Healthy, living soil is a vital component of an organic garden. Soil covered with growing things draws carbon out of the atmosphere and stores it in roots, stems, and leaves. In sustainable, earth-friendly gardening /farming we do not allow land to fallow, but keep our soil covered with actively growing or dormant plants all year. This is a win-win situation for your garden; you can have abundant crops yet at the same time help reduce the amount of CO2 in earth’s atmosphere.
Cover crops build and protect your soil by increasing humus and nutrients. Cover crops will add organic matter, smother weeds, prevent erosion, break up compacted soil, provide livestock forage and attract beneficial insects. In dry summers or dry climates cover crops provide a kind of living mulch, preserving soil moisture.
The timing of planting and turning under of cover crops is varied, depending on climate, soil requirements, and time of year. Here in the north (Western Montana) our growing season is short so we have to be precise in our timing of planting, cutting and turning under cover crops. After vegetables are harvested in the fall a cover crop can be planted immediately. First, remove vegetable debris; second, till or rake and prepare a six-inch deep seedbed; third, irrigate to moisten soil; and finally, plant your cover crop. Fall planted cover crops might include: hairy vetch, field peas, crimson clover, summer alfalfa, Mammoth red clover, annual rye grass, winter cereal rye, winter barley, winter wheat, winter oats ,or winter triticale. Peas or hairy vetch planted together with one or another of the fall/winter cereal grains will add nitrogen and organic matter to the soil. Oats and peas planted together or hairy vetch and winter rye together are quite effective combinations for fall cover crops. The grain will protect the legumes somewhat from winter damage, though peas will probably winter kill. Hairy vetch will vigorously regrow in the spring and can be mown and turned under. Any cover crop of the grass family, such as cereal grain or annual ryegrass needs to be turned under at least three weeks before a vegetable crop is planted. Some cereal grasses are allelopathic, which means they release growth inhibiting chemicals that prevent seed germination of other species. It takes three or more weeks for the chemicals to dissipate. Since our spring is often late, and short, and sometimes wet, the timing of turning under your cover crop is very important. In a home garden, if you plan to plant some cool-weather crops early, such as onion sets, cole crops, or spinach, you can pull up or chop out a (non-grass family) cover crop in early spring just before you plant. It is easy to hoe out hairy vetch by cutting it off below the crown. You can then prepare the soil and plant immediately.
Several cover crops can be planted in spring if you have about four to six weeks’ time before you plan to plant a crop. An example might be if you plan to put in a warm-season crop such as peppers or tomatoes. A cool-weather cover crop could be put in as soon as the soil could be worked (sometimes as early as April 10 or as late as May 5). Some useful, early spring cover crops include: crimson clover, medium red clover, Mammoth red clover, white clover, or field peas. If you have four to six weeks growing time plus three weeks’ time after taking down a cereal grass before you plant you could put in spring oats and field peas, or annual ryegrass. The plot would be ready to plant your vegetable crop by mid to late June in our Montana climate. This would limit your choices to fall-maturing cool-weather crops such as cauliflower, Brussels sprouts, lettuce, carrots, etc.
If you plan to have a cover crop during the warm season, buckwheat , mustard, turnips or phacelia all work well in our climate. Remember to cut mustard or buckwheat when in flower so it will not reseed. Phacelia will reseed also, but has a long blooming season and is a great pollinator attractant. Turnips are biennial, so will not reseed the first year.
In a home garden cover crops need to be considered in crop rotation. It is best not to plant a cover crop of the Brassica family (Cole crops, such as turnips, mustard, radish, cabbage, etc.) where cole crops are to be planted the following season. The rule of thumb is to rotate crops so that members of the same plant family are not grown in the same spot for three years or more. In our certified organic market garden we use a nine year crop rotation plan. A more practical home garden rotation plan might be only four-years.