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Cover Crops for Vegetable Growers

Cover crops for mid to late summer

Late season buckwheat and sudangrass; annual ryegrass

Early vegetable harvest begins in mid to late summer. Rather than leave the ground open to grow weeds, the land can be improved by holding it with a cover crop. Reducing weed seed production is an obvious advantage that will be appreciated the following year. Using the great growing conditions of late summer also allows you to address soil health issues by choosing a cover crop that directly targets properties that are a concern. Hard surface soil and subsurface compaction are both common vegetable production limitations. Both inhibit crop growth directly and create favorable conditions for soil-borne diseases by reducing percolation. Late summer cover crops also scavenge leftover nitrogen and add a lot of organic matter. These processes provide both organic nitrogen reserves and food for the microbes that increase the soil’s potentially mineralizable nitrogen.

For planting in July, the main choices are buckwheat and sudangrass. These two cover crops are at their best when sown in July, and can be sown into early August. Annual ryegrass becomes a possibility beginning in August.

Buckwheat and sudangrass
Last month’s article described making the choice between buckwheat and sudangrass. Buckwheat works in a shorter window (40 days), is excellent on weeds, and leaves the soil mellow and ready to plant. Sudangrass and its relative, sorghum-sudangrass, are great for producing organic matter and penetrating a plow layer, but need a longer window (70 days), higher fertility, and more management. The following section recaps how to choose between buckwheat and sudangrass.

Buckwheat: One of the big advantages of buckwheat is that it works in little more than a month and leaves the ground ready to plant. The short cycle lets you use buckwheat between summer crops. It can even fit between peas and broccoli. It works especially well before fall grains - it is done in time to sow grain at the ideal time. Buckwheat can be sown as late as mid-August, but growth slows down at its normal mid-September ripening time. There are many other cover crop choices available later in the summer.

Buckwheat seeded in July and early August sets seed quickly. If spring buckwheat volunteers are hard for you to control, make sure to stop the cover crop at the right time. If it is mowed too early (30 days), it will regrow and make seed in less than a month. If it is mowed or incorporated too late (45 days), it will already have viable seed that will sprout in the spring. Buckwheat volunteers tend not to be a problem with row crops that have early cultivation or post-emergence herbicide programs.

Sudangrass: Sudangrass needs over two months before the following crop can be planted. If you are seeding in late July, it will best be used as a winter-killed cover. Early July seedings may leave enough time to establish winter rye.

Sudangrass has a coarser and deeper root system that is valuable for correcting plow-pan compaction. It is also one of the best cover crops for resupplying the soil with active carbon. Active carbon is important to have in your soil because it provides sustenance for beneficial microbes. In particular, farmers with low-quality muck soil have seen great improvements by using sudangrass.

Annual ryegrass
Annual ryegrass can be grown into the fall but is more commonly grown on through the winter. If you want an August-seeded, overwintering cover crop, the choice is annual ryegrass. It corrects hard surface soil by creating a sod. It does more for hard surface soil than buckwheat, but takes much longer and requires some time for decomposition. Annual ryegrass is also good for weed suppression. It grows vigorously enough to outcompete late summer annuals as well as winter annuals that start in the fall.

Annual ryegrass is a good cover crop choice on heavier ground because it has better flooding tolerance than other cover crops.

For weed control, a rapid start and vigorous growth are essential. A rapid start depends on good soil moisture. Vigorous growth depends on available nitrogen. There is often a substantial amount of nitrogen left in the soil after vegetables. If there is not, 30 lb/ac of nitrogen can double fall growth of the ryegrass. With these conditions, annual ryegrass will outcompete weed seedlings, but established weeds must be killed before sowing the cover crop.

The key to successful seeding is getting the seed in contact with moisture. Under dry conditions, drilling is essential. If the soil surface is moist, broadcasting without covering is effective. Good seedling growth requires continued moisture. July and early August tend to be too dry to support July-sown annual ryegrass seedlings. July seedings can succeed with irrigation or in wet years, but buckwheat and sudangrass are much more reliable in July. Weak seedling growth can be an advantage when established between beds on plastic mulch.

Annual ryegrass will mostly overwinter, especially where there is good snow cover. Winter survival varies considerably among varieties: Southern types will kill, whereas Midwestern ones will survive. The variety is often not specified in the New York market, so winter survival my not be consistent. For more predictable performance, specific varieties can be ordered from Oregon through your dealer.

August seeded annual ryegrass makes a dense sod by spring. This is valuable for improving the soil condition and for nitrogen scavenging. However, it takes some decomposition after spring tillage to break it up.

The major concern with annual ryegrass is that it not go to seed in the spring. [WHY?] It can be controlled by tillage. Once the air is warm, glyphosate also works. Glyphosate is not effective until the daytime temperature has reached the 50s. In vegetables, the common grass herbicides control annual ryegrass. Annual ryegrass volunteers are particularly undesirable in small grains, so special care must be taken if they are part of your rotation.

When planting annual ryegrass in August, it is worth having a plan for how the field will be prepared in the spring. The plan should include a method for killing the sod, and should allow time for it to decompose before the field is fitted and planted.

Annual ryegrass can be interseeded into crops that will be harvested in late August or September. If conditions are moist, simply broadcasting seed on the surface is enough. Interseeding is best done just before the crop fills the canopy. The crop will begin to establish a shallow root system, but will not be competitive in the shade of the crop. This early sod could reduce damage from harvest traffic, and could help speed the recovery of the soil after harvest. Interseeding into a vegetable crop allows the cover crop to establish better in the fall than does a grain sown well after vegetable harvest.

For August seeding, the planting rates are 10 lb/ac if drilled into reasonably moist soil, and 15 lb/ac in dryer soil. Broadcasting requires 15-20 lb/ac. On dry soil, rates can go as high as 30 lb/ac. Seed cost is usually under $1/lb. Seedway distributes annual ryegrass seed in New York.

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