Cover crops are crops you grow to benefit the soil. You do not harvest them like other crops for consumption by you and your family but grow them because of the many good things they do for your soil.
Cover crops protect the soil surface from the impact of rainfall, reducing runoff and erosion. Cover crops break up compacted soil and capture nutrients that otherwise would be lost through leaching. Cover crops choke out weeds and when turned under, add fresh organic matter to the soil.
Cover crops typically are planted in the late summer and early fall since soil compaction, erosion and loss of nutrients are highest in the winter due to rainfall. Some gardeners, however, use cover crops as part of a summer rotation.
Cover crops can be any type of plant but are generally legumes, grasses or grass/legume mixtures.
Legumes fix atmospheric nitrogen into a form more usable by plants through a symbiotic relationship with Rhizobia, a bacterium that forms nodules on legume roots. When the roots decompose in the soil, the nitrogen in the nodules will be released.
Legumes generally grow slowly in cool weather and need to be planted in September. Because of their slow growth, they are not as good at suppressing weeds as other cover crops.
Legumes commonly used as cover crops are hairy vetch, crimson clover, and red clover. Fava bean, another legume, grows well in cool weather and can be planted later than most legumes (in early October). Its deep taproot is ideal for opening up heavy, compacted soil, but its long stalk can become woody and requires chopping before incorporation into the soil.
Most grass cover crops are annuals that grow vigorously and provide quick ground cover, making them good at suppressing weeds. They do not fix nitrogen like legumes, but their extensive root systems grow deep, capturing nitrogen already in the soil that might otherwise be lost to leaching. Grasses yield large amounts of above ground plant material.
Grasses commonly used as cover crops include cereal rye, winter wheat, and annual rye grass. They establish fairly well in cool weather and can be planted in September and early October.
Cover crops are commonly grown as mixtures that can provide a wider range of benefits than a single cover crop. One commonly grown mixture contains a cereal grain (which germinates and grows readily through the fall and winter) and a legume (which establishes more slowly but can fix atmospheric nitrogen).
The following tips will help you use cover crops most effectively:
• Select a cover crop (or crop combination) based on the desired benefits and the crop that best fits into your overall garden plan.
• Avoid sowing cover crop seeds too thickly. If sown too close together, the seedlings form a thick mat that is difficult to mow and turn under.
• In the spring, as soon as the ground dries enough for tilling or plowing, turn the cover crop under. Alternatively, mow or cut the crop with a scythe and leave the cuttings in place without incorporating them into the soil. The latter reduces soil disturbances and works well in climates where the soil stays wet well into the growing season.
• Do not allow a cover crop to go to seed. If a cover crop reseeds, it will become a weed in subsequent seasons.
• After mowing and digging in a cover crop, wait three to four weeks to plant. Freshly turned cover crops often utilize soil nitrogen to begin their decomposition process, causing a temporary unavailability of this nutrient.
Cover crop seeds are often unavailable late in the growing season (the time most gardeners typically plant them). If you hope to plant a cover crop in late summer, plan ahead and purchase the seed well before you need to sow it.
Jeanette Stehr-Green and Audreen Williams are WSU-certified Clallam County Master Gardeners.