Spring Cover Crops for Vegetable Rotations

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

One principle of managing soil for improved health is to always have a crop growing on the soil. This will maintain or add organic matter, provide benefits from the action of growing roots, and recycle nutrients.

Where fall cover crops were not planted due to late harvest, spring cover crops can be planted in early April to provide soil health benefits where vegetables and field crops are not scheduled until late May or the month of June.

The most common cover crop options for late March or early April planting include spring oats, mustards and annual ryegrass. Plant oats at 90-120 lbs per acre, mustards at 10-20 lbs per acre, and annual ryegrass at 20-30 lbs per acre.

Field peas are another option; however, we are somewhat south of the best zone for spring planting. One type of field pea is the winter pea which is often fall planted in our area but can be spring planted. It has smaller seed so the seeding rate is 30-60 lbs per acre. Canadian or spring field peas are larger seeded and used as a spring cover crop planted alone at 120-140 lb/A.

Mixtures can also be used. Field peas are well adapted to mixing with spring oats or with annual ryegrass. Reduce seeding rates of each component when using in mixtures. Recommended seeding rates are 70 lbs of oats per acre and 40 lbs/A of Austrian winter peas or 80 lbs/A of Canadian or spring field peas.

Many mustard family crops have biofumigation potential. When allowed to grow to early flower stage and then incorporated into the soil, they release compounds that act as natural fumigants, reducing soil borne disease organisms. Some biofumigant mustard varieties and blends include ‘Pacific Gold’, ‘Idagold’, ‘Caliente’, ‘Trifecta’, and ‘Kodiak’. Other mustard family crops serve as non-hosts, trap crops, or deterrents for pests. In research at the University of Delaware biofumigation using early spring planted biofumigant crops such as ‘Image’ radish, ‘Dwarf Essex’ rapeseed, or ‘Nemat’ arugula showed potential for managing root knot nematode populations. When used as a biofumigant, mustard family cover crops should be grown to achieve maximum biomass by adding 60-100 lbs of nitrogen per acre. Nitrogen is also required to produce high biomass with spring oats and annual ryegrass at similar rates. When planting mixtures with peas, nitrogen rates should be reduced.

An often-forgotten spring seeded legume crop that can also be used is red clover. Red clover can be frost seeded into small grains, seeded alone, or mixed with spring oats or annual ryegrass. Seeding rates for pure stands would be 10-16 lbs/A, for mixtures 6-10 lbs/A.

Spring planted radishes and mustards as cover crops.

Are You Growing Cover Crops for Maximum Benefits?

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

For most of Delaware, the optimal window for cover crop planting is in the month of September and we are nearing the end of the planting window where soil health benefits can be maximized. As we move into October, cover crop selection becomes limited due to reduced daylength and lower temperatures thus limiting potential soil health benefits.

Vegetable growers understand the benefit of growing cover crops to maintain soil heath. Most vegetable production systems are tillage intensive and organic matter is oxidized by soil microorganisms at a high rate. Cover crops are an important means to add organic matter back into vegetable production systems.

Cover crop acreage has been growing in the region, largely due to nutrient management efforts and an emphasis on growing cover crops for soil health benefits. Successful programs have been implemented by the USDA-NRCS and Conservation Districts to increase cover crop plantings for soil improvement.

Nutrient management goals and soil health goals are not necessarily the same. You can think about this with the question are you growing cover or crops?

In nutrient-management based cover crop programs, the goals are to have crops that can take up residual nitrogen and provide cover to reduce erosion losses. Non-legumes predominate, with most of the acres planted in small grains such as rye with some recent use of radishes. No fertilizer or limited fertilizer can be used with these cover crops. In this case, the answer to the question above is that a cover is being grown. While there will be soil health benefits, they are not maximized.

In contrast, when soil improvement is the primary goal, the cover crops are grown as crops. You are growing plants to maximize the benefits they provide. To increase organic matter and improve soil health the main goal is to produce maximum biomass above ground and below ground. A secondary goal would be to provide different types of organic matter with cover crop mixtures to support a diverse soil microbial environment.

In other situations, the goals will be different. With leguminous cover crops a goal may be to maximize the amount of nitrogen fixed. With soil compaction reducing crops such as radishes, the goal is to maximize the amount of “biodrilling” – the amount of tap roots being produced. With biofumigant crops, the goal is to maximize the production of fumigant-like chemicals the crops produce. With mulch-based systems, the goal is to maximize above ground biomass.

What these soil improvement and specific use goals have in common is the need to treat the cover crop as a crop to optimize plant growth. This would include seeding at the proper rate to achieve optimal stands, planting at the right time, using seeding methods to get maximum seed germination and plant survival, having sufficient fertility to support good plant growth, providing water during dry periods, managing pests (insects, diseases, weeds), and inoculating legumes. If cover crop mixtures are being used, the ratios of seeds being planted must be considered to have the best balance of plants in the final stand.

The best cover crop stands are obtained with a drill or seeder that places the seed at the proper depth, at the proper seeding rate, with good soil to seed contact. Fertilization and liming programs should be used to support season-long growth – fertilizers and other soil amendments will be necessary in most cases. Nitrogen will need to be added for non-legumes.

When the crop is terminated is also key. The cover crops should be allowed to grow to the stage that maximizes the benefits they offer before killing the crops. Allowing a winter cover to grow for an extra week in the spring can make a large difference in the amount of biomass produced.

Again, consider the question are you growing a cover or a crop? The answer is important to achieve your cover crop goals.

Above-ground biomass for a mulch-based vegetable production system after spring burn-down. Note the differences between the cover crop strips.

Spring Planted Cover Crops for Vegetable Rotations

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

One principle of managing for improved soil health is that you should always have a crop growing on the soil. This will maintain or add organic matter, provide benefits from the action of growing roots, and recycle nutrients.

Where fall cover crops were not planted due to late harvest, spring cover crops can be planted and provide soil health benefits where vegetables are not scheduled until late May or the month of June.

The most common grass family cover crop options for mid to late March or early April planting are spring oats, and annual ryegrass. Plant oats 90-120 lbs per acre and annual ryegrass at 20-30 lbs per acre.

Mustard family (Brassica) cover crop options for late March or early April planting include yellow mustards, white mustards, brown mustards and oriental mustards. Companies also offer blends of several mustard species. Mustards are generally planted at 10-20 lbs per acre. Rapeseed and canola are another mustard family option for spring planting at 5-12 lbs per acre. Forage radishes and oilseed radishes can also be spring planted at a rate of 4-10 lbs per acre. Arugula is an additional mustard family option planted at 4-7 lbs per acre.

In the legume family, field peas are another option for spring planting. One type of field pea is the winter pea which is often fall planted in our area but can be spring planted. It has smaller seed so the seeding rate is 30-60 lbs per acre. Canadian field peas are larger seeded and used as a spring cover crop planted alone at 120-140 lbs per acre. An often-forgotten spring seeded legume crop that can also be used is red clover. Red clover can be frost seeded into small grains, seeded alone, or mixed with spring oats or annual ryegrass. Seeding rates for pure stands would be 10-16 lbs per acre, for mixtures 6-10 lbs per acre.

Mixtures also can be used. Research has shown that you get the best soil health benefits from mixing three species from different plant families. Commonly a grass is mixed with a legume and with a mustard family crop. Examples would be spring oats, field peas, and forage radish; or annual ryegrass, red clover, and mustard. Reduce seeding rates of each component when using in mixtures. Companies often offer preblended mixture for these uses.

Many of the mustards have biofumigation potential. When allowed to grow to early flower stage and then incorporated into the soil, they release compounds that act as natural fumigants, reducing soil borne disease organisms. Some biofumigant mustard varieties include Pacific Gold, Idagold, and Kodiak. Biofumigant blends include Caliente and Mighty Mustard. Biofumigant rapeseed varieties include Dwarf Essex and Bonar.

To use as a biofumigant, mustards will be allowed to go to full growth (early flowering) and then are chopped with a flail chopper (cut fine) and incorporated with a tractor mounted rototiller or other tillage tool for complete incorporation. Chopping releases the biofumigant compounds in the plants. Ideally the area then should be rolled with a cultipacker or overhead irrigated to seal in the biofumugant.

Finely chopped biofumigant cover crop ready for incorporation. Chopping releases the biofumigant compounds in the plants.

When used as a biofumigant, mustards should be grown as a crop. You need to add 60-100 lbs of nitrogen per acre to produce the maximum biomass. Nitrogen is also required to produce spring oats and annual ryegrass at similar rates. When planting mixtures with peas, nitrogen rates should be reduced.

Several spring-planted cover crops have been used specifically to address nematode infested soils. This includes “Nemat” arugula and “Image” radish. Mustards such as Caliente 199 have been used to reduce Phytophthora infestations.

Spring planted cover crops shown including mustards, rapeseed, radishes, and arugula.

Spring Biofumigants for Late Spring Planted Vegetables?

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

In fields with heavy vegetable rotations that have built up diseases (including nematodes), or fields known to have soil borne pest problems in the past, use of a biofumigant mustard or rapeseed crop planted in March ahead of late spring planted vegetables can help to reduce disease levels.

Mustard family plants produce chemicals called glucosinolates in plant tissue (roots and foliage). These glucosinolates are released from plant tissue when cut or chopped and then are further broken down by enzymes to form chemicals that behave like fumigants. The most common of these breakdown products are isothiocyanates. These are the same chemicals that are released from metam-sodium (Vapam) and metam-potassium (K-Pam), commonly used as chemical fumigants.

You should plant biofumigant rapeseed or mustard as soon as the ground is fit in March. They take 50 days to produce full biomass. Planting rates are 15 lbs/ A for most biofumigant mustards and 7-10 lbs/a for rapeseed. Add 60-80 lbs of nitrogen per A to grow the crop (the higher N level on sandy soils). Mustards are not winter hardy; however, biofumigant rapeseed is and can be fall or spring planted.

The goal is to produce as much biomass of the biofumigant crop as possible. This requires that you have a good stand, fertility, and sufficient growing time. The more biomass that is produced and that is incorporated, the more chemical is released.

The plant material must be thoroughly damaged so that enzymes can convert glucosinolates into isothiocynates. This means that you need to chop the material as much as possible and work it into the soil as quickly as possible, so as not to lose the active compounds to the air. A delay of several hours can cause significant reductions in biofumigant activity. The finer the chop, the more biofumigant is released. A flail mower is ideal.

The material should be incorporated as thoroughly as practical to release the biofumigant chemical throughout the root zone of the area that is to be later planted to vegetables. Poor distribution of the biofumigant crop pieces in the soil will lead to reduced effectiveness.

Sealing with water (or plastic for smaller areas) after incorporation will improve the efficiency (as with all fumigants). Soil conditions should not be overly dry or excessively wet.

Allow 10-14 days after incorporation before planting the next crop.

A March 15 planting will be ready to incorporate in mid-May and can be planted with the vegetable crop in late May (around Memorial Day). April 1 plantings be ready 2 weeks later.


Kodiak biofumigant mustard ready to incorporate.


Pacific Gold biofumigant mustard ready to incorporate


Caliente biofumigant mustard ready to incorporate


Dwarf Essex rapeseed a biofumigant option for fall or spring (it will overwinter)

Cover Crop Decisions for Vegetable Growers Part 2

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

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:

Small Grains
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.

Ryegrasses
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.

Brassica Species
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.

Cover Crops – Are You Growing Cover or Crops?

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Cover crop acreage has been growing in the region, largely due to nutrient management efforts and cost share programs. In the last year, there has been an emphasis on growing cover crops for soil health benefits and programs are underway from NRCS and Conservation Districts to increase cover crop plantings for soil improvement.

Nutrient management goals and soil health goals are not necessarily the same. You can think about this with the question are you growing cover or crops?

In nutrient management based cover crop programs, the goals are to have crops that can take up residual nitrogen and also provide cover to reduce erosion losses. Non-legumes predominate, with most of the acres planted in small grains such as rye with some recent use of radishes (Maryland programs are non-legume based while Delaware conservation district programs allow for the use of legumes). No fertilizer can be used with these cover crops. In this case the answer to the question above is that a cover is being grown. While there will be soil health benefits, they are not maximized.

