Cover Crops for Vegetable Growers

Why cover crops?
Soil Health

Decision Tool

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Early summer
Mid and late summer
Late summer legumes
Early fall
Fall
Early spring

Cover crop information

Annual ryegrass
Buckwheat
Sudangrass
Mustard, spring
Mustard, fall (includes radish, rape, kale)
Red clover
Hairy vetch

Oats
Wheat (includes spelt, triticale)
Rye

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Seed sources

Research

Buckwheat planting date
Buckwheat field preparation

Fertility

Cover crops for early summer
Buckwheat and sudangrass

Summer may seem an odd time to use  cover crops, because that is the time when the real crops are growing. But summer may be the right opportunity to improve fields with a cover crop. If the soil is wearing out, summer is when a soil-building crop can do a lot more work. Also, if the rotation leaves an opening in the summer, a short cycle cover crop will be much better than leaving the field open, to suffer erosion from rain and have weeds to go to seed. Buckwheat sown in late May or early June, can be used before vegetables such as pumpkins, broccoli, late cucumbers. There is another opportunity for summer cover crops after lettuce, peas, early beans, spinach or small grains.

For planting in June, there are really only two choices. One is sudangrass, or sorghum-sudangrass, and the other is buckwheat. Both grow rapidly in the summer warmth. 

Making the choice

The two cover crops have different properties, so the management goal and field condition will determine which is the right one.

Sudangrass is often chosen for improving soil organic matter. It produces a strong root system and lots of biomass. The deep root system is helpful for reducing subsurface hardness. It is also a good choice for reducing root-knot nematode pressure.

Buckwheat is best known for weed suppression and mellowing the soil.

If weed suppression is the main purpose, buckwheat is preferred. It covers the ground earlier than sudangrass, especially in early June, and outcompetes weeds that may establish in sudangrass. Sudangrass requires a higher seeding rate for effective weed suppression.

The amount of time until the fall crop is to be planted is a significant decision factor. Buckwheat is in the ground for 35 to 40 days when used as a cover crop. It can be sown as early as May 20th. Sudangrass needs 60 to 70 days to be effective, and is best planted once June has become thoroughly warm. Both of these cover crops should be mowed after about 40 days. That is the end of the season for buckwheat, but the beginning of major root growth for sudangrass. Sudangrass needs a final flail mowing and immediate incorporation to suppress nematodes.

The condition of the field will determine which crop is suitable. If the soil is hard, or the field is prone to standing water, sudangrass is a good choice, while buckwheat will do poorly. However, if the field is low in nitrogen and phosphorous, buckwheat will do well without additional fertilizer, while sudangrass needs about 40 lb of N to give satisfactory performance.

If the crop to follow needs a fine seedbed, that will be easier to produce after buckwheat. It mellows the soil for easy working, and decomposes quickly after incorporation. Sudangrass crowns take some time to break down, so the following crop needs to be one that can be sown in a somewhat lumpy field.

The main production risks with buckwheat are a failed stand and letting it go to seed. The failed stand usually follows a heavy rain around emergence. It will be obvious two weeks after planting. If the seedlings are not doing well then, till them in and plant again. To avoid volunteer buckwheat seed, kill the crop before there are filled green seeds on the plant. That takes about 40 days from a July planting or 50 days from a June planting.

The main production risk with sudangrass is that the crop gets too big to mow, or to incorporate after frost has killed it. This crop grows very fast, so keep an eye on it. Mow the first time at about 3 feet and the second time while the flail mower can still chop it well. If sudangrass gets too big to control, it will be killed by frost and make a nice winter mulch. However the biofumigant effect will be lost.

Seed sources

Buckwheat is available from some local farm seed retailers. The variety does not matter, and many suppliers don’t identify any variety. Regional suppliers include The Birkett Mills in Penn Yan, Ernst Conservation Seed in Meadville, PA, AgriCulver in Trumansburg, and Lakeview Organic Grain in Penn Yan. A short crop in 2006 has raised the seed cost for this season, with prices ranging from $15 to $25 per 50 lb bag. A bag is enough to seed an acre. 

Sorghum and sorghum-sudangrass are widely available. Varieties suitable for cover crops must be selected carefully. Grain types are inappropriate and some new forage varieties, described as sweet or with brown midrib are low in dhurrin, which is the biofumigant in sudangrass. Piper sudangrass is readily available, and has a similar composition to Trudan 8, the classic sudangrass for biofumigation. Sorghum-sudangrass hybrids are more vigorous, and will produce more biomass than sudangrass, but the seed is also more expensive. Appropriate varieties that are available locally include Sordan 79, Green Grazer and Special Effort. With a modest seeding rate of 30 lb/ac, sudangrass can cost as little as $10 to $20 per acre. Weed suppression requires 50 lb/ac. Regional suppliers include Seedway in Hall, Agriculver in Trumansburg and UAP in Sodus.