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

Cover Crop Decisions for Vegetable Growers Part 2

Gordon Johnson, Extension Vegetable & Fruit Specialist;

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.

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 Crop Decisions for Vegetable Growers I: Basic Considerations & Winter Killed Cover Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist;

With cover crop season coming up, vegetable growers will have decisions to make on what cover crops to plant and how best to grow and use them. The following is the first in a series revisiting this topic.

Cover crop acreage has been growing in the region, largely due to nutrient management efforts and cost share programs. In the last 2 years, 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. 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 cover crops in these programs. 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 in order to optimize plant growth. This includes 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.

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 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 sun hemp 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 late July or during 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.

All of 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 radishes 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 in mild winters they may not fully winter kill.

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 also in 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 opens up dozens of options for strip tilling (seed or transplanted) spring vegetables.

No-Till and Vertical Tillage for Processing Vegetables

Gordon Johnson, Extension Vegetable & Fruit Specialist;

No-Till Processing Vegetables
There is increased interest in no-till production of processing vegetables. No-till production is possible for most of our common processing vegetables. However, success will depend upon a number of factors. For spring planted crops, soil temperature and emergence will be the dominant issue. For summer plantings, especially into small grain stubble, soil seed contact and stand emergence will be a major issue. When planting any vegetable crop no-till into cover crops, residue management is a major challenge and for all mechanically harvested crops, contamination by previous crop residue at harvest is a common concern along with harvest recovery.

No-Till Peas
Research at the University of Delaware in 2012 showed that peas planted into winter-killed cover crops yielded equal to or better than peas planted conventionally (March planting). Yields were highest in plots where forage radish or oil seed radish winter killed. No-till peas after winter killed mustards also performed well. In contrast, peas no-tilled after winter killed spring oats did not perform as well as conventionally planted peas. It should be noted that 2012 was exceptionally warm in March. Success with early no-tilled peas will depend upon soil temperature and ground cover. Lower residue systems such as winter killed radishes or soybean stubble would be best adapted for no-till peas.

No-Till Sweet Corn
Sweet corn can be successfully no-tilled. However, a major concern for early planted no-till sweet corn into decaying crop residue or killed cover crops is seed corn maggot. Even with insecticidal seed treatments, seed corn maggot can overwhelm early plantings in some conditions and reduce stands significantly. In a 2012 experiment, April planted processing sweet corn planted into killed forage radish cover crop performed poorly when compared to conventional plots due to losses to seed corn maggot. Another issue is cold soils and delayed emergence. While most processing sweet corn varieties compensate well for reduced stands, early no-till plantings are still at risk for reduced yields. No-till sweet corn will be most successful from mid-May onward. Use of row cleaners can help to make no-till more successful in early planted systems.

No-Till Lima Beans
Lima beans have been successfully no-tilled in the past. The main issue has been with residue at harvest. In 2012 trials, no-till lima beans did not perform as well as conventionally planted lima beans after wheat. This difference was most pronounced where stubble was close mowed prior to planting versus planting into standing stubble. Trials in 2013 will focus on stubble height in no-till systems after small grain with lima beans.

No-Till Snap Beans
Snap beans have been successfully no-tilled. In discussions with growers and green bean processors, green beans no-tilled into barley stubble performed very well. We will be evaluating no-till snap beans in 2013 in different stubble heights after barley. Early planted snap beans (April and May) also have the potential to be no-tilled into areas with winter killed cover crops and we will be evaluating no-till snap beans after winter killed forage radishes in 2013. Early terminated small grain cover crop would also be a possibility with no-till snap beans.

Using Vertical Tillage with Processing Vegetables
Growers are interested in the use of vertical tillage tools with processing vegetables. The success of this system will depend on the type of cover prior to planting. In 2012 research, vertical tillage prior to pea planting performed as well or better than conventional tillage in areas with winter killed forage radishes. Processing sweet corn performed equal to conventionally planted sweet corn in vertically tilled ground again after a winter killed forage radish crop. Research in 2013 will look at vertical tillage in a number of crop residues and processing crops.