In contrast, when soil improvement is the primary goal, the cover crops are grown as crops. You are growing plants to maximize the benefits they provide. To increase organic matter and improve soil health the main goal is to produce maximum biomass above ground and below ground. A secondary goal would be to provide different types of organic matter (such as with cover crop mixtures) to support a diverse soil microbial environment.

In other cases the goals will be different. With leguminous cover crops a goal may be to maximize the amount of nitrogen fixed. With soil compaction reducing crops such as radishes, the goal is to maximize the amount of “biodrilling” – the amount of tap roots being produced. With biofumigant crops, the goal is to maximize the production of fumigant-like chemicals the crops produce. With mulch based systems, the goal is to maximize above ground biomass.

What these soil improvement and specific use goals have in common is the need to treat the cover crop as a crop to optimize plant growth. This would include seeding at the proper rate to achieve optimal stands, planting at the right time, using seeding methods to get maximum seed germination and plant survival, having sufficient fertility to support good plant growth, providing water during dry periods, managing pests (insects, diseases, weeds), and inoculating legumes. If cover crop mixtures are being used, the ratios of seeds being planted must be considered to have the best balance of plants in the final stand.

The best cover crop stands are obtained with a drill or seeder that places the seed at the proper depth, at the proper seeding rate, with good soil to seed contact. Fertilization and liming programs should be used to support season-long growth – fertilizers and other soil amendments will be necessary in most cases. Nitrogen will need to be added for non-legumes.

When the crop is terminated is also key. The cover crops should be allowed to grow to the stage that maximizes the benefits they have to offer before killing the crops. Allowing a winter cover to grow for an extra week in the spring can make a large difference in the amount of biomass produced.

Again, consider the question are you growing a cover or a crop? The answer is important to achieve your cover crop goals.

Cover Crop Mixtures

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

There is significant interest in cover crop mixtures, and in some cases up to 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 cover crop 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 or functions they perform. 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 erosion control.

The first step in creating a mixture is to list the available species that can be used for the time of the year they will be planted in and the season(s) they will be growing in. For late summer and fall planting this would include small grains (wheat, barley, rye, winter oats, triticale), ryegrasses, rapeseed, other hardy Brassicas and winter annual legumes (crimson clover, hairy vetch, winter hardy field peas, subclover, many other clovers) for overwintering. 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 with strong tap roots 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. Cover crop mixture research has been very limited. With that said, there are some guidelines to follow.

As a general rule, you should reduce the seeding rate from a stand-alone (one cover crop) rate by the percentage that you want to see in the final stand. For example if you want to have at least 50% small grain cover in a mixture with radishes once the radishes winter kill you would reduce you small grain seeding rate by 50% (from 120 lbs to 60 lbs per acre). Radish seeding rates would also be reduced by 25-50%. In a 6 species cover crop mix if you wanted to have equal amounts of all components in the final stand, then start with a seeding rate at 1/6 (17%) of a stand-alone rate.

While this is a good place to start, the rates may need to be modified additionally based on how competitive (or quick growing) each component is. Quick growing species such as ryegrasses or radishes may need to be reduced relative to slow growing species such as the winter annual legumes to allow the slower growing species to become established. The slow growing species are often are maintained 75%-100% of stand-alone rates in mixtures with quick growing species.

Overseeding rates will need to higher than drilled rates and adjusted also allow for the potential for establishment as a overseeding. Species with lower establishment rates as overseedings will need to have additional seed in the mixture to compensate for lower emergence.

Further modifications both in seeding rate or species selection may be necessary based on residual fertility or planned fall fertility programs. If there is significant residual nitrogen in the soil or if fertilization is planned (to give some components more growth potential) then non-legume species will be favored and rates should be adjusted or legumes should be left out of the mixture.

Another issue is timing for seeding. Some species should be planted 4-6 weeks before a killing frost in September. This would include rapeseed, radishes and other Brassica species as well as winter annual legumes. Small grains can be planted through October. The mixture needs to be planted in the time period that best meets the earliest needs of the species in the mix.

A practical consideration for cover crop mixtures is how they should be best seeded when seed sizes between species are considerably different. Drills with both large seed and small seed boxes are good tools to address this issue. Small seeded species would be mixed and placed in the small seed box and larger seeded species in the large seed box in the appropriate ratios. Alternating rows by blocking off some drill spouts is another option. Spinner spreaders may not work well where there are large differences in seed sizes. Do not forget to inoculate all legumes in the mixture. Seeding rates may need to be adjusted based on the planting equipment available.

