Posts Tagged ‘cover crops’

Using Winter Kill Cover Crops as a Part of Your Vegetable Cropping System

Friday, August 24th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Cover crops that will put on significant growth in the fall and then die during the winter can be very useful tools for vegetable cropping systems and the University of Delaware, University of Maryland, and other universities in the region have been conducting research on a number of these winter killed crops for use with vegetables.

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. Spring oats, mustards, and radishes can be planted from late August through September. Once into October, they do not put on adequate fall growth. Summer annuals should be planted in late July or during August for use in a winter killed system to obtain sufficient growth.

The winter of 2011-2012 was extremely mild and gave us a good look at issues that occur when crops that normally winter kill do not. In our plots at the Georgetown, DE research farm last winter, forage radish, oilseed radish, spring oats, and edible greens type mustard (Tendergreen) did not winter kill completely. All the biofumigant mustards (Pacific Gold, Idagold, Caliente, and Kodiak) winter killed completely (as did summer annuals).

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 peas, early sweet corn, and early snap beans.

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. 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.

A Quick Note on Cover Crop Management

Friday, March 2nd, 2012

Richard Taylor, Extension Agronomist;

In many areas of Delaware this winter has not only been lacking in significant snow cover but also marginal to deficient in rainfall, resulting in soil moisture levels that are at risk of becoming short as we move to corn planting season in April. Although the forecast does call for rainfall this week, growers who have established cover crops on their 2012 corn fields will need to carefully monitor their soil moisture levels. When spring growth begins, cover crops can remove a large amount of soil moisture in a relatively short space of time. This is not only because of their rapid growth rate in the spring but also because they have a well-developed and often deep root system already established. Cover crops can remove not only surface moisture but the subsoil moisture we often depend on to hold corn during early- to mid-summer drought conditions.

If rainfall between now and early corn planting time remains below normal, growers should think seriously about killing cover crops early, before too much soil moisture is removed. If using a systemic herbicide to kill the cover crop, you should also account for the week to two weeks it will take for the crop to die when determining the timing of herbicide application versus soil moisture levels.

Finally, a number of growers around the state planted the tillage radish or daikon radish as a cover crop this past fall. Although the weather was cold enough on some fields in New Castle County to winter kill the tillage radish, not all fields were completely killed. I suspect that the same is true in the lower counties of Delaware. You should carefully monitor these fields so you can make the decision on whether or not you will need to spray these fields with an herbicide to clean them up in time for corn planting time. Again, you should also monitor the subsoil moisture levels since this crop can send roots very deep into the soil. If it remains alive, a large amount of the subsoil moisture may be lost through transpiration as the radish enters the reproductive stage later this spring.

Cover Crops that Did Not Winter Kill

Friday, March 2nd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

With the mild winter, many cover crops have not fully winter killed including fall planted spring oats, forage radish, and some mustard species. For vegetable growers seeking to have early areas for spring planting, this will require that these cover crops be killed by herbicides, low mowing, or tillage. This will also limit the potential to no-till vegetables into these areas.

Cover Crops, Winter Annual Weeds and Spring Vegetables

Friday, September 23rd, 2011

Mark VanGessel, Extension Weed Specialist;

Using cover crops prior to early-spring vegetables like peas and sweet corn is becoming more common. One issue for growers who want to no-till into that killed cover is managing winter annual weeds, like henbit and chickweed. In this situation, glyphosate is usually used to kill the cover crops (such as barley, wheat, rye). But glyphosate often will not provide the necessary control of henbit or chickweed because the weeds have not started actively growing that time of year. One way around this is using a herbicide in the fall to “clean up the cover crop”. Using a broadleaf herbicide such as Harmony Extra or 2,4-D in the fall will control many of the broadleaf weeds and not limit crop rotation in the spring (replant intervals are 1.5 to 3 months). Then, when burning down the cover crop in the spring, the concern is killing the cover crop, and not worrying about the winter annual broadleaves that can be tough to control that time of year.

Cover Crops for Vegetable Rotations Revisited

Friday, August 12th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist;

August is here and it is time to consider late summer and fall cover crop options for vegetable rotations. Cover crop planting windows vary with crop and timely planting is essential to achieve the desired results. Here are some reasons to consider using cover crops in vegetable rotations:

Return 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.

Provide 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.

Reduce certain diseases and other pests. Cover crops help to maintain soil organic matter. Residue from cover crops can help 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.

Provide 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.

Improve 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.

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 levels of certain nematode 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.

Mixtures of rye with winter legume cover crops (such as hairy vetch) have been successful and offer the advantage, in no-till systems, of having a more rapidly decomposing material with the longer residual rye as a mulch.

Burning Down Cover Crops

Thursday, April 14th, 2011

Mark VanGessel, Extension Weed Specialist;

When burning down cover crops this spring, be aware of a few things. Gramoxone Inteon or glyphosate are your two best options, Gramoxone Inteon can be inconsistent with many of the grass cover crops, particularly with annual ryegrass. The addition of atrazine, simazine, or metribuzin will improve the control of Gramoxone Inteon for burndown; furthermore, Gramoxone is not as effective on cloudy, overcast days. Glyphosate is more consistent for control of grass cover crops, however, annual ryegrass is difficult to control and may require higher glyphosate rates. Tankmixing glyphosate with atrazine (or atrazine containing herbicides) can reduce the activity of glyphosate and tankmixing should be avoided when treating annual ryegrass. Glyphosate can also be inconsistent with some of the broadleaf cover crops. The addition of 2,4-D will improve control of legumes, rape or canola.


Spring Cover Crops for Vegetable Rotations

Friday, April 8th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist;

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 some benefit where vegetables are not scheduled until late May or June. Cover crop options for 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 if drilled. Increase seeding rates by at least 50% if surface broadcast. These cover crops can be no-tilled into soybean or corn stubble.

Spring Cover Crop Managment

Friday, March 11th, 2011

Richard Taylor, Extension Agronomist;

Although it seems like ancient history, many years ago when no-till technology was first beginning, Delaware and Maryland farmers were rapid adopters of cover crops for no-till grain production. Farmers mostly used cereal crops as winter cover crops. At the time, we learned some important lessons that we should remember this year because of the weather pattern that has occurred in a number of areas in Delaware.

Because there are a number of perceived environmental benefits with cover crops, government programs, as well as many environmentally-conscious growers have moved production agriculture back into heavy reliance on cover crops. Wheat and cereal rye are two popular cover crops, although some growers are using legumes, legume-cereal combinations, and even some other broadleaf crops such as the forage or Daikon radish. These cover crops are designed to protect the soil, add in organic residues, or supplement the soil with legume-derived nitrogen (N).

For any cover crop, whether it’s the grass cereals used for ground-covering, water-conserving mulch or legumes for spring N-fixation as well as for residue, I have found that there is a tendency to allow these crops to grow as much as possible by delaying herbicide or tillage or other cover crop control method as late as possible. In years when adequate rainfall occurs or good early season rainfall keeps the crop supplied, cover crops are not very harmful to soil moisture reserves or actually may be very helpful in drying out the surface soil. However, the season to be extra cautious in is the year when winter rainfall is below normal and this is followed by a dry early spring. The combination of lower than expected subsoil moisture level and rapid cover crop growth with heavy water use by the cover crop can lead to excessively dry sub-soil conditions.

The latter weather pattern seems to be developing in many areas of Delaware since winter rainfall has been below normal or the ground has been frozen during precipitation events. Growers need to monitor their subsoil moisture levels closely this spring and be prepared to terminate their cover crops earlier than normal if the subsoil becomes too dry. Early termination of the cover crop will allow time for subsequent rainfall to percolate into the subsoil and for the killed mulch to protect the soil from excessive water loss through evapotranspiration.

Growers or their consultants can check the subsoil moisture level with either the standard soil testing probe or with one that has an extended handle to make deep probing physically easier. It still is much of a “feel method” that depends on the experience of the person testing the soil. As a general rule, if subsoil is formed into a ball by squeezing it together in one’s hand and then the hand is opened and the ball easily falls apart with the least touch and no hint of moisture is present on the hand after making the ball, then the soil is on the dry to very dry side. The cover crop should be killed before the subsoil drops to the very dry state.

Late Summer and Fall Cover Crops for Vegetable Ground

Friday, August 28th, 2009

Gordon Johnson, Extension Ag Agent, Kent Co.;

Vegetable growers should make plans to put in late summer or fall cover crops after summer vegetables are harvested. Cover crops help to maintain organic matter, recycle nutrients, reduce compaction, and maintain overall soil health. These benefits far outweigh the cost of establishing the cover crops.

The following are some cover crops to consider:

Winter Annual Legumes
These cover crops will produce significant biomass (organic matter) and, at the same time, provide nitrogen for the following crop through biological nitrogen fixation: hairy vetch, crimson clover, field peas (winter peas). Hairy vetch makes an excellent mulch for no-tilling vegetables into. Plant by September 30.

Small Grains
These winter annual grasses will provide significant biomass, recycle nutrients (especially nitrogen), and produce excellent mulch for no-tilling vegetables in the spring: rye, triticale, wheat, barley, winter oats. Spring oats can be used where you want to get fall cover but need the crop to winter kill for early spring vegetable crops. Plant by the end of October.

