   Plant a Cover Crop - October 12, 2005 Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County October is prime time for planting a cool season cover crop in your garden. Cool season cover crops are often annual plants grown to add organic matter and nutrients to the soil while protecting it from wind and water erosion. Cool season cover crops grow slowly during the cold months of winter, but growth accelerates as days lengthen and temperatures climb during spring. Most cover crops are not allowed to mature and then tilled into the soil as “green manure” prior to summer planting. Once incorporated, the cover crop decomposes in three to four weeks releasing the organic matter and nutrients over the summer growing season. Cover crop plants can be a single species or a combination of species suited to your climate and gardening objectives. Legumes are almost always used as green manure cover crops because of their ability to convert unavailable nitrogen in the atmosphere into plant-available nitrogen in the soil. Legumes are plants such as alfalfa, peas, beans, clover, vetch, and their relatives (including mesquite and palo verde trees). Many gardeners are aware of this phenomenon, but for those that aren’t, the process is called nitrogen fixation and carried out by bacteria (Rhizobium) that live in the roots of legumes. This is a symbiotic relationship where the legume receives nitrogen from the bacteria and the bacteria receive sugars from the plant. Quite often, cover crops also include grasses to increase soil organic matter. Grasses have fibrous root systems that utilize nitrogen released by the legumes and decompose readily to contribute organic matter. When tilled in, the tops of the grass plants also contribute nutrients and organic matter. Legume cover crops can add up to 300 lbs per acre of nitrogen. Some cool season annual legumes suitable for cover crops in north central Arizona include common vetch, hairy vetch, sweet clover, red clover, medic, and fava beans. Alfalfa and other perennial legumes are sometimes used as cover crops. These plants penetrate more deeply in the soil, but may become persistent and difficult to control. All legume seeds should be inoculated with the proper strain of Rhizobium bacteria to ensure successful nitrogen fixation. This information is available in seed catalogs that sell cover crop seeds. Before planting, inoculate your seeds with the appropriate Rhizobium strain for hairy vetch (inoculant is usually available in catalogs that sell cover crop seeds). Make sure you buy fresh inoculant each year. Studies have shown this to improve nitrogen fixation. The Rhizobium inoculant is packaged in a mixture containing finely ground peat. To inoculate, place the seeds in a bowl with just enough milk to barely moisten the seeds then add the entire package of inoculant. Stir and sow immediately. After broadcasting, cover the seeds with soil as soon as possible using a garden rake. A good rule of thumb is to try and cover the seeds with soil to a depth three times the diameter of the seed. Last October, I sowed a mixture of 85% hairy vetch and 15% cereal rye in my garden. By early May, the rye was five feet tall and the vetch was trailing on the ground and trying to climb up the rye. I had to chop down the rye before incorporating it into the soil. During the growing season, pull up a legume every now and then to observe the nodulation of the roots by the Rhizobium. Pink nodules on the roots indicate colonization by Rhizobium and effective nitrogen fixation. Cover crops can also be grown to meet other objectives. Mustards are planted to decrease root knot nematode populations. Root knot nematodes are plant parasitic, microscopic roundworms that can severely stunt plants. Borders can also be planted with plants that attract and feed beneficial insects such as predatory mites and wasps, lady beetles, lacewings, syrphids, tachnids, and predaceous beetles. Some of the species in these seed mixes include: clovers, gypsophila, buckwheat, alyssum, nasturtium, yarrow, carrot, dill, daikon, celery, radish, fennel, caraway, chervil, and coriander. Cover cropping is sustainable and economical. It can be done in the warm or cool season and, best of all, its fun to watch these plants grow then do their magic. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |