How Much N Could You Expect from Your Cover Crop?

Amy Shober, Extension Nutrient Management and Environmental Quality Specialist; and Jarrod O. Miller, Extension Agronomist,

Small grains or legumes are commonly planted as cover crops in Delaware. While small grains are good at scavenging left over soil nitrogen (N) in the fall, they are not as likely as a legume cover crop to release that N early in the spring. It is helpful to remember that the C:N ratio of a cover crop can predict N availability to crops in the spring. A cover crop with a C:N < 20 at termination will be easily broken down by soil microbes and release N to the soil. In contrast, a cover crop with a C:N >30 at termination could result in tie up (immobilization) of soil N. If N is tied up by microbes, it will not be available for the emerging crop in the spring but may be available later in the season as that residue breaks down.

The timing of cover crop termination will directly affect the C:N ratio and N availability to the emerging crop. For small grain cover crops, the C:N ratio will increase past the boot stage, as more N is moved from the stem to the developing head. Therefore, earlier termination of small grain cover crops will result in more N released from the cover crop residue in the early spring. However, early termination may reduce the ability of small grain cover crops to provide other benefits, like weed suppression. So even though the C:N ratio will increase as the small grain cover crop develops, you may choose to terminate late to get these other benefits. Leaving high C:N ratio biomass on the surface (no-till) will reduce the N tie up by microbes. However, if you decide to incorporate residues with tillage, you may increase early season N tie up because the residue has more contact with soil microbes following incorporation.

Later termination of a legume cover crop has the opposite effect on soil N availability. Terminating a legume cover crop prior to bud stage (March to early April) will result in little to no N contribution from the cover crop. This is because N fixing bacteria do not have enough time to form nodules on the roots of the legume cover crop prior to bud stage. Legumes with more biomass will contribute more soil N in the spring. Termination at flowering stage will result in the most available N. A red clover or crimson clover cover crop terminated at flower stage could contribute 40-80 lb N/acre depending on the quality of the stand. If the stand is poor (<2 ton/ac), you can expect available N to be on the lower end of the scale. In contrast, a good stand (>3 ton/ac) will provide N toward the upper end of the scale. Similarly, hairy vetch, while tricky to manage, could contribute between 50-100 lb N/acre if terminated late. If termination occurs at bud stage, you can expect available N to be approximately 50% of what you could get with termination at flowering.

Spring Cover Crops for Vegetable Rotations

Gordon Johnson, Extension Vegetable & Fruit Specialist;

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;

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.

What to Think About When Using Cover Crops for Weed Control

Mark VanGessel, Extension Weed Specialist;

Cover crops can provide many benefits, including preventing soil erosion, improving soil structure, alleviating compaction, nutrient management, and weed control. The most successful approach with cover crops is to identify your goals, and then select the best cover crop(s) and actively manage the cover to reach the goal(s). When considering cover crops to help with weed control, consider a cover crop species that will not only produce a lot of biomass, but also produce biomass that does not breakdown quickly. Local research has shown that weed density can be reduced by more than 50% with a cover crop and cover crops can outcompete weeds for light resulting in small weeds that are easier to control with other tactics, including herbicides. Use of cover crops has shown promise for resistance management by helping to reduce selection pressure when herbicides are used. Most cover crops in Delaware are planted in the fall and terminated in the spring, so they have the potential to affect both winter and summer annual weeds.

For fall planted cover crops, cereal rye or triticale are the best two species for weed control from our research (annual ryegrass also has many similar attributes, but it can be difficult to kill in the spring). They both shade the ground quickly in the fall because they emerge fast and grow late into the fall, and they grow more prostrate than other grass species to provide ground cover very soon after emergence. Legume cover crops are not as effective for winter annual weed control because they are slow to establish and often do not produce significant amount of biomass until the spring. In addition, legume biomass tend to breakdown much quicker than winter cereals, reducing their ability to suppress late emerging summer annual weeds. Winter sensitive cover crops, such as forage radish, produce a significant amount of biomass in the fall, and are a good option for control of winter annuals such as henbit or chickweed. However, the biomass breaks down quickly in the early spring and they provide very little suppress of summer annual weeds.

