Cover crops are plants grown for the direct benefit of the soil rather than human consumption. They were known in ancient China and India and have been standard practice in many agricultural regions of the world, including colonial America. This changed in the 1940s when our WWII infrastructure was redirected toward the production of fertilizers, pesticides, and other chemicals that replaced traditional land stewardship practices.
In its own way, the “Green Revolution” cushioned humanity from nature’s attempts to sabotage our tomato beds, and it certainly increased global food production. Unfortunately, the environmental and human health costs begged for better solutions. Before long, we saw the return of traditional (but improved) cultural practices such as cover cropping, which offers better living through biology.
Building soil health and fertility are the main reasons for using cover crops, but the benefits don’t stop there. The soil’s many organisms play a major role in plant vitality, and they love cover crops, which they use as food. This recycles important nutrients, particularly nitrogen, back into the soil for future plant use. Over time, the increased biological activity improves soil’s structure, and water and nutrient holding abilities.
Creating balance is another key to healthy soil, which cover crops achieve in several ways. They break up the cycle of pests and diseases that accumulate when plants within the same family (e.g. tomatoes and eggplant or cucumbers and melons) are grown too many times in a row. The habitat cover crops provide is beneficial to a diversity of creatures, preventing any one species from becoming a pest. You even get a natural weed block, as many weed seeds cannot germinate through the thick mat of cover crops.
Selecting your cover crop takes some research, but it’s usually best to choose a mix to counterbalance drawbacks of any given species. At minimum, you should plant legumes (e.g. fava bean, cowpeas) for a quick release of Nitrogen, and grasses (e.g. winter rye) to decompose more slowly, providing a longer-lasting mulch and slower Nitrogen release. Other cover crops to consider are oilseed radishes whose taproots break up heavy, compacted soil, and buckwheat, which establishes quickly, providing rapid weed suppression.
Using cover crops takes some planning, as you will be growing them in place of food crops. They are an excellent alternative to letting soil lie fallow over winter when rains can leach nutrients out of the root zone or cause erosion. Select a site in full sun and sow seeds in September or October. Water until the rains arrive, and then only as needed during winter dry spells. They won’t need any other maintenance until March, when you cut them down in preparation for your food crops. The exact timing depends on how quickly they begin to set flower, which is when you want to kill them.
Tradition calls for tilling cover crop biomass into the soil. For soil microbes, this is like taking a bulldozer to your house and hosting a dinner party the same day: possible, but not conducive to comfort. Bacteria, fungus, worms, and many other organisms feeding on organic matter exist in different strata of the soil, depending on what stage of decomposition their food needs to be in. Some organisms, especially bacteria, are immobile, so disrupting the soil layers moves many to where they won’t be able to access the food they need. If you feel more comfortable incorporating the cover crops into your soil, organisms will eventually repopulate, but try to disturb them as little as possible.
Four to six weeks before planting your food crop, kill the cover crop with a lawn mower, weed whacker, or string trimmer, cutting as close to the ground as possible. Spread the biomass evenly on the ground as a mulch. At planting time, use a mattock to create planting strips. Food crops started from large seeds or transplants work best.
Other benefits of the “no-till” method include keeping weed seeds buried, and saving you a lot of work. As long as you have legumes, the temporary nitrogen deficiency mentioned above should not be an issue. Selecting annual grasses rather than perennials will help ensure they won’t stick around when it comes time to plant your food crops. Finally, be sure to kill cover crops before they set seeds to maximize nutrient recycling and prevent weed problems.
See the Sacramento County Master Gardener website at <sacmg.ucanr.edu/covercrops> for information on traditional cover crop methods. If you have a gardening related question you can contact the UC Master Gardeners at 209-953-6112. More information can be found on our website.