Fall Cover Crops

Home Grown  by Keith Reed
Cover, or green manure, crops are usually grown when the garden soil is idle. They can also be planted between rows of fruits or vegetables to serve as living mulch. Cover crops are also called “catch crops” because their deep roots absorb nutrients from the soil that could otherwise leach away or be unavailable to garden crops with shorter roots. When tilled under, cover crops decompose and release those “caught” nutrients.

There are two main types of cover crops, warm season and cool season. Early September through the end of this month is the time to plant the cool season crop. The idea is they are planted early enough so their roots develop before winter but late enough so they do not complete their growing cycle (and die) before the weather gets cold.

Some cover crops (those from the legume family) even trap and transform atmospheric nitrogen in their roots. This nitrogen serves as a fertilizer source for future crops. Cover crops in the grass or grain family don’t actively fix nitrogen, but usually create a thick mulch and produce a large amount of organic matter to be tilled under. They also have deep roots that loosen compacted soils, thereby improving drainage and aeration.

Legumes need certain strains of bacteria to enable them to convert nitrogen gas from the air into a form that plants can use. The bacteria needed by various kinds of legumes may or may not already be in your garden soil. To be certain, legume seeds should be coated with an inoculant powder that contains living Rhizobium spores. Commercial inoculant is usually inexpensive and widely available. Some legume seeds are sold pretreated with the proper bacteria

Because they are used in rotation with other crops in the same garden location, cover crops can help suppress harmful soil nematodes. Nematodes, which are microscopic worm-like parasites, tend to be host-specific, attacking just one crop or crop family. They do not “like the taste” of other plant families and their numbers will decline without the preferred food source.

Broadcast the seed evenly when planting. Two to four times the rate may be used to assure a good stand. Cover the seed with a thin layer of soil by raking over it. Keep the area moist until the seedlings emerge. Light watering may be needed twice a day, or more, in hot weather. Mow and harvest the cover crops before they flower and produce seeds. Till under at least ten days to two weeks before planting garden crops.

For a recommended list of cover crop species and their seeding rates, see
OSU Fact Sheet HLA-6436 Healthy Garden Soils.

For more information on this or any other horticultural topic, you can contact Keith Reed, the Horticulturist in the Payne County Extension office. Keith can be reached via email at
keith.reed@okstate.edu, phone at 405-747-8320, or in person at the Payne County Extension office, located at 315 W. 6th in Stillwater.

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