In hearing talk about cover crops, what are the benefits?

Cover or green manure crops are usually grown when the garden soil is idle but are also sometimes planted between rows of fruits or vegetables to serve as a living mulch.

Cover crops are sometimes called "catch crops." Their deep roots absorb nutrients from the soil that could otherwise leach away or are unavailable to garden crops with shorter roots. When tilled under, cover crops decompose and release those "caught" nutrients.

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, produce a large amount of organic matter to be tilled under, and have deep roots that loosen compacted soils, thereby improving drainage and aeration.

Cover crops are divided into two categories: warm-season and cool-season, based on the optimum times to plant and grow.

Warm-season types will not tolerate freezing temperatures and should be planted after all danger of frost. Most take six to eight weeks (or longer) to grow large enough to turn under. An exception is buckwheat, which may need only four weeks under good growing conditions.

Cool-season cover crops will survive through the winter. They are planted in the fall, from mid-September until the end of October, and left over the winter to provide protection from soil erosion. They need to be 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.

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 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.

Some cover crops, just like any other crop, may attract insects that could harm other garden crops. Gardeners should watch for pest insects in cover crops and other crops and be ready to use various IPM or best management methods while the pest problem is in its early stages.

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.

Cover Crop Planting Guidelines

  *   Prepare the soil as you would as if planting vegetables. Legumes will produce the nitrogen they need, but non-legume crops will need to have nitrogen fertilizer (1 to 1-1/2 pounds of actual nitrogen per 1000 square feet) added to the soil to produce maximum yields of organic matter.

  *   Inoculate legume seeds by moistening them, draining the excess water, adding the inoculant powder, and mixing well.

  *   Broadcast the seed evenly.

  *   Cover seed with a thin layer of soil by raking it in or going over the area with a rototiller set very shallow.

  *   Keep the area moist until seedlings emerge. Light watering may be needed twice a day, or more, in hot weather.

  *   Mow and harvest cover crops before they flower and produce seeds, and till under at least ten days to two weeks before planting garden crops.

For more information about these species and the warm-season species see OSU fact sheet HLA-6436<> Earth Kind Gardening Series: Healthy Garden Soils.   This article can be found on our web page, along with a list of cover crop recommendations:

If you would like more information on cover crops, a Soil Health - Cool Season Cover Crop Field Day is scheduled for Dec. 6th.  More details will be on our web page when available.