Both have cereal rye and triticale have a wide window for planting and quite adaptable to a range of soil conditions. Forage radish has a very short window for successful planting, while legumes have a wider window, though is not as wide as cereal species. Getting a uniform stand across the field is critical for maximum weed control or suppression. Where the cover crop stand is not uniform, the weeds are not subjected to the plant competition, the weeds are more numerous and larger, and can be harder to control than those plants growing amongst the cover crop. Achieving this uniform stand is best with a drill due to the improved soil to seed contact. Other seeding methods can sometimes provide good stands, but the results have not been consistent. Many farmers are compensating for the challenge of achieving a good stand by increasing their seeding rates compared with rates used when drilling.

Most of the local research has used 2 bu/A of cereal rye (or triticale) because that was the seeding rate for cost share programs, but experience tells us we can reduce that seeding rate without sacrificing weed control. We do not have enough local research to say how low we can plant rye and still get the same level of weed suppression. We are asking that question with ongoing research.


Herbicide carryover: what herbicides were used throughout the summer (and when)? Is there a risk that the herbicide can slow the growth or reduce the stand of the selected cover crop species? Forage radish and legumes tend to be more sensitive to herbicide carryover than the cereal species.

Cover crop termination (when): the maximum benefit of the cover crop for weed control occurs when the cover crop is terminated as late as possible. Terminating the cover crop at or close to flowering/heading time allows for the high levels of biomass, but also allows for the maximum production of lignified tissue (which is the tissue most resistant to breaking down). Terminating the cover crop more than 2 weeks before planting lessens the weed control benefits. Earlier termination of the cover crop results in less biomass production, less lignified tissue produced, and longer opportunity for the cover crop to breakdown (and less weed suppress after planting). UD Weed Science is examining the weed control benefits of planting green, and we have some promising results, but more work is needed.

Cover crop termination (how): ask yourself how you will terminate the cover crop in the spring. It is best to know how difficult it might be to kill the cover crop; what herbicides are needed to control the cover crop; and will the herbicides used to kill the cover crop interfere with planting your spring crop. If planting a cover crop mixture, consider how you will kill all the species. Do not plant something that you cannot successfully terminate in the spring. Annual ryegrass is one of the species that can be challenging to kill in the spring.

Is an herbicide needed at planting: Are weeds present when you are planting your cover crop, how big are they? If winter annual weeds have emerged before you plant your cover crop, you may see benefits from a burndown herbicide. Winter annual weeds emerging after planting are seldom a problem with a competitive stand of cereal rye or triticale, but weeds up before planting could compete with the cover crop and limit it from reaching its full biomass potential.

Fertility levels: some cover crop species’ growth can be reduced if there are low levels of soil nutrients (including micronutrients). Tillage radish is more sensitive to fertility levels than some of the cereal cover crop species.

Multi-species mixtures: using more than one species is a great strategy for some soil health objectives. However, as more species are added to the mixture (and the amount of any one species is reduced) the benefits for weed control could be reduced. Terminating a multi-species cover crop could be difficult.

In the early spring newsletter we will discuss related topics to using cover crops for weed control, including challenges:

  • Cover crop/cash crop competition (moisture/light/nutrients)
    ● Volunteer cover crops
    ● Pests (slugs)
    ● Additional expense / management
    ● Establishment a good crop stand

Winter Killed Cover Crops for Vegetable Cropping Systems Revisited

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Cover crops that put on significant growth in the fall and then die during the winter can be very useful tools for vegetable cropping systems. These winter killed cover crops add organic matter, recycle nutrients, improve soil health, and allow for earlier spring vegetable planting.

Winter killed cover crops that are late-summer and early-fall planted include spring oats, several mustard species, and forage and oilseed radish. Earlier planted summer annuals (millets; sorghums, sudangrasses, and hybrids; annual legumes such as cowpeas or forage soybeans; buckwheat and many others) can also be used as winter killed species. Timing of planting will vary according to the species being used and winter killed species selection will depend on when fields will be available for seeding. Summer annuals should be planted in August for use in a winter killed system to obtain sufficient growth.

Spring oats and mustard species can be planted from late August through September. For best effect, forage and oilseed radishes should be planted before the middle of September. Spring oats, radishes and mustards are not suited for October or later planting because they will not produce adequate fall growth.

The winter killed non-legumes mentioned above will benefit from the addition of 30-60 lbs of nitrogen.