Some seed companies offer mixes that have already been blended at certain proportions. While some of these may be suitable for Delaware, growers should look at the specific proportion of each component in the mix to see if it will meet the guidelines listed above. Consideration also should be given to whether or not the mixture can be effectively seeded without seed separation with available equipment

A successful mixture used in Delaware in the past has been to plant rye and crimson clover with hairy vetch (seeding rates of 30, 10, and 15 lb./A, respectively). 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, planting windows and rotations.

Fall Strawberry Planting Season is Approaching

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Fall planting season for plasticulture strawberries is approaching in September. Growers who root their own tips will need four weeks depending on tray size. While 72 cell trays root faster, plants grown in 50 cell trays will be larger and will grow more quickly after transplanting. Rooting tips requires misting for several days while roots are established. Growers who buy plug plants should have already put in orders as nurseries only root limited amounts over what they have orders for.

The main source for strawberry tips that growers or nurseries root for fall plasticulture planting in the Eastern US has been Canadian nurseries. In 2012 there was a problem with viruses in one nursery source that a number of Delmarva growers received. Efforts have been made to reduce the potential for viruses in the source tips. Ideally, a tip producing nursery should receive virus free foundation material, increase that material by tissue culture and then increase in the field for a minimum of generations to take tips. Attention needs to be paid in tip growing areas so that enough rotational ground is available to avoid recontaminating the planting stock with viruses and tip producers should have a vigorous testing program for viruses. The take home message is know your tip source or ask the nursery you are working with for plug plants about their tip source.

Strawberries are very prone to root diseases. This is why, in major strawberry producing areas, fumigation has been a common practice (along with weed control). Fumigated strawberry beds almost always are higher producing than non-fumigated areas. There has been extensive research on fumigation alternatives to methyl bromide which is being phased out. Other chemical fumigants are still available but are somewhat less effective. Use of special plastic mulches that retain fumigants – virtually impermeable (VIF), or totally impermeable (TIF) mulch films – have improved fumigant effectiveness and reduced setback restrictions. However, these mulches will keep fumigants in the soil for a longer period making in necessary to have longer plant back periods in some cases. When using such mulches with available fumigants, make sure to follow manufacturer’s recommendations for waiting periods between fumigation and planting. The following site at the University of Florida has a good overview of research with these mulches http://edis.ifas.ufl.edu/in403.

For small growers who don’t want to use fumigation, do not have the capability for fumigation, or can’t meet setbacks, there are some alternatives. One of the more promising has been mustard seed meal incorporated prior to laying the plastic. While fairly costly, it does provide some fumigation action and also is a nitrogen source. Another alternative is to grow a crop of sorghum/sudangrass prior to the strawberry crop then chopping it and tilling into the soil. This should be done several weeks ahead of time also (it will take 4-6 weeks to grow the crop).

We still do not have many choices for varieties that work in our region. The majority of our plasticulture strawberries have been Chandler with some Sweet Charlie for extended (early and late) production. In discussions with Barclay Poling (NC State) last week, he stated that the North Carolina industry is largely planting Camarosa now. Experience in our region with Camarosa has been mixed in the past. However, after listening to Barclay, I would suggest that growers try more Camarosa on Delmarva. The key with Camarosa is that it needs to be planted earlier than Chandler. Camarosa has the potential to give longer production seasons than Chandler and will out-yield Sweet Charlie. While in the past we have thought of Camarosa as a shipping berry, Barclay stated that it is the most popular direct market berry in North Carolina now.

On the Chandler variety, planting date is so important. Target the first 2 weeks in September. Later plantings will not yield as well. However, research has shown that early row covers, while not increasing crown number, can increase flower number if Chandler is planted later and compensate somewhat for the losses. Adequate fall crown number is important with Chandler. However, early planting may produce too many crowns and lead to smaller berries next year.

