cover crop will provide all these benefits since different types of cover crops and management approaches offer different types of benefits. Many gardeners plant a mixture of cover crops to increase the number of benefits to their garden, but cover crops do require some management. Seed must be pur-chased and planted at the right time, cover crops may need some irrigation during establishment, and the right tools and techniques are needed to terminate cover crops.

• Cereal rye (Secale cereale) is one of the most commonly grown cover crops in the Northwest (Figure 1). This crop is vigorous, very cold hardy, and can germinate and establish in cool weather. When planted by early September, rye will capture some of the available nitrogen in the soil and recycle it for the next crop. Rye grows rapidly in the spring, so it can become difficult to turn under by the time gardeners are ready to work their gardens. Thus, mowing or weed whacking may be necessary before incorporating a heavy cover crop of cereal rye.
• Winter wheat (Triticum aestivum) is most suitable for mid- to late-season planting. Planting winter wheat before late September makes it more susceptible to disease and premature death. Wheat covers the ground quickly in the fall but does not grow as vigorously as rye in the early spring, making it a good choice for late-planted gardens. Wheat is more herbaceous than rye, so it may decompose faster when it is turned back into the soil.
• Oats (Avena sativa) are a good choice for an early cover crop because they grow vigorously when the soil is warm (Figure 2). Oats also tolerate wet and heavy soils better than many other cover crops. Spring oats (i.e., the varieties Cayuse and Monida) should be planted by early September in the Puget Sound area to get good growth. They may experience winterkill, depending on location and weather, but with enough fall growth, killed spring oats can still provide soil-protecting mulch that is then easy to reincorporate in the spring. Winter oats (i.e., the varieties Amity and Walken) are more winter hardy than spring oats and can be planted in early spring, and they are unlikely to experience winterkill.
• Barley (Hordeum vulgare) (Figure 3) is similar to oats in growth characteristics. It produces a large number of leaves, but most of its growth comes late in the spring; although, spring varieties are suscep-tible to occasional winterkill. Barley uses less water in later growth stages compared to other cereals, which reduces the need for irrigation if it is grown as a summer cover crop.
• Annual ryegrass (Lolium multiflorum) is a turfgrass rather than a cereal grass (Figure 4). It has a vigorous root system that is effective at reducing soil erosion, and it is reasonably tolerant of short-term flooding. However, once it is established, annual ryegrass can be harder to till under compared to cereals. Annual ryegrass regrows readily after mowing, but it can also persist like a perennial, making it a potential weed when used as a winter cover crop.

• Hairy vetch (Vicia villosa) is one of the more aggressive legumes. When the weather warms in the spring, hairy vetch will grow quickly. Its tendrils help the plant climb up the stems of other plants, making it a particularly good companion crop for cereals. However, the tendrils can also wrap around tiller tines, making it more difficult to turn the crop under with a rototiller. Hairy vetch should be terminated before it sets seed to prevent it from becoming a weed.
• Common vetch (Vicia sativa) (Figure 5) is similar to hairy vetch but is less winter hardy, although it generally overwinters well west of the Cascades. It also has fewer tendrils than hairy vetch, which makes it easier to till back into the soil.
• Crimson clover (Trifolium incarnatum) (Figure 6) is easier to turn under, and it is less likely to become a weed compared to vetches. It is a good option for areas where early crops will be planted the follow-ing year, but it does not compete well with weeds. It also does not compete as well with cereals and is often grown alone or with annual ryegrass. It grows best when planted in the first half of September.
• Austrian winter pea (Pisum sativum) is a cover crop that can be grown alone or mixed with cereal rye. It competes poorly with winter weeds, which can choke it out during mild winters. Gardeners who raise animals can graze them on the Austrian winter pea before turning the crop back into the soil. Some winter pea varieties have the added benefit of producing delicious pea shoots early in the spring.
• Fava or bell bean (Vicia faba) is a good legume for October plantings because it grows faster during cool weather compared to other legumes. It is not as winter hardy as many other legumes, but plant-ing later decreases the risk of winterkill. Fava beans can be grown alone or mixed with rye or wheat. When grown as a summer cover crop, fava beans compete well with weeds. Small-seeded fava beans (also called bell beans) are most commonly used for cover crops, rather than the large-seeded varieties grown primarily as a food crop.
• Red clover (Trifolium pretense) (Figure 7) is a short-lived perennial cover crop, but it is sometimes used in annual crop rotations when there is a longer win-dow for cover cropping. For example, red clover is a good crop to plant in July or August for termination the following spring. Red clover also tolerates some shade and foot traffic, so it can do well when relay-seeded into a late-season vegetable crop, such as late sweet corn, peppers, or tomatoes. Red clover consis-tently overwinters well west of the Cascades, but it establishes more slowly than many annual legumes.

