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Ask a Master Gardener: Understanding cover crop planting techniques

by Peter Sutter | October 16, 2022 at 4:00 a.m.

Although I have some crops in the high tunnel, my outside gardening is done. Today, I got my garlic put in and I planted hairy vetch and rye cover crop in the last section of the garden. Speaking of cover crops, Justin Key (MU horticulture specialist) had and excellent article on cover crops in the Ag Connection. Here are some highlights:

Soil management of vegetable crops takes on added importance because of their high-dollar value. Therefore, soil improvement via the use of cover crops is an important management consideration for vegetable growers. Cover crops represent an effective way to improve both the physical and chemical properties of soils dedicated to vegetable production.

A number of questions should be considered before planting cover crops including:

• Is the primary goal of the cover crop to fix nitrogen, suppress weeds or add organic matter to the soil?

• When will the cover crop be planted (fall, spring or summer), and how long will it take to mature?

• Does the cover crop chosen need to be able to withstand freezing temperatures?

• How will the cover crop be terminated (e.g., mowing, herbicides, tillage, etc.) so that the field may be planted?

• Will the cover crop seed itself before termination and risk becoming a weed?

• What cash crop will be planted after the cover crop?

Additionally, growers must be aware of the advantages and disadvantages of various types of cover crops. For example, legumes, such as clover, hairy vetch, Austrian winter pea and others do a good job of fixing nitrogen, but they do not leave high amounts of residue or contribute greatly to soil organic matter. However, since most vegetable crops require between 100-220 pounds of nitrogen per acre, the use of legumes as cover crops can help supply a substantial portion of nitrogen requirements.

Alternatively, grasses such as cereal rye, Japanese millet and others produce a lot of biomass and build soil organic matter more rapidly, but do not fix nitrogen. Additionally, they can tie up nitrogen in soils due to their high carbon-to-nitrogen ratio. Tillage radish and buckwheat neither fix nitrogen or produce abundant residue. However, each serves a function of its own.

The majority of vegetable growers employ fall-planted cover crops, since most of their available land is occupied during the growing season by vegetables. There are several major considerations for fall-planted cover crops. For example, for larger, transplanted crops such as tomato and pepper, a cover crop that can be terminated and left on the soil for mulch and weed suppression might be the best choice. Alternatively, low residue cover crops such as tillage radish can be planted directly into the soil the following spring or incorporated into the soil and then planted.

Fall seeding of cover crops most generally occur from mid-August until mid-September, which allows for an adequate time to achieve the maximum amount of growth. Cereal rye can be planted until Thanksgiving. Some simple options like tillage radish are worth trying, especially for growers who have not planted cover crops before. Tillage radish leaves very little if any residue left on the soil.

Oats are another common cover crop used in vegetables systems; they will winter kill and leave low residue. They often are combined with Austrian winter pea, which is able to fix nitrogen. Hairy vetch is a winter annual that can be planted in the fall and terminated in the spring, using a variety of methods. Cereal rye is a winter annual often used as a companion plant to hairy vetch.

The establishment of a cover crop is just as important as the establishment of a cash crop. Cover crop seed germination will be enhanced through good seed-to-soil contact. This can be accomplished through mechanical preparation of the soil or through the use of hand tools to help rough up the soil surface. Additionally, it is important to remove or kill weeds before planting cover crops, since cover crops find it difficult to compete with already established weeds. When legumes are used as cover crops, it is important to remember different legume species might require different bacterial inoculants. Therefore, make sure to purchase the appropriate inoculant for the cover crop being seeded to ensure maximum nitrogen fixation.

Cover crops may be seeded by broadcasting seeds by hand, through the use of a belly seeder or a fertilizer spreader. Drop seeders/spreaders and powered broadcast spreaders represent another option. In any case, covering the seed will also help to enhance seed germination and establishment. This can be accomplished through the use of tractor tools such as light harrow, cultipacker or a firmer/roller. Growers also might consider spreading a light layer of compost on top of the seeds, if covering the seeds using the above-mentioned options is not possible. Seeds should be watered in well, especially if the soil is dry. In the absence of rain, irrigation should be considered to help seeds germinate.

Happy gardening!

Peter Sutter is a lifelong gardening enthusiast and a participant in the MU Extension's Callaway County Master Gardener program. Gardening questions can be sent to [email protected]

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