I think there is great potential to replace some of our Chandler production with 2 day-neutral varieties: Albion and San Andreas. Both have excellent berry quality and when planted in the fall like Chandler will produce good spring crops (will start earlier than Chandler) with extended production into early summer. I have found the trick with Albion is to plant at a higher density. This is a variety that can be planted with 3 rows per bed, or with in-row spacing down to 8-9 inches. Both also give some potential for fall production in the planting year or the following year. For first year fall production, planting as early as possible is recommended.

Mustard Seed Meal as a Chemical Fumigation Alternative

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

With September strawberry planting season approaching for the annual plasticulture system, growers will be preparing beds and fumigating in the next 2 weeks. While several chemical fumigants are registered for strawberries, new fumigant use restrictions will make their use more of a challenge. In addition, strawberry growers that are organic or are using high tunnels with limited rotation are looking for effective fumigation alternatives.

One natural fumigant alternative that has shown great promise is mustard seed meal. According to researchers Dean Kopsell and Carl E. Sams, “studies conducted at The University of Tennessee showed that mustard seed meal has extremely high concentrations of isothiocyanates (ITCs). The seed meal is also a fertilizer source of nitrogen and other nutrients. When incorporated into the soil, ITCs act as effective biofumigants, reducing populations of pathogenic fungal species (Sclerotium, Rhizoctonia, Phytophthora, and Pythium), nematodes, weeds, and certain insect species.” ITCs are the same compounds found in some commercial chemical fumigants.

Specific studies with strawberries showed yield increases of as much as 50% compared to untreated controls using mustard seed meal. Additional research is going on in the region (Virginia, Maryland, Delaware, and Pennsylvania) with this material.

For mustard seed meal to be effective as a fumigant it has to be thoroughly worked into the bed area and plastic layed immediately after incorporation. The bed must remain evenly moist so the meal can break down (dry pockets will have delayed break down and can cause problems later) so a moist soil is important. A waiting period of 20 days is advised similar to a commercial fumigant before planting.

Current supplies of mustard seed meal come from Tennessee and costs $1.00-1.20 per pound. Recommended rate is 1000 lbs per mulched acre.

Because mustard seed meal is a natural compound, fumigant restrictions do not apply. It is also OMRI certified for organic production.

Biofumigant Mustards for Spring Vegetable Plantings

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

In fields with heavy vegetable rotations that have built up diseases (including nematodes), or fields known to have problems in the past, use of a biofumigant mustard crop planted in March ahead of spring planted vegetables can help reduce disease levels.

Mustard family plants produce chemicals called glucosinolates in plant tissue (roots and foliage). These glucosinolates are released from plant tissue when cut or chopped and then are further broken down by enzymes to form chemicals that behave like fumigants. The most common of these breakdown products are isothiocyanates. These are the same chemicals that are released from metam-sodium (Vapam) and metam-potassium (K-Pam), commonly used as chemical fumigants.

Three mustard varieties that have been successfully used for this purpose are Pacific Gold, Idagold, and Caliente 99.

You should plant these mustards as soon as the ground is fit in March. They take 50 days to produce full biomass. Planting rates are 10-15 lbs/ A for Pacific Gold and 15-20 lbs/A for Caliente 99 and Idagold. Add 40-80 lbs of Nitrogen per A to grow the crop (the higher N level on sandy soils).

The goal is to produce as much biomass of the biofumigant crop as possible. This requires that you have a good stand, fertility, and sufficient growing time. The more biomass that is produced and that is incorporated, the more chemical is released.

The plant material must be thoroughly damaged so that enzymes can convert glucosinolates into isothiocynates. This means that you need to chop the material as much as possible and work it into the soil as quickly as possible, so as not to lose the active compounds to the air. A delay of several hours can cause significant reductions in biofumigant activity. The finer the chop, the more biofumigant is released. A flail mower is ideal.

The material should be incorporated as thoroughly as practical to release the biofumigant chemical throughout the root zone of the area that is to be later planted to vegetables. Poor distribution of the biofumigant crop pieces in the soil will lead to reduced effectiveness.

Sealing with water or plastic after incorporation will improve the efficiency (as with all fumigants). Soil conditions should not be overly dry or excessively wet.

Allow 2-3 weeks after incorporation before planting the next crop.

A March 15 planting will be ready to incorporate in early May and can be planted with the vegetable crop in late May (around Memorial Day).