Other Broadleaf Crops

• Phacelia (P. tanacetifolia) is a winter annual native to the southwestern United States and Mexico. It is a non-legume broadleaf plant in the Boraginaceae family. Phacelia can survive most winters south of Salem, Oregon, but can be killed by hard freezes when planted further north, especially if the plant is more than 3 or 4 inches tall during a cold snap. Their stems are hollow, and the plant is easily killed if the stem is broken, making it fairly easy to incorporate back into the soil in the spring. Phacelia produces beautiful blue flowers, which are not only attractive in a garden but can provide an important spring source of pollen and nectar for honeybees, native pollinators, predators, and other beneficial insects. Phacelia grows well with other cover crops and is often grown in combination with legumes.
• Brassica family cover crops are crops that include mustards (Figure 8), oilseed radish, and canola. These crops have variable winter tolerance and are more often grown as short-season summer cover crops, similar to buckwheat. They are very effec-tive at smothering weeds, and their residues can inhibit weed seed germination and some diseases, but they can become troublesome weeds themselves if allowed to go to seed. They can also harbor club root, an important soilborne disease in brassica veg-etables. Growing these cover crops is restricted in areas where commercial brassica seed (such as cab-bage seed) is grown. Restricted areas include parts of northwest Washington, Oregon’s Willamette Valley, parts of central Washington, and central and eastern Oregon, thus limiting their use in the Northwest.

To determine if you are located in a restricted area, check the following sites: for Washington go to http://apps.leg.wa.gov/wac/default.aspx?cite=16-326&full=true#16326-010 and for Oregon go to http://www.oregon.gov/ODA/PLANT/canola_summary.shtml#Maps.

Cold-sensitive cover crops

The following cold-sensitive cover crop species can only be grown during the summer. However, they can provide cover for a garden space that is not planted in the current year, or they can be part of a planned garden rotation. These crops can break up soil compaction, help suppress weeds, and improve the tilth of a new garden. The use of both summer and winter cover crops further reduces weed problems in a new garden space by interfering with weed lifecycles.

• Buckwheat (Fagopyrum esculentum) is a very popular summer cover crop and is well suited to the North-west (Figure 9). It grows quickly in warm weather and in a wide range of soils. Buckwheat begins flow-ering in 4 to 6 weeks and is usually ready to turn under in 30–50 days. It is possible to grow several crops of buckwheat in a single summer. Buckwheat can go to flower within 20–30 days of seeding and can set mature seeds within 40 days. However, if crops are drought stressed, growth may be restrict-ed. Termination must occur before seed is set to avoid reseeding. Although buckwheat is a broad-leaf cover crop, it does not fix nitrogen. Its prolific growth will smother weeds, and it has been shown to suppress weed emergence for short periods after being turned back into the soil. But if allowed to grow too large, it becomes difficult to turn under and may begin to produce seed. If planted after days begin to shorten in July, it flowers before produc-ing much plant material. Buckwheat is sensitive to frost, so it should not be planted in the spring until the danger of frost has passed. The white flowers attract pollinators, predators, parasitoids, and other beneficial insects.
• Sorghum-Sudangrass (Sorghum bicolor) hybrids (Figure 10) are hot weather, drought-tolerant rela-tives of corn that can provide good mid-summer cover and weed suppression. These hybrids need soil temperatures in the 60s or higher to establish good germination, and they do best when planted in June or early July. They have vigorous root growth and will produce large quantities of plant material if they have adequate heat, nutrients, and water. Although summers west of the Cascades are cooler than is ideal for sorghum-sudan hybrids, these hybrids have been reliable in the Puget Sound region of Washington. Sorghum-sudan hybrids do not grow as quickly in and around the Puget Sound as they do in warmer regions, but they still produce enough plant mate-rial to make them difficult to incorporate back into the soil. Because they are sensitive to frost, they are sometimes left to freeze. The killed plants provide a weed-suppressing mulch over the winter, and in the following spring, they are easier to incorporate back into the soil.