Cover Crops

Cover Crops

  • By Cleve Campbell
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  • September 2015 - Vol.1 No. 9
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  • 1 Comment

The gardening season is winding down, the days are getting shorter and the nights are getting cooler. After an intense spring and summer of planting, harvesting, weeding and fighting that endless war on pests, it’s time to take a gardening break. That was my gardening philosophy for many years, and unfortunately, I was missing out on an opportunity to improve my garden. As Ben Franklin said, “Watch the pennies and the dollars will take care of themselves.” Just like taking care of pennies, it’s often the little things we do in the garden that make a world of difference.

One of the most important things a gardener can do to improve the soil is to add organic matter in the form of compost, manures or other organic materials, such as leaves, straw, or grass clippings. Earthworms, bacteria, fungi, nematodes and other forms of life utilize the organic matter to build a healthy or “happy” soil. I have found that one final push in the fall can make a big difference in my garden soil–planting a cover crop or “green manure” crop.

Instead of being harvested, a cover crop is grown to provide vegetative cover for the soil. It can be left on the surface as mulch or tilled while it is still green into the soil, becoming a green manure. (Brady & Weil) Garden soil can be abused during the growing season from tilling, weeding, harvesting, and foot traffic. Over the years I have found that planting cover crops is an economical and easy way to improve soil structure and overall soil quality.

Cover crops have a place in the home garden regardless of garden size and provide many benefits:

Soil erosion

The roots of a cover crop stabilize the root zone or surface of the soil, reducing the risk of erosion from wind and rain. The leaves and stems of the cover crop also decrease soil erosion by reducing the impact of rain and potential runoff. I think of soil as an investment in the future. After years of testing, analyzing, tweaking the pH levels, adding nutrients and organic matter, it just makes sense to protect my investment and decrease the risk of my valuable soil being washed or blown away. Regardless of the time of year, I want to protect my soil by keeping it covered, either with a vegetable crop or a cover crop, and in our area we can grow cover crops year around to protect our soil.

Soil Compaction

Cover crop root systems can be used to combat both shallow and deep compaction. Cover crops with taproot (forage radishes) reach deep into the soil and can break up compacted soil layers. Likewise, the extensive root systems of grass cover crops (cereal rye) reduce surface compaction making it easier for vegetable roots to access essential water and nutrients that may previously have been unavailable.

 Soil organic matter

Cover crop residues increase soil organic matter, providing numerous benefits to the soil and successive crops. Increasing organic matter improves soil structure, soil water holding capacity and infiltration, and soil aggregate stability. Decaying plant material contributes nutrients back to the soil to be used by future crops.

Weed Suppression

Cover crops can provide an incremental benefit of weed control by out-competing weeds for light, water and nutrients. Research has found that cereal rye also exhibits an allelopathic effect on weeds, i.e., acts as a natural herbicide,

The Allelopathic Effect

Cereal rye produces several compounds in its plant tissues and releases root extractions that inhibit germination and growth of weed seeds. These allelopathic effects, together with cereal rye’s ability to smother other plants with cool weather growth, makes it an idea choice for winter weed control. Cereal rye can be planted in our area up to the end of October.

However, be aware that in addition to suppressing weed growth, allelopathic compounds may suppress germination of small seed vegetable crops (large seeds are rarely affected) if they are planted shortly after the incorporation of cereal rye residual. Cereal rye should be incorporated 3-4 weeks into the soil before small seed vegetables, such as lettuce, are planted. Cereal rye may grow as tall as 3-4 feet by early spring and will need to be cut with either a weed-eater or lawn mower before being incorporated into the soil.

Disease and Pest Management

Many articles have been published recently about cover crops being another tool for use in disease and pest management. Particular members of the brassicas family, certain mustards, and rapeseed varieties help control soil-borne pathogens such as root knot nematodes and verticillium wilt.

Root knot nematodes are microscopic worms that attack many summer vegetables including cucumbers, tomatoes, eggplants, okra, and cantaloupes.

Plants infected with root knot nematodes will be stunted, wilt frequently and produce little or no harvest. When you pull up these plants, look for knotted and bumpy roots. These characteristics indicate the presence of the destructive root knot nematode parasite.

Verticillium wilt affects tomato, potato, pepper and eggplants in the vegetable garden. The pathogen usually enters the plant though young roots and then grows into and up the water-conducting vessels of roots and stem. As the vessels become plugged and collapse, the water supply to the leaves is blocked. With a limited water supply, the leaves begin to wilt on sunny days and recover at night. This process may continue until the entire plant is wilted, stunted or dead.

If your cover crop goal is to help control soil pathogens such as root knot nematodes or verticillium wilt, ‘Dwarf Essex’ rapeseed (canola), and ‘Caliente 119’ mustard have proven effective in various field trails. In Central Virginia, these cover crops need to be planted in mid-September to early October and tilled into the soil two to three months later. Mustard and rapeseed contain a chemical and an enzyme in the plant cell wall. When the cell wall is damaged, the chemical and the enzyme come into contact and the enzyme breaks down the chemical into a compound that behaves as a fumigant. The folks at Penn State recommend that these cover crops be finely chopped before they are incorporated into the soil, and should be incorporated into the soil immediately after mowing because the bio-active compounds that help control soil pathogens are unstable. “As much as 80% can be lost if the cover crop is not incorporated (into the soil) within 15 minutes of mowing.”

 Low Maintenance

Cover crops require very little maintenance, and additional nutrients are seldom needed to support them since cover crops scavenge nutrients already present in the soil, and may even “fix” additional nitrogen from the atmosphere.

Having talked a bit about all the marvelous things cover crops can do for our garden, the next step is selecting which cover crop to plant. Unfortunately, this is not