As peas, garlic, summer squash and many other vegetables come out, it is time to think about putting a cover crop in. For late July and early August plantings in most of Michigan, oats, buckwheat and mustard family cover crops are among the best choices for protecting soil and suppressing weeds.

Is it worth it?

This is a very busy time of year, so clear benefits are needed to justify the time and expense of planting a cover crop. Although there are many potential benefits of cover-cropping, probably the most important benefits in late summer are protection of soil from heavy rainfall and wind events, and weed suppression.

Soil protection. On Monday, July 11, I visited several trials after the heavy rains that hit many parts of the state. Not surprisingly, plots without cover crops suffered much more soil erosion than those with covers. This is, of course, especially obvious on sloped ground, but even on flat ground these types of pounding rains can leach nutrients and pesticides, destroy soil structure and, ultimately, reduce future crop productivity.

Weed suppression. A well-established smother crop can take the place of one or more herbicide applications or tillage passes that might otherwise be required to prevent weeds from going to seed. After harvest, weed seeds and seedlings that are released from competition with the crop can quickly establish a foothold. Many species, including pigweed and lambsquarters, are programmed to rapidly produce seeds as days shorten in the late summer. Although they may not produce much biomass, such weeds can produce hundreds of thousands of seeds that will create weed management headaches for years to come.

Which cover crop to plant?

In deciding whether to plant a cover crop and which cover crop to plant, consider the purpose, timing and expense. A new resource that can help with cover crop decisions is now available through the Midwest Cover Crop Council. This electronic cover crop decision tool allows you to search for cover crop options based on your county and desired cover crop characteristics.

For late July and early August plantings in most of Michigan, cover crop choices which have proven ability to suppress weeds and protect soil are listed below.

Oats. Oats are inexpensive, easy to establish, provide good weed suppression and reliably winter-kill in Michigan. Oats can also be undersown with clovers that can fix abundant N next spring or summer if a late planted vegetable is planned. Another combination that can work well is oats with hairy vetch. Oats will winter kill, allowing vetch to grow rapidly next spring. Although hairy vetch can become a weed problem in crops like asparagus or winter wheat, it is one of the most efficient N-fixers in Michigan and can be a useful tool for reducing fertilizer costs and suppressing weeds for growers who grow mostly summer annual vegetables.

Buckwheat. Extremely fast, early growth provides great weed suppression and soil protection. Buckwheat will flower in 35 to 40 days and attract many beneficial insects (although may temporarily pull some pollinators away from crops that need them). Like vetch, buckwheat can become a weed problem in subsequent crops if it is not mowed before seed set. This can occur within 10 days of flowering, so be prepared to mow soon after the white flowers appear.

Mustards. Several mustard family cover crop options are available, including the increasingly popular tillage radish (oilseed radish) and yellow mustard. Mustard family cover crops do best when planted after August 1, but will continue to produce abundant biomass well into the fall. Many have known benefits for suppressing pathogens and weeds. MSU horticulturist Mathieu Ngouajio has documented benefits of mustard cover crops prior to several vegetables including celery and onions. However, caution should be used to avoid planting these where mustard family vegetables, including broccoli, cabbage or radish, are an important part of the rotation; some vegetables including sweet corn and peas are also occasionally suppressed by mustards. For example, in one of our trials this spring, we observed increased incidence of seed corn maggot where yellow mustard had been planted the year before.

Of course, other excellent cover crop options are available following early harvested vegetables, including sorghum-sudangrass, Japanese millet and cowpea, as well as a wide range of mixtures of two or more of these and other species. In addition to the MCCC website, more detailed information on cover crop choices can be found in the book Managing Cover Crops Profitably available online.

Dr. Brainard's work is funded in part by MSU's AgBioResearch.