Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
Vegetable growers should take time to revisit their rotations and plans for the next growing season. Decisions on fall rotational crops or cover crops will need to be made soon.
Start by listing your goals. Some possible goals for vegetable rotations include:
- Returning organic matter to the soil. Vegetable rotations are tillage intensive and organic matter is oxidized at a high rate. Cover crops help to maintain organic matter levels in the soil, a critical component of soil health and productivity. Brassicas and winter legumes provide the most biomass followed by ryegrasses and then rye.
- Providing winter cover. By having a crop (including roots) growing on a field in the winter you recycle plant nutrients (especially nitrogen), reduce leaching losses of nitrogen, reduce erosion by wind and water, and reduce surface compaction and the effects of heavy rainfall on bare soils. Cover crops also compete with winter annual weeds and can help reduce weed pressure in the spring.
- Providing fall and early winter cover and then winter killing. The use of winter killed cover crops are very useful when early spring (March or April) plantings of vegetable crops such as potatoes, peas, cole crops, early sweet corn, or early snap bean crops are being planned. By winter killing, cover crop residue is more manageable and spring tillage and planting can proceed more quickly.
- Reducing certain diseases and other pests. Cover crops help to maintain soil organic matter. Residue from cover crops can help to increase the diversity of soil organisms and reduce soil borne disease pressure. Some cover crops may also help to suppress certain soil borne pests, such as nematodes, by releasing compounds that affect these pests upon decomposition. One system would be planting mustards in August or early September, tilling them into the soil to provide some biofumigation in October, and then planting a small grain crop for winter cover. Spring planted mustards can also work ahead of later spring planted vegetables.
- Providing nitrogen for the following crop. Leguminous cover crops, such as hairy vetch or crimson clover, can provide significant amounts of nitrogen, especially for late spring planted vegetables. Hairy vetch is particularly well suited for no-till systems and can provide full nitrogen requirements for crops such as pumpkins and partial requirements for crops such as sweet corn, tomatoes, or peppers.
- Improving soil physical properties. Cover crops help to maintain or improve soil physical properties and reduce compaction. Roots of cover crops and incorporated cover crop residue will help improve drainage, water holding capacity, aeration, and tilth. The use of large tap rooted cover crops such as forage radish or oilseed radish are particularly well adapted to these uses.
- Setting up windbreaks in the fall for spring planted vegetables. Small grain crops will overwinter and grow tall enough in to provide wind protection for spring planted vegetables. Rye has been the preferred windbreak because tall types are still available and it elongates early in the spring. While barley is also early, tall varieties are not generally available. Wheat and triticale are intermediate and later.
- Developing no-till, bio-strip-till, and bio-bed preparation systems. There is much opportunity to increase the amount of no-till and bio-tillage systems. The key will be selecting the right cover crop for the desired system. Rye, crimson clover, subclover, tillage radish, spring oats, and other cover crops have been used successfully for no-till vegetables. One innovative system that uses a combination of winter killed covers and standard covers is bio-strip-till. In this system, a high biomass cover crop such as rye or vetch is planted with strips of forage or oilseed radish in rows where spring planting will occur. Another system uses rye strips with forage radish planted where the beds will be next year.
Cover crop planting windows vary with crop and timely planting is essential to achieve the desired results. There are many cover crop options for late summer or fall planting including:
Rye is often used as a winter cover as it is very cold hardy and deep rooted. It has the added advantage of being tall and strips can be left the following spring to provide windbreaks in crops such as watermelons. Rye makes very good surface mulch for roll-kill or plant through no-till systems for crops such as pumpkins. It also can be planted later (up to early November) and still provide adequate winter cover. Wheat, barley, and triticale are also planted as winter cover crops by vegetable producers.
Spring oats may also be used as a cover crop and can produce significant growth if planted in late August or early September. It has the advantage of winter killing in most years, thus making it easier to manage for early spring crops such as peas or cabbage. All the small grain cover crops will make more cover with some nitrogen application or the use of manure.
To get full advantage of small grain cover crops, use full seeding rates and plant early enough to get some fall tillering. Drilling is preferred to broadcast or aerial seeding.
Both perennial and annual ryegrasses also make good winter cover crops. They are quick growing in the fall and can be planted from late August through October. If allowed to grow in the spring, ryegrasses can add significant organic matter to the soil when turned under, but avoid letting them go to seed.