The following are several options for using winter killed species with vegetables:

1) Compaction mitigation for spring planted vegetables. Where there are compacted fields, the use of forage radish has worked very well as a winter killed cover crop by “biodrilling”. The extremely large taproot penetrates deep into the soil, and after winterkilling, will leave a large hole where future crop roots can grow. Oilseed radish also provides considerable “biodrilling”. Winter killed radishes works well with spring planted crops such as spinach, peas, early sweet corn, and early snap beans. One issue with radishes is that on mild winters they may not fully winter kill.

Fall growth on mustards and radishes that will then winter kill. A potential winter kill mix would include a radish, a mustard, and spring oats.

2) Early planted vegetables. A wide range of early planted vegetables may benefit from winter killed cover crops. For example, peas no-till planted or planted using limited vertical tillage after a winter killed cover crop of forage radish, oilseed radish, or winter killed mustard have performed better than those planted after conventional tillage. Early sweet corn also has potential in these systems as do a wide range of spring vegetables including spinach, potatoes, and cabbage. Winter killed radishes and mustards also have the advantage of outcompeting winter annual weeds leaving relatively weed free fields and recycling nutrients from the soil so that they are available in the spring for early crops (decomposition has already occurred).

3) Mixed systems with windbreaks for plasticulture. By planting planned plasticulture bed areas with winter killed cover crops and areas in-between with cereal rye you can gain the benefits of these soil improving cover crops and eliminate the need make tillage strips early in the spring. The winter killed areas can be tilled just prior to laying plastic.

4) Bio-strip till. By drilling one row of forage or oilseed radish and other adjacent rows with rye or other small grains, you can create a biodrilled strip that winter kills and that can be no-till planted into the spring without the need for strip-till implements. This presents dozens of options for strip tilling (seed or transplanted) spring vegetables.

Considerations for Postemergence Herbicides and Planting Cover Crops this Fall

Mark VanGessel, Extension Weed Specialist;

Most of the herbicides we use in corn and soybeans are compatible with planting cereal rye cover crop in the late summer or fall. However, use of residual herbicides such as Dual, Zidua, Outlook, or Warrant postemergence could pose a risk; particularly if the cover crop is planted prior to mid-September. Broadleaf cover crops such as forage radish or legumes are more likely to show injury from herbicides, and the sooner after application these cover crops are seeded the greater the risk for injury. We have seen stunting in the forage radish from Valor or Spartan. Postemergence applications of ALS herbicides (Group 2) or fomesafen (Reflex or FlexStar) have the greatest risk of injuring forage radishes.

Hairy vetch appears to be more tolerant of herbicide carryover than crimson clover. Halex, Zidua, and atrazine in corn and fomesafen, Warrant and ALS herbicides in soybeans are the greatest risk to crimson clover.

Irrigated fields allow more herbicide degradation by microbes and less the risk of injury. Herbicide evaluation for carryover to cover crops is still an active area of research at UD and other universities and with more research, we hope to provide detailed guidance on this topic. However, if cover crops are an important part of your cropping system, give careful thought to the herbicides you use this spring to reduce the risk of cover crop injury this fall.

Summer Soil Building Crop Options for Delmarva Vegetable Growers

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Where possible, vegetable growers should consider the use of summer soil building crops. This can be between spring and fall crops, prior to mid-season plantings or anytime there is about 6-8 weeks of fallow time. Use of these summer soil improving crops can help maintain or increase organic matter levels, address certain soil disease issues (fungal pathogens, nematodes), add nitrogen to the soil (in the case of legumes), reduce weed pressure, and improve soil physical characteristics.

The following are some soil building crops for summer use that I recommend:


Cowpea (Vigna unguiculata)
Also known as blackeye or southern pea, this crop is underutilized in our area. It is fast growing with peak biomass often in 60 days. Cowpeas can fix up to 100 lbs of N per acre with biomass of 3000-4000 lbs/a. Cowpeas grow well in poor soils and can handle droughty conditions. Drill at 40-50 lbs per acre. Certain varieties such as California Blackeye #5 and Mississippi Silver are poor nematode hosts and will be beneficial in systems where root knot nematode is a problem. See this site for nematode ratings of different cowpea varieties Cowpeas also can be harvested in the immature pod stage as a fresh legume so can serve dual purpose in small farms.

