What is a cover crop and why might you consider growing one? Cover crops provide a way to add nutrients into the soil while also controlling weeds. Improving soil health is one of the best ways to improve plant growth and production as regular planting depletes soil of essential nutrients. Farmers frequently use cover crops, but many people don’t realize that they can enhance home gardens, too. They can be considered a living mulch because of their thick growth. Other benefits of growing cover crops include reducing erosion, maintaining soil structure, and managing weeds. As a ‘green manure’, cover crops serve to provide organic matter and nutrients back into the soil. Cover crops can support a wide range of soil microorganisms and importantly, given the drought conditions of the past few years, they help the soil retain water.

Cover crops can be legumes such as vetch, clover, beans, and peas; grasses/cereals such as annual ryegrass, oats, rapeseed, winter wheat, and winter rye; and buckwheat. Selection will depend upon when you are planting your cover crop, where the cover crop will be planted in your garden, and your purpose in planting, e.g., add nitrogen, suppress weeds. The UMN Extension website provides numerous resources to help you make the best selection for your garden (see references at the end of this article).  There is also a comparison guide of different cover crop options and information on seeding rates, (Comparisons and Planting Rates). Figure 1 below provides a snapshot of the table that can help you make the best selection, with planting times.

Figure 1: Planting timing and seeding rate

When selecting which cover crop to grow, you also may want to consider a few key factors. If you choose a cover crop that dies over the winter, it will be easy to work into the soil in the spring. If you select a plant that will survive Minnesota’s winters, you will need to have a way to kill the plant and then work it into the soil. You will also want to carefully consider the various benefits to growing a cover crop and determine which ones are of greatest importance. Some cover crops provide pollinators with food, help prevent weeds from growing, prevent erosion, or add nitrogen.

Figure 2: Selecting cover crops based on effectiveness of providing various benefits

When to plant cover crops is the next major decision. In Minnesota, our growing areas often sit empty after the harvest has occurred. Cover crops can be sown in early spring as a cool season crop in an area where you might plant tomatoes or pumpkins, which need warmer soil and temperatures to grow. If you grow cool season vegetables like lettuce or spinach, you could grow a cover crop during the summer months when it is too hot for these plants. Cover crops can also be grown in late summer after harvesting early vegetables that might be done by August. Late Fall before freezes occur is another option, once all the vegetables beds have been emptied. Figure 3, from the University of Minnesota Extension (Cover crop selection for vegetable growers), outlines these plant timing options, accompanied by illustrations of vegetables that might be grown before or after planting a cover crop. Please note the timing in the graph is approximate.

You can find seeds for cover crops at garden stores, farm stores, and vegetable seed catalogs. To derive the full benefit, you will usually want to plant your cover crops densely, unlike the usual vegetable garden practices of spacing seeds. You will want to fertilize and water your cover crop to encourage a thick growth of biomass. You then work this back into the soil, adding organic matter.

References

Cover Crops and green manures in home gardens https://extension.umn.edu/managing-soil-and-nutrients/cover-crops-and-green-manures

Cover crop selection for vegetable growers https://extension.umn.edu/cover-crops-and-soil-health/cover-crop-selection-vegetable-growers

Cover crops improve soil health, even on a small scale https://extension.umn.edu/yard-and-garden-news/cover-crops-improve-soil-health-even-small-scale

Cover crop comparisons and planting rates https://extension.umn.edu/cover-crops/cover-crop-options#cover-crop-guides-and-resources-2174260

PHoto credits: University of Minnesota Extension (all)