Winter Annual Legumes
Hairy vetch, crimson clover, field peas, subterranean clover, and other clovers are excellent cover crops and can provide significant nitrogen for vegetable crops that follow. Hairy vetch works very well in no-till vegetable systems where it is allowed to go up to flowering and then is killed by herbicides or with a roller-crimper. It is a common system for planting pumpkins in the region but also works well for late plantings of other vine crops, tomatoes and peppers. Hairy vetch, crimson clover and subterranean clover can provide from 80 to well over 100 pounds of nitrogen equivalent. Remember to inoculate the seeds of these crops with the proper Rhizobial inoculants for that particular legume. All of these legume species should be planted as early as possible – from the last week in August through the end of September to get adequate fall growth. These crops need to be established at least 4 weeks before a killing frost.
There has been an increase in interest in the use of certain Brassica species as cover crops for vegetable rotations.
Rapeseed has been used as a winter cover and has shown some promise in reducing certain nematode levels in the soil. To take advantage of the biofumigation properties of rapeseed you plant the crop in late summer, allow the plant to develop until early next spring and then till it under before it goes to seed. It is the leaves that break down to release the fumigant-like chemical. Mow rapeseed using a flail mower and plow down the residue immediately. Never mow down more area than can be plowed under within two hours. Note: Mowing injures the plants and initiates a process releasing nematicidal chemicals into the soil. Failure to incorporate mowed plant material into the soil quickly, allows much of these available toxicants to escape by volatilization.
Turnips and mustards can be used for fall cover but not all varieties and species will winter over into the spring. Several mustard species have biofumigation potential and a succession rotation of an August planting of biofumigant mustards that are tilled under in October followed by small grain can significantly reduce diseases for spring planted vegetables that follow.
More recent research in the region has been with forage radish. It produces a giant tap root that acts like a bio-drill, opening up channels in the soil and reducing compaction. When planted in late summer, it will produce a large amount of growth and will smother any winter annual weeds. It will then winter kill leaving a very mellow, weed-free seedbed. It is an ideal cover crop for systems with early spring planted vegetables such as peas. Oilseed radish is similar to forage radish but has a less significant root. It also winter kills. Brassicas must be planted early – mid-August through mid-September – for best effect.
Cover Crop Mixtures
There is significant interest in cover crop mixtures to the point where 6 – 8 different species are being mixed together. As fall cover crop season is upon us, there are a number of considerations that growers interested in using mixtures should be aware of.
Cover crop species are commonly grouped into six major categories: 1) cool season grasses; 2) cool season legumes; 3) cool season broadleaves 4) warm season grasses; 4) warm season legumes; and 6) warm season broadleaves. In theory, a successful mixture will combine species from as many categories as practical based on the planting season. For late summer/fall planting we will be limited to 1, 2, and 3 above.
In addition, cover crop species can also be placed into groups based on the benefits they offer. This includes nitrogen fixation, nutrient (particularly nitrogen) uptake and recycling, compaction reduction, disease suppression, biofumigation, weed control, biomass accumulation, use as a mulch, winter killing to facilitate early spring plantings, and other benefits.
The first step in creating a mixture is to list the available species that can be used for the time of the year. For example, for late summer and fall planting this would include small grains (wheat, barley, rye, winter oats, triticale), ryegrasses, rapeseed, winter annual legumes (crimson clover, hairy vetch, winter hardy field peas, subclover, many other clovers). If winter killed crops with extended fall growing seasons are desired then radishes, mustards, and spring oats would be examples of selections.
The second step would be to list what soil health attributes or other cropping system needs should be prioritized. For example, if a mulch for no-tilling vegetables into next spring is a priority then high biomass cover crops that decompose more slowly such as cereal rye or triticale should be in the mixture. Conversely, if early spring planting is the goal then winter killed cover crops should be in the mixture. If compaction needs to be addressed then radishes or other species in the Brassica family should be in the mix. If nitrogen fixation is a priority then a high N fixing potential legume such as hairy vetch should be included.
The final step would be to develop seeding rates for each mixture component. This is critical because too much of one component can outcompete other components and limit their survival or limit their usefulness in the mixture. Unfortunately there is little actual science to guide seed rate determinations for complex mixtures. A number of seed companies supply mixtures and can be consulted.
An example of a potential September seeded cover crop mixture for Delaware with many winter hardy species is: rapeseed, ryegrass, cereal rye, crimson clover, and hairy vetch. A multi-species example with combinations of winter killed and winter hardy species is: radish, mustard, spring oats, triticale, crimson clover, and field peas.
Growers will need to do some experimentation on their own farms with different mixtures and seeding rates to determine what works best for their farm, growing conditions, and rotations.