Soybean can also be a good cover crop drilled at 60 lbs per acre. Forage-type soybeans produce considerable biomass and make excellent cover crops. For nematode suppression, use of root knot nematode resistant varieties may be beneficial. Edamame types can be harvested and sold in green pod stage and the residue returned to the soil for soil building, again serving a dual purpose on small farms.

Sunnhemp (Crotalaria juncea)
Sunnhemp is a tropical legume that is used extensively for soil building in countries such as Brazil and India. Drill 20-30 lbs of seed per acre. Sunhemp can produce very high amounts of biomass (10 ton biomass is not unheard of in Florida – amounts will be lower here on Delmarva, expect 3-4 tons). It is a high nitrogen fixing legume and can contribute over 100 lbs of N to a following crop. Sunhemp grows very fast in the summer, reaching 6 feet or taller in 8 weeks. However, a better way to manage sunnhemp is to let it grow to about 1-3 feet tall, then mow it and let it regrow again. If allowed to get too tall and old the stems will become tough and fibrous and will not decompose rapidly. Sunnhemp is a day length sensitive crop. It will grow any time during the summer, however it will not flower and go to seed until the days start getting shorter in very late summer.

Non Legumes

Sudangrass and Sorghum-Sudangrass hybrids (Sorghum bicolor x S. sudanense)
Sudangrass is a forage crop in the Sorghum family. Sorghum-sudangrass is a cross between forage or grain sorghum and sudangrass. These are warm-season annual grasses that grow well in hot conditions and produces a large amount of biomass. Plant at 20-40 lbs per acre drilled. Of all the non-legumes, it is the most useful for soil building. Sorghum-sudangrass will often reach 6 ft in height. Like sunnhemp, it can be mowed and allowed to regrow to enhance biomass production and have younger material that decomposes more quickly. Expect 3-4 tons of biomass addition per acre. As a grass, to get the most growth you will need to add nitrogen fertilizer (40-80 lbs/a). If incorporated at a young stage, the nitrogen will be re-released for the following crop. Sorghum-sudangrass is very effective at suppressing weeds and has been shown to have allelopathic and biofumigant properties. Research on nematode suppression by sorghum-sudangrass is mixed with some studies showing that sorgum-sudangrass suppresses nematode levels. Choose finer stemmed, leafy varieties when available. Brown midrib types will decompose more quickly because they have less lignin.

Forage-type Pearl Millet (Pennisetum glaucum)
Pearl millet is a tall summer annual grass that grows 4 to 8 ft. tall. It is well adapted to sandy and/or infertile soils and does well in the summer heat. Forage types are better adapted for soil improvement than the grain types. Seed at 20-30 lbs/a drilled. Expect 3-4 tons of biomass addition per acre. Again, as a grass, to get the most growth you will need to add nitrogen fertilizer (40-80 lbs/a). Pearl millet has been shown to suppress some nematodes. Forage pearl millet can make a good mulch for late-summer planted crops no-till or strip till.

All these crops above can be planted from late May through late July for soil improvement use.

There are many other possibilities for summer soil improving including several other millets, brassicas, and buckwheat, however the ones listed above are my recommendations for growers on Delmarva to try.

Issues to Consider with Cover Crop Management

Mark VanGessel, Extension Weed Specialist;

With brassica cover crops in full bloom and crimson clover starting to bloom, remember why you planted the cover crop in the first place.

Most benefits of cover crops are maximized by allowing cover crops to develop later into the spring. As cereal rye gets taller, the stems develop more lignin and are more resistant to breaking down and they last longer on the soil surface. The more “lignin-ified” the tissue becomes, the longer it provides weed suppression and the better it will prevent moisture evaporation from the soil. The longer the legumes are allowed to grow, the more nitrogen being produced for the cash crop. Be aware that by allowing cover crops to reach the flowering stage, the cover crop is producing viable seeds that could cause an issue in subsequent crops. Volunteer cover crops are the biggest challenge if small grains will be planted in the fall.

While more cover crop biomass is better for some objectives, later terminated cover crops can pose some challenges. At this time of year, cover crops are using a lot of soil water. They can deplete soil moisture quickly, leaving a dry seedbed for the cash crop. In addition, by depleting the soil moisture, the cover crops themselves will undergo stress and maybe more difficult to control. So, if you have not killed your cover crop yet, be sure to look at the weather forecast for rain, and do not put yourself into a difficult situation by allowing the cover crop to use all the soil moisture if there is a poor chance of rain. On the other hand, you may consider delaying terminating the cover crop until rain comes and replenishes soil moisture.

We have done some work with planting green, meaning not killing the cover crop until after planting. This has potential, particularly if the cover crop was planted late and an additional 10 to 14 days of growth will provide substantially more biomass. Be sure you have a planter outfitted with plenty of weight and sharp cutting coulters to provide a good seed furrow. Your planter should be outfitted with a guidance system because it will be very difficult to use row markers. Apply your burndown herbicide after planting and use high gallonage to achieve good coverage of the cover crop and weeds present. We use 20 gallons per acre and that seems to be adequate.

Planting is best if the cover crop is either dead or not sprayed yet. Cover crops that are in the process of dying can be challenging to plant into because the stems are harder to cut with a coulter. We typically plant into a standing cover crop and have been very successful in getting a good crop stand. In addition, we typically do not roll our cover crop when planting soybeans. We plant in 15-inch rows and all the planter units are on a single tool bar. This single line of planter units knocks over most of the tall rye. However, if 15-inch planter units are on two tool bars, I have seen more of the rye to remain standing after planting.

There is still a need for residual herbicides for most fields with cover crops. The benefit of the cover crop for weed management is fewer weeds present (weed density is reduced) and growth of the weeds is slower, so weeds are smaller when postemergence herbicide applications are made. So, cover crops improve overall weed control; but in fields with heavy weed pressure, they do not replace herbicides.

Guess the Pest! Week #4 Cover Crop ID Answers

Bill Cissel, Extension Agent – Integrated Pest Management;

Congratulations to John Swaine for correctly identifying the cover crop species and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Photo 1: Austrian Winter Peas

Photo 2: Hairy Vetch

Photo 3: Winter Rape

Photo 4: Forage Radishes. This was a little misleading since we had a good winter kill. If you look carefully, you can see the divots where the radish roots once were.

If you didn’t guess all the species correct or if you would like to find out about some of the research and demonstrations that are going on in Delaware, join us at one of the Cover Crop Twilight Tours this coming week.

You will have an opportunity to walk through the demonstrations planted last fall including cover crop species, cover crop mixes, and cover crop planting date comparisons. Discussions will also include issues associated with terminating cover crops, slugs and insect issues, effects on weeds, soil health, and planting.

For more specifics visit

Strip-Till, Biological Strip-Till and No-Till Systems Using Cover Crops for Seedless Watermelon Production

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Strip-Till, Biological Strip Till, and No-Till Systems Using Cover Crops for Seedless Watermelon Production

Seedless watermelons are the most important fresh market vegetable crop on the Delmarva Peninsula with over 5,000 acres grown annually on over 150 farms

Considerable production costs are incurred to grow seedless watermelons including transplants, plastic mulch, drip tape, irrigation (pumping), fertilizers, and pest control. Over 95% of seedless watermelons are grown on black plastic mulch in a tillage and input intensive system.

Current systems require several tillage operations prior to laying plastic. Heavy tillage reduces organic matter levels in the soil by increasing decomposition rates, destroys soil structure, and negatively affects soil health. Compacted areas between beds allow water to accumulate and can increase disease pressure in wet years as evident with the high amounts of Phytophthora fruit rot in watermelon fields on Delmarva in 2017.

Plastic mulch use adds extra cost to production, requires addition labor and time to apply, requires hand labor and machine use for removal, and must be disposed of in landfills. Degradable mulches are available and do offer another option for watermelons, however there is a high up-front cost in their use.

In a standard production system, over 130 lbs. of nitrogen are applied using inorganic nitrogen sources, another input cost (manufactured from fossil fuels), There are a minimum of 4 trips across the field with tillage and plastic laying equipment with associated fuel cost.

There is increased interest in no-till and strip till systems using killed cover crops for seedless watermelon production for later season plantings (late May and June) to reduce costs, reduce the risk of Phytophthora fruit rots, and maintain soil health. Another option is to transplant into barley stubble after harvest in June. These systems will not produce early watermelons but can improve the economics of later plantings.

No-till production of transplanted vegetable crops has been researched and demonstrated on-farm over the last two decades and no-till systems have been shown to be as productive as plasticulture based systems.

Research by Johnson and Taylor in Delaware in the 1990s showed the potential for no-till transplanting vegetable crops into rye cover, using a rolling corn stalk chopper to roll kill the rye (newer systems use a chevroned roller/crimper specifically designed to roll kill cover crops). Vegetables successfully grown with this method included pumpkins, cantaloupes, watermelon, tomatoes, and peppers. Additional studies looked at cover crop systems and no-till transplanting of vegetables into hairy vetch, crimson clover, hairy vetch-rye-crimson clover mix, and subterranean clover cover crops. This research showed that crops of squash could be grown with no additional nitrogen in killed legume covers.

Chevron bladed roller crimper for rolling cover crop prior to transplanting.

The University of Delaware conducted additional research evaluating no-till and biological strip till methods for seedless watermelon production. The goal was to reduce input costs while maintaining productivity, eliminate plastic mulch in production, maintain or improve soil organic matter and soil health, provide a portion of nitrogen fertilizer biologically, decrease fruit rots and other diseases, and decrease machine and labor costs.

Use of forage radish in a biological strip till system (winter killed forage radish strips with rye in between) was demonstrated for seedless watermelon and cantaloupe production at the University of Delaware in 2013. Additional research was conducted at the University of Delaware in 2014 with biological strip till using rye, hairy vetch, crimson clover and mixed systems with winter killed forage radish strips.

Biological Strip Till Systems in 2015
A one-acre plot was dedicated to this study. Cover crops were planted in early September 2014 for the 2015 study. A biological strip till system uses a one row strip of forage radish surrounded by the cover crop on either side. This is accomplished by blocking or dedicating seed meters in a drill. A diagram is shown below:


(C = Cover Crop. R = Forage Radish)

Cover crop combinations are given in the treatments below. The forage radish winter killed and deteriorated, leaving a strip with holes (the biological strip till). Cover crops were rolled using a roller crimper after rye headed but before anthesis and when full biomass was achieved with legumes. Additionally, non-selective and pre-emergence herbicides were applied after rolling. Seedless watermelons and pollinizer plants were set by hand. It has been shown that transplants can be set directly in the hole left by the forage radish that winter kills. Drip irrigation was used in both the plasticulture and biological strip till systems.

Treatments with the single row of tillage radish in the middle and cover crops on either side included:

1)       Roll killed rye

2)       Roll killed vetch

3)       Roll killed crimson clover

4)       Killed subterranean clover

5)       Roll killed rye-vetch

6)       Roll killed rye-crimson clover

7)       Black plastic mulch (control)

Results indicate that biological strip till systems, when planted later in the season, can be a viable alternative to plasticulture systems. The best cover/radish combination for weed management was the rye/crimson clover mix.

Yield of seedless watermelons in a biological strip till system by variety and cover crop, Georgetown, DE 2015

Tillage Based Strip Till
Tillage based strip till systems can also being used to grow seedless watemelons. In this system strips are tilled using a strip tillage implement with coulters or with mini rotavators. Transplants are set with a transplanter designed to go through some trash or that punches holes in the ground for the transplant.

Strip-till Implement

No-till for Seedless Watermelons
No-tilling into rolled cover crop or into barley stubble can also be successful with seedless watermelons. The key to success with this system is to have soils in good condition that will allow a no-till transplanter to function properly (cut a slot and then close around the transplant). To make this function, soils need to have a sufficient moisture level at transplanting.

All Systems
In each of these systems, addition of a legume cover crop such as hairy vetch or crimson clover can provide a portion of the nitrogen to grow the watermelon crop (credit 60-90 lbs of N/acre). Thick cover crop stands producing high amounts of biomass will serve as a mulch for weed control and will also serve to keep fruit off the ground, limiting fruit diseases. Good transplant to soil contact at planting is essential and equipment must be set up correctly to achieve this. Additional fertilizers can be applied before or at planting and can be sidedressed.

Strip-till and no-till production systems are adapted to overhead irrigation. Drip tape can be applied in strip till systems using properly modified equipment to place in the ground next to plants. Surface applied drip tape is not recommended.

The biggest challenge in each of these systems is weed management, especially in the row. Non-selective herbicides are used before transplanting along with a residual program. Other residuals can be applied between rows with a shielded sprayer. Post emergence applications are limited to grass materials or shielded applications. Irrigation is necessary to activate residual herbicides. See the 2018 Mid-Atlantic Commercial Vegetable Production Recommendations for specific guidance