There are different reasons for the home gardener to grow cover crops in their plots. These plants, also referred to as green manures, can add a lot of nitrogen to the soil. Nodes on the roots of certain plants help pull nitrogen from the air and make it into a form a plant can use. Nitrogen helps stimulate leafy growth, so growing a cover crop in winter will give a nutrient boost to spring greens like spinach, kale and lettuce. Different nitrogen fixing green manures like alfalfa, clove, hairy vetch, and winter rye grow quickly and then break down, providing nutrients for the soil.
Cover crops also have other uses, such as protecting the soil from blowing away in the winter. A closely planted ground cover will also choke out weeds. Plants with deep roots, like alfalfa, can break up impacted soil. They can be sown at any time that the ground needs refurbishing. If preparing for a main crop in early spring, “dormant seed” the area, which is to spread the seed in winter. As soon as the temperature warms up, the seed will germinate. For a wider range of benefits, different types of plants can be mixed together in the same plot, such as with hairy vetch and winter rye. Consider one of the following green manures the next time the garden soil is spent.
Alfalfa
Alfalfa has deep roots so it can work to break up hard soil. The roots can reach to almost 15 feet deep. Annual varieties will grow as long as the weather is warm enough for it. Besides producing nitrogen, alfalfa is also a source of protein and calcium. Perennial types go dormant in the fall and mowing it down in the fall can damage it. Otherwise, the clippings make an excellent addition to the compost pile.
Clover
Clover also makes a good lawn replacement. It also can germinate in the shade and works well on compacted soil. It needs less moisture than some other cover crops. If the vegetable garden is still underway clover can be over-seeded in the bed and grow as the vegetables wane. As a winter cover crop it is best planted in September. Both red and white varieties are available, but some gardeners report better results from the red.
Hairy Vetch
Vetch is best planted in the fall and turned under in spring. It is called “hairy” because it has fuzzy vine-like leaves. It also bears small blue flowers. Hairy vetch has a shallow root system and is known for slow, early growth. This plant overwinters well and can adapt to the cold. It does best in well drained or sandy soil. Don’t allow it to grow too late in the spring or it can rob the soil of moisture that the next crop will need. Also, don’t mow it back too close unless it is time to kill it off or it will damage it so much that it won’t come back.
Winter Rye
Winter rye is quick growing it is easy to get a quick nitrogen boost in the garden before spring fully arrives. It has a large root system will help break up soil. Winter rye is the most cold tolerant of the nitrogen fixing cover crops and can germinate as long as the temperature is above freezing. It is good for rejuvenating spent beds because it can grow in poor soil that does not tolerate much.
Removing the Cover Crop
Once the cover crop is no longer needed, kill it off before using the bed again. Don’t let the crop go to seed or there may be problems in the future. Mow the taller crops, like alfalfa, and put the excess clippings on the compost pile. Then, turn the soil over so the remains of the plant break up and rot, fertilizing the soil. If the cover crop is low growing, like clover, simply turn under the whole thing. It is best to wait two or three weeks before planting the bed but planting immediately generally won’t hurt anything.
Resources
UC SAREP Online Cover Crop Database
The Microbial World: The Nitrogen Cycle and Nitrogen Fixation by Jim Deacon
