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Using Cover Crops: Species Selection for Gardens

Hairy Vetch


During my time at Iowa State University, a professor of mine told me that agriculture is “the management of photosynthesis”. This quote holds true to gardening, and like any successful manager worth their salt, “hiring” the right cover crop for the job is key.

Even in Minnesota, there are quite a few cover crop species for you to choose, each with their own pros and cons. To narrow your possible choices, it is incredibly important to know the needs of your garden. I have condensed cover crop uses broadly in this paper—there will be a few left out— but your specific need will likely fall into these four categories.

Nitrogen Adders/“Green Manures”

As I mentioned in the previous article, “A History of Green Manure”, adding nitrogen through cover crops was likely their earliest use. Legumes such as hairy vetch (Vicia villosa) and field peas (Pisum sativum) are prime examples of cover crops than can work in a garden rotation in Minnesota. As they fix a high percentage of their nitrogen from their bacterial friends (rhizobia), these crops can cut your fertilizer requirements for many of your high feeders such as corn and tomatoes.

To get a rough estimate for exactly how much nitrogen they would pump, cut a square foot of your green manure cover crop and let it dry in the sun for a few days. After it dries, weigh the material and find out how many pounds it is, now you have lb. / sq. ft.

Once you have a weight per square foot, you can then find a rough estimate of how much nitrogen you added by using an equation. Most legumes have between 3% and 4% nitrogen in their leaves and stems.

Total nitrogen in cover crop (lb. N / 1000 sq. ft.) = Cover crop weight (lb. / sq. ft.) x percent N of cover crop (%) ÷ 100 x 1000 sq. ft.

A few caveats for their use, however. Be sure to terminate these plants right around or slightly before flowering to get the maximum amount of available N for your crop. Next month we will get into more detail about when you should do this step. Also, make sure to add inoculum (the rhizobia unique to the plant) if possible to ensure these plants have all they need to start fixing nitrogen. Many times major seed companies will sell inoculum alongside their legumes.

Soil Builders

These cover crops are especially useful in beds with poor soil structure. Signs these cover crops can be useful include areas that have standing water, a lack of earthworms, and rock hard surfaces after a good rainfall. Species such as rye (Secale cereal), oats (Avena sativa), and sorghum sudangrass (Sorghum bicolor x S. bicolor var. Sudanese) add quite a bit of organic matter through their rooting systems. This organic matter serves a couple of purposes, one of which is to serve as a kind of glue that increases the amount of soil aggregates. These aggregates “fluff up” the soil to help make larger pores, which allow plant roots to explore their surroundings without being choked by compacted areas. Organic matter also can serve as a storage facility for many nutrients such as potassium, calcium, and certain types of nitrogen.

Word of caution, these plants tend to have a lot more carbon than nitrogen. What this means is that microbes will break down these plants slower than legumes such as vetches and beans. The microbes will hold nitrogen released by these grasses greedily, only being available to your main crop after about one to three weeks after the cover crop is killed. In some instances, you may need to apply some nitrogen fertilizer if you cannot wait that long.

Weed Suppressors

Made up of grasses, legumes, and herbaceous plants, these species thrive on smothering weeds in the garden. Rye, hairy vetch, oat, and buckwheat (Fagopyrum esculentum) are great selections due to their quick growth. This quick growth can shade out and rob weeds of their nutrients. Some of these cover crops, notably rye, put out chemicals in the soil, which can interfere with weed seed germination. This process is called allelopathy, and is most effective when rye is in a garden bed with small weed seeds like common lambsquarters (Chenopodium album).

Just as these species can pound weeds, they can also pound your own garden if you are not careful. Do not allow any of these species to set seed for the next year, especially rye or buckwheat, or you will be back to square one with weed issues.

Pollinator Hotels

Full of beautiful blooms and attractive smells, pollinator-friendly cover crops can really bring palpable “buzz” to your garden. Buckwheat and phacelia (Phacelia tanacetifolia) are great annual cover crops that can really help attract both native and honey bees, wasps, and butterflies. These creatures not only increase fruit set from your squashes and tomatoes, but can also serve as a secondary food source for insects that prey on your garden pests.

Two things to keep in mind for these plants. First, if the only thing within two miles of your garden is corn and soybeans, expect the benefits to your garden to be less than average. Many of these insects cannot travel large distances from their nests, and may not notice your little oasis of flowers. Second, be aware of bloom times for both the cover crop and your vegetables. Your goal is to provide food for pollinators for as long as possible to maximize any benefit. Front or back loading your blooms can leave long stretches where your garden is not as attractive to pollinators or insects that control pests.

Remember, before you purchase cover crop seeds, you must have an idea of what your garden needs. If you read closely, you may have recognized that one or several species may take care of several issues at once (rye, oats, hairy vetch, buckwheat). Next month I will talk more about timing the planting of your cover crops correctly to maximize their usefulness as well as taking into account your typical garden planting schedule.


Works Cited

Clark, A. 2007. Managing cover crops profitably. College Park, MD: SARE.
Fortier, J. M., and M. Bilodeau. 2014. The Market Gardener: A Successful Grower's Handbook for Small-scale Organic Farming. New Society Publishers.


Shane Bugeja
Extension Educator, Ag Production Systems
(507) 304-4325
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