Plant a Cover Crop - October 2, 2002
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

October is a good time to start your fall garden cleanup and plant a fall/winter cover crop. Cover crops are planted in vacant space and worked into the soil after they grow instead of being eaten. They provide many benefits to the vegetable garden: increased soil organic matter and essential nutrients, protection from erosion, reduced compaction, and improved tilth. Cover crops also capture leftover fertilizer and out compete cool season annual weeds.

Cover crops fall into two categories: legumes and grasses. Legumes include peas, beans, clover, alfalfa, vetch, trefoil, and medic. Legumes can be annuals, biennials, or perennials and have the added advantage of being able to fix nitrogen (converting nitrogen in the atmosphere to plant available nitrogen in the soil). Some legumes can fix as much as 200 lbs of nitrogen per acre per year. To ensure an adequate level of nitrogen fixation, legumes must be inoculated with the appropriate strain of Rhizobacteria. These inoculants are inexpensive and the process is simple.

Annual legumes are ideal for short-term plantings in fall or early spring. Biennials grow much more vigorously in the second year and are best planted on areas that will not be cultivated for one year. Perennial legumes, such as alfalfa, grow deep taproots that are very good at improving compacted soil conditions but can be difficult to eliminate from the garden once they have served their purpose.

Grasses do not fix nitrogen, but have a fine textured, fibrous root system that is efficient at stabilizing soils and is easily decomposed to add organic matter to the soil. Some common annual grass cover crops are barley, oats, rye, and wheat. Perennial grasses are usually used in orchards, vineyards, and other areas where tillage is unlikely to occur. These include fescues, orchardgrass, bluegrass, ryegrass.

Many varieties of cover crop seeds may be purchased from organic gardening suppliers through catalogs or the Internet. A simple mix of annual grass (rye or barley) and clover is a great place to get started with cover crops. Annual grasses germinate quickly and act as a nurse crop for the legumes. As time goes on, the grasses provide a scaffold for the legumes to grow upward and spread. Plant about one ounce of annual grass seed mixed with one half ounce of clover seed per 100 square feet. Inoculate clover seed (and other legumes) or purchase freshly inoculated seed. Broadcast onto raked soil, and then cover the seed to a depth of 2-3 times the width of the seed and firm the soil with a rake. Irrigate 1-3 times after planting to establish the cover before cold weather sets in.

Before the cover crop has started to form seed, it should be tilled into the soil as green manure. Green manure adds organic matter, nitrogen and other nutrients that were contained in the leaves, stems and roots of the cover crops. The green manure should be allowed to decompose for at least three weeks before planting. Legumes add nitrogen and decompose easily while grass roots add easily decomposed organic matter. The grass leaves are less easily decomposed and will contribute organic matter over a longer period.

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 or E-mail us at 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:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: September 27, 2002
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