Cover Crops

By Pat Hitchcock, U. C. Master Gardener

In October I planted a portion of my vegetable patch to Asian greens, lettuce, broccoli and other cool-season edibles that will do well during our wet and relatively cool winter weather. When I stood back to admire my efforts, I couldn't help but notice the fading tomato plants and the bare rows where the melons grew during the summer.

In fact, my winter vegetables take up a fraction of the space that spring and summer crops utilize. Most winters, the unused space produces some outstanding weeds which are setting nice crops of seeds by the time the soil dries out enough in spring for turning. This year, however, we're giving cover crops a try.

Farmers have used cover crops for centuries to improve the soil. In home gardens, they can be equally useful in restoring the organic matter that's gradually depleted as we cultivate and harvest. Along with mulches and amendments such as compost, cover crops can improve the overall productivity of your garden soil.

They also provide nutrients. When cover crops are growing, plant roots go down into the soil and take up nutrients necessary for plant growth and development. When the cover crop is later turned into the soil and begins decomposing, the nutrients are in a form that is easily available to the next plant grown there. Legume cover crops are known for capturing atmospheric nitrogen, and all plant roots bring up phosphorus, potassium and micronutrients from the soil.

Cover crops also reduce soil erosion by slowing water flow across the soil surface, allowing better water infiltration and less runoff. A vigorously growing cover crop can compete successfully with the weeds. Cover crops also capture leftover fertilizer, preventing it from being leached away during rainy weather.

Although you can grow cover crops whenever you have bare ground, most of us have idle vegetable patches in winter. Among the winter cover crops used for years by farmers are vetches (Vicia genus), of which fava beans are one well-known example. Fava beans are legumes that fix nitrogen from the air and grow well once the rainy season has started. The best time to plant is early November.

There are many varieties of favas, but the ones used for cover crops are small-seeded types that should be planted one to three inches deep in rows thirty inches apart, with six inches between plants.

Vetches with smaller seeds should be planted more shallowly, one to two inches deep. All legume seed should be inoculated with the appropriate nitrogen-fixing bacteria before planting; ask for the inoculant when you buy seed. Other possible leguminous cover crops include several varieties of clover. Inquire about varieties from your seed source.

Also consider non-legume cover crops such as mustard, rye or barley. Some farmers sow mixed cover crops—rye with vetch, for example—to try to capitalize on the benefits of each plant.

Once you have planted the cover crop, winter rains should keep the plants going, but if we get a dry spell, you may need to irrigate. Let the cover crop grow as late in the spring as you can, but dig it in three to four weeks before you want to plant in that spot again. You may need to mow the cover crop first. Fava beans are typically turned under when they start blooming.

Favas bean seeds are available at local nurseries, and farm supply stores carry a wide variety of cover crop seeds. Plant now to have a crop that will be steadily improving your soil all winter while you're curled up with the seed catalogs.