If you hear gardeners talk about green manure, you may wonder just what type of animal produces green manure?
It is not produced by an animal. The gardeners are talking about cover crops. These are crops planted and tilled back into the soil, primarily for the purpose of adding organic matter, but they also provide additional benefits. These other benefits include the reduction of soil erosion from wind and water, relief of soil compaction by crop roots, discouragement of weeds by shading, and decreased nutrient loss over the winter by temporarily tying up of nutrients.
There are different types of cover crops, including grains, grasses, legumes and broadleaf crops, each with different benefits and management techniques
I do not know many vegetable gardeners who plant cover crops, but they should consider it. Late summer through early fall is a good time for gardeners to seed a cover crop. This is early enough to give the plants time to grow before cold weather begins and while irrigation water is still available.
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There are different types of cover crops, including grains, grasses, legumes and broadleaf crops, each with different benefits and management techniques. For home gardeners, an important consideration is how easily it can be worked into the soil the following spring.
Gardeners will want to pick a crop that grows quickly and can be easily worked into the soil. Some cover crops may get too tall to easily work into the soil without cutting them first. Depending on the crop, gardeners can use a rotary mower (without the mulching attachment), a string trimmer, scythe or grass whip for cutting the crop down.
Late summer through early fall is a good time for gardeners to seed a cover crop.
Cover crops can be planted between rows or plants, so you don’t have remove still-producing plants. If you have areas that are finished producing, remove the plants and seed the entire area.
Leguminous cover crops (vetch, clover, beans, peas) are desirable because they add nitrogen and organic matter to the soil. However, legumes work in association with a special soil bacteria called Rhizobia that take or “fix” nitrogen that is in the air. If you select a leguminous cover crop, you will need to inoculate the seed with the right Rhizobium species for that crop, or buy seed that is pre-inoculated.
When deciding what cover crop to grow, consider the advantages and disadvantages of each. For more information on selection, consult the Washington State University Extension fact sheet at bit.ly/cover_crop. In addition, a good resource on the use and management of cover crops is at bit.ly/manage_covercrop.
Gardeners will want to pick a crop that grows quickly and can be easily worked into the soil.
Once you decide what cover crop to plant, ask your local garden or feed stores if they carry the seed. If not, there are online sources, including Territorial Seed Company (territorialseed.com), High Mowing Organic Seeds (highmowingseeds.com) and Peaceful Valley Farm & Garden Supply (groworganic.com).
Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.