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Become a Smarter Gardener in 2019: Plant a cover crop over your vegetable garden!

A fall-seeded rye/red clover cover crop 
mixture in May, shown after the winter. 
Photo: Annie Klodd.
As the weather gradually grows colder in Minnesota,  you may find that some of your early summer garden plots are ready to be cleaned up and put to bed for the winter. But you can give your garden one last boost before you say "goodbye" 'til spring.

Did you know...this could be a perfect time to plant something called a "fall-seeded cover crop"?!

What's a cover crop?

These arer specific plant or crop varieties that are grown and turned into the soil to improve its overall quality. Farmers often plant "cover crops" in a field in the fall (off-season).  For home gardeners, they're started by seed in an empty garden bed like a vegetable garden plot or a new garden bed.

Sometimes these plants covering your garden are killed by frost, and some last the whole winter and remain until the snow melts away in the spring. So why do it?

Reasons to add a 'living mulch'

Think of it as a "living mulch." Basically, you're planting your garden in the fall to enrich and protect the soil. Here are some of the specific benefits:

  • Adds soil organic matter, which supports healthy plant growth.
  • Suppresses weeds 
  • Protects soil from erosion
  • Soaks up excess nutrients in the soil
  • Prevents nutrients from leaching away, among other things. 
  • Legume cover crops like field peas and clovers add nitrogen to the soil, too.

For some gardens, beds that were planted early in the season lie bare over the winter. That makes them more vulnerable to erosion and your garden can lose nutrients. Some gardeners mulch heavily in the fall with straw to prevent these issues but that can be a lot of work and straw mulch is not always available.

Again, to sum it up: planting a cover crop protects the soil and nutrients in it, might give you a head start on improving your soil for the next season, suppresses weeds, and those living roots in the soil may support beneficial soil organisms that help plants grow.

How to plant a cover crop

The goal is to plant a cover crop in the fall in time to cover the soil before the temperatures drop and snow covers your garden. There are two main types of fall cover crops: winter-killed and over-wintered.

Winter-killed cover crops

Winter-killed cover crops, as the name suggests, are cover crops that are not winter-hardy, and will die off in the late fall. Common species mixtures for winter-killed cover crops include:

A common winter-killed mixture is oats and field peas.

Overwintered cover crops

Overwintered cover crops consist of winter-hardy species that will lay dormant over the winter and continue growing in the spring. The most common overwintering mixture is:

  • Rye and vetch or field peas, if you have a relatively protected area. This can reliably survive the Minnesota winter. 
  • Certain clovers, like red clover, can overwinter in Minnesota, but need to be planted in late summer in order for them to establish before winter.

Several cover crop species growing in a high tunnel 
at the HAFA farm in Farmington, MN. 
Photo: Annie Klodd

How to choose a crop 

The process for managing your cover crop will depend on whether you choose an overwintering cover crop or a winter-killed cover crop. This in turn depends on what you want to use the bed for next year, and what sort of equipment you have for killing the cover crop before you plant your garden in the spring.

Winter hardy cover crop: If you plant a winter-hardy cover crop, here's the trick: you have to kill it before you plant your summer vegetable crops. It is critical that you plan ahead for how to kill it! The most common methods for gardeners include mowing them and tilling them under.

If you have a small enough bed, you may be able to chop down large grasses like oats or rye by hand, but otherwise, stick to field peas or another low-growing species that can be mowed.

You may also want to plan ahead for how to use the "biomass" (stems, leaves, flowers) from the cover crop. For example, maybe the dead cover crop material can serve as a mulch for a no-till bed.

Winter-killed cover crop: If you want to use the garden bed early next spring, you should plant something that will die from the cold and begin to decompose over the winter. Otherwise, your cover crop will be competing with your early season vegetables.

Winter-killed species are also a good choice because they don’t require special tools for killing them since they die on their own over the winter.

Ready to get some seed? 

Choose a species or mixture and calculate quantities to buy using this cover crop chart. Estimated costs are provided per acre and per 1000 square feet. Many seed companies which sell garden vegetable seeds also sell cover crop seed in garden-sized quantities.

Resources for more information

For more information, go to our web page:
Cover crops and green manures in home gardens

NC-SARE has many resources on specific cover crop species and mixes, and suggestions for planting rates, dates, and termination. In particular, Managing Cover Crops Profitably is an invaluable resource for the novice or expert cover crop user, and is available via a free online version.

Authors: Vivian Wauters, UMN Horticulture Graduate Student 

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