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.
Cover crops can be an effective way to maintain soil organic matter, supply and retain nitrogen (N), reduce soil erosion, and suppress weeds. However, successful cover cropping requires advanced planning, selecting the right cover crops, and planting and terminating them at the right time. Cover crops will provide the greatest benefit if they are treated as an integral part of garden planning and not just as an afterthought.
Cover crops require very little maintenance, and additional nutrients are seldom needed to support them since cover crops scavenge nutrients already present in the soil, and may even “fix” additional nitrogen from the atmosphere. Winter cover crops seldom need irrigation in areas west of the Cascades and only need irrigation for crop establishment in areas east of the Cascades. Cover crops do not require weeding since they will compete with, and even smother, surrounding weeds.
Planning for Cover Crops
Plan cover cropping in advance by selecting the best species for your garden conditions, and obtain seed early, so you can be ready to plant at the proper time. Gardeners plant and harvest different vegetable crops at different times. Planning ahead makes it easier to stagger the planting and termination of cover crops with the planting and harvest of vegetables in the garden.
Decide what vegetable crops you want to plant in the spring, so you can plan your winter cover crops accordingly. If you intend to plant early vegetables in March, plant a cereal cover crop, such as rye, oats, or wheat, the preceding fall. These cover crops can provide sufficient soil cover for the winter and significant growth before they need to be terminated to make room for vegetable planting. If you intend to plant vegetable crops in April or early May, a legume winter cover crop will have enough time to grow, so it can provide a nitrogen benefit. Legumes will provide even more nitrogen if the vegetable crop
Try to create niches in your garden for cover crops. You can reap the benefits of earlier fall-planted cover crops by using them in areas of the garden where the vegetable harvesting will be complete by August or early September. In garden areas where the vegetable crop is not harvested until October or later, choose a cover crop that is suitable (one that germinates at low soil temperatures) for later planting, such as rye, wheat, or fava beans. You can also relay seed cover crops into the garden while the vegetable crop is still growing. (See Advanced Cover Crop Techniques in this publication for more information on relay seeding.) This planting system is suitable for vegetable crops that remain in the garden until October, November, or even December. If you have vegetable crops that overwinter, such as kale, consider planting cover crops in the spring after harvest.
Decide how you will terminate a cover crop when you are planning your garden. Crimson clover, for instance, is easier to incorporate (turn back into the soil) than hairy vetch, so it may be a better choice if you are terminating a large crop area with hand tools. If you will be transplanting vegetable seedlings or planting vegetable crops with large seeds, such as beans or corn, you may be able to leave the mowed cover crop on the surface as mulch, rather than incorporating it. (See Reduced Tillage below for details on using cover crops as mulches.)
Another issue to consider when planning for cover crops is how wet your soil becomes in the winter. Some cover crops can tolerate short periods of standing water, while others cannot. For example, hairy vetch will survive better in wet soils than crimson clover or winter peas, while annual ryegrass is more tolerant of wet soils than is winter wheat.
You can also plan for cover crops outside the normal winter season. See Cover Crop Niches below for ideas and guidance on summer- or “shoulder-season” (spring and fall) cover crops.
Planting Cover Crops
Cover crops are typically planted directly from seed. They can be seeded in spring, summer, or fall, depending on your gardening goals and vegetable rotations. Seeding cover crops during periods when environmental conditions are less favorable, such as cold soil or wet conditions, may require additional steps to improve seed germination and establishment. Each cover crop responds best to a certain seeding method, so using the proper method will improve crop establishment. While cover crops do not require much ground