Time to Think of Cover Crops

No one wants to think of harvest’s end as the vegetable garden reaches peak, but now’s the time to plan for cover crops to be planted in fall. 

Cover crops, also called green manure, include grains like winter oats and cereal rye. Legumes, such as commonly used crimson clover, Austrian field pea and common vetch, are nitrogen “fixers.” Beneficial bacteria in legume root nodules take nitrogen from the air and supply it to the plant. When the cover crop decomposes, some of the nitrogen becomes available to other plants. 

If you’re not acquainted with cover crops, here’s the rundown: These hardworking plants can add organic matter and aerate the soil, protect it from compaction caused by rain, suppress weeds and reduce erosion, according to Nick Andrews, organic vegetable advisor for Oregon State University Extension Service. As a bonus, their blooms provide nectar and pollen for pollinators and other beneficial insects. 

Not a bad deal for an almost no-maintenance plant. All that’s needed is to seed it in, water a couple of times until the rains start, leave it through winter and dig or till it in spring. However, timing is key. Plant seeds of overwintering cover crops by September or early October so they get established before the weather turns cold and wet. 

Make sure when you plant that the seed has good contact with the soil. Larger seeds like peas, vetch and cereals should be raked in lightly. Mix small seeds with sand to make them easier to broadcast and then use a sprinkler to water in. If the weather is still dry, keep the area irrigated. 

It’s also important that plants are cut or mown down in spring before they set seed. In our climate, most gardeners incorporate cover crop residue to promote decomposition. Do this about four weeks before planting vegetables so the crop decomposes well, otherwise it can promote some soil-borne diseases and attract some insect pests. If you don’t have four weeks for the cover crop to decompose, you can remove the stems and leaves and apply them somewhere else as a mulch or compost them. For vegetables you’re harvesting after early October, consider inter-seeding during the summer. 

“It all depends on when you need to get your vegetables in,” Andrews said. “It’s best to let cover crops decompose before you plant, especially for vegetables that you start from seed.” 

When it comes time to incorporate the cover crop, till shorter plants right into the soil. If the plant is too tall to turn under easily, mow first or use a weed trimmer. Tough-stemmed plants can be cut and left to decompose above ground. Or you can put the tops in the compost pile and dig in the roots. No matter the method, let the turned-under material sit until it’s time to plant. 

For beginners, Andrews advises: 

  • Start with a cover crop that is easy to grow and manage. For example, crimson clover and phacelia are relatively easy to incorporate into the soil. 
  • Be sure to prepare your field well and have sprinklers available if the weather is dry. It’s a good idea to water the soil a bit before preparing the seedbed if the soil is very dry. 
  • The first time you try cover crops, plant them in an area of your garden that you can leave for vegetables typically planted in late spring or early summer. This will buy you time to learn how to manage the cover crop residues in spring. 
  • After you have successfully used one cover crop, try another in a different area of your garden. Then when you gain experience, experiment with mixtures, reduced tillage and other innovative practices. 
  • Consider inter-seeding cover crops during the summer into late-harvested crops like sweet corn, winter squash and tomatoes. 

For more information, refer to these publications Cover Crops for Home Gardeners East of the Cascades and West of the Cascades; and Methods for Successful Cover Crop Management for Gardeners, publications by Washington State University that Andrews collaborated on. 

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