Mustard Family Cover Crops
These include both fully hardy overwintering species and species that will winter kill. They provide significant organic matter, recycle nitrogen, can reduce compaction, and offer the potential for biofumigation. Plant by September 15. Included are:

Rapeseed and Canola – overwinter and are good biofumigants

Forage Radish, Oilseed Radish, and Daikon Radish – very good for reducing compaction in soils; forage radish winter kills, oilseed radish is more hardy

Mustards (brown and yellow mustards as well as garden mustard) – offer good biofumigant potential; half hardy

Turnips (forage and garden types) – good biomass production; half hardy

Kale (forage and garden types) – winter hardy; good biomass production

Hybrid Forage Brassicas (such as ‘Typhon’) – these are hybrid crosses of two or more species that will produce excellent fall growth and some will overwinter

Annual Ryegrass
This winter annual grass offers easy establishment, even when overseeded, and puts on significant fall and spring biomass. It scavenges nitrogen and is a quick decomposer in spring. Plant by October 15.

For seeding rates, contact you County Extension Office.

It is often advantageous to plant several of these cover crops together and most will mix well. Use the planting deadline for the species that has to be planted the earliest. Reduce the rate of each component in the mix by 1/3 to ½. I particularly like a rye-hairy vetch-crimson clover mix.

Fumigation Alternatives – Biofumigants

Friday, April 17th, 2009

Gordon Johnson, Extension Ag Agent, Kent Co.;

Crop rotation is a key to successful vegetable production. However, land limitations often make long term rotations difficult. Shorter rotations lead to a buildup of pests, with soil borne diseases being a major challenge for many vegetable crops. One approach to tighter rotations is to fumigate soils using commercially available chemical fumigants. Methyl bromide has been phased out and can only be used for certain exempted critical uses in specified states as listed by the EPA (go to for more information). Other fumigants such as chloropicrin; dicloropropene + chloropicrin; metam-sodium, metam-potassium, and iodomethane + chloropicrin are being used in this region where labeled. Numerous experimental materials are being studied for use as soil fumigants and may receive labels in the future. A major drawback to chemical fumigation is material cost. There are also application requirements and equipment considerations to take into account.

There has been considerable interest in the use of certain crops as biological fumigants ahead of vegetable production to reduce the need for chemical fumigation, especially in tight rotations. These are crops that would be grown for their naturally occurring compounds that kill soil borne pests. Plants in the mustard family, such as mustards, radishes, turnips, and rapeseed, and Sorghum species (sudangrass, sorghum-sudangrass hybrids) have shown the potential to serve as biological fumigants. Research in this region and across the country has shown some promise in using these crops to reduce soil borne pests. Plants from the mustard family produce chemicals called glucosinolates in plant tissue (roots and foliage). These glucosinolates are released from plant tissue when it is 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. Sorghums produce a cyanogenic glucoside compound called Dhurrin that breaks down to release toxic cyanide when plant tissue is damaged.

While rapeseed, mustards, radish species, sudangrass, sorghum-sudangrass hybrids and other related species have shown some promise as biofumigants, results in Delaware have been inconsistent, often with minimal benefits. It is important to note that success with biofumigant crops depends on a number of factors. The following are some suggestions to achieve the best results:

● Plant biofumigant crop varieties selected or bred for higher levels of active compounds if available.

● 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. However, as plants mature, the will reach a point where levels of these active chemicals will decline and you should not let the plants go to seed. There is also the practical consideration that it is difficult to do a good job of incorporation with too much biomass. With a crop like sudangrass, this means you cannot let it get too tall.

● The plant material must be thoroughly damaged so that enzymes can convert glucosinolates into isothiocynates or so that the Dhurrin is converted into cyanide. 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 to not 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.

● 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 efficacy (as with all fumigants). Soil conditions should not be overly dry or excessively wet.

Biofumigant crop systems include overwintering rapeseed; early spring planted mustards, cover-crop type radishes, or turnips; and late spring planted sorghum/sudangrass. Each has the potential to fit different vegetable cropping sequences for early, mid, or late season production.

Different species and cultivars of biofumigant crops contain varying levels of active compounds. There have been significant efforts to evaluate species, and varieties within a species, for levels of biofumigant chemicals produced, as well as some breeding and selection efforts to increase levels of these compounds. This work is on-going in different parts of the US. Where varieties with higher levels of active chemicals are available, they should be used.

A major limitation to biofumigant crop use is the fact that you cannot deliver high enough levels of the active chemicals to do a complete fumigation job and the biofumigation effectiveness is also limited by the depth of incorporation. However, you are adding organic matter and do get the benefits associated with that addition.

The bottom line: biofumigant crops can suppress soil borne pests but are not a full replacement for chemical fumigants, nor are they a substitute for adequate rotations. However, using biofumigant crops in combination with disease tolerant vegetable varieties can allow for a greater chance of success in tight rotation situations.

There also has been considerable research into the effect of cover crops (such as hairy vetch), compost, and other organic additions on disease suppression in vegetable crops with some positive results. The mechanisms for these benefits may include some chemical suppression. However, other mechanisms such as increasing diversity of the soil microbial community (resulting in increased competition with pathogens) and promoting growth of beneficial microbes that serve as biocontrol agents may be more important.