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Cover Crops for Vegetable Growers

Cover crops for early summer

Buckwheat and sudangrass

Summer may seem an odd time to use cover crops because it is the time when the main 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 be a lot more beneficial. Also, if the crop rotation leaves an opening in the summer, using a short cycle cover crop is much better than leaving the field subject to rain erosion and weeds that are going to seed.

There are two early summer opportunities to sow cover crops: one is in late May or early June before vegetables such as pumpkins, broccoli, or late cucumbers. The other is after lettuce, peas, early beans, spinach or small grains are sown.

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
Buckwheat and sudangrass have different properties, so the management goal and field condition will determine which is the right one to use.

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

If weed suppression is the main goal, buckwheat is preferable. Buckwheat is best known for weed suppression and mellowing the soil. 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.

When will the cover crop be planted? The amount of time until the fall crop is to be planted is a significant decision factor. As a cover crop, buckwheat is in the ground for 35-40 days. It can be sown as early as May 20. Sudangrass needs 60-70 days to be effective, and is most worthwhile if planted once June has become thoroughly warm. Both cover crops should be mowed after about 40 days. This 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.

What is the current condition of your soil? If the soil is hard or the field is prone to standing water, sudangrass is a good choice, but 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 nitrogen to give satisfactory performance.

What are the needs of the fall crop? If the crop to follow the cover crop needs a fine seedbed, it will be easier to produce after buckwheat. Buckwheat 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.

What production risks are you willing to take? The main production risks associated with buckwheat are a failed stand and letting it go to seed. The failed stand usually follows a heavy rain around the time of 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. This takes about 40 days from a July planting or 50 days from a June planting.

The main production risk associated 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 when it reaches 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 seed 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, NY, Ernst Conservation Seed in Meadville, PA, AgriCulver in Trumansburg, NY, and Lakeview Organic Grain in Penn Yan, NY. Buckwheat generally costs between $15 - $20/50 lb bag. A bag is enough to seed an acre.

Sorghum and sorghum-sudan grass 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, locally available varieties include Sordan 79, Green Grazer, and Special Effort. Regional suppliers include Seedway in Hall, NY, Agriculver in Trumansburg, NY, and UAP in Sodus, NY. With a modest seeding rate of 30 lb/ac, sudangrass can cost as little as $10-$20/ac. Weed suppression requires 50 lb/ac.

This article is intended for publication in Cornell Cooperative Extension newsletters, and similar outlets, that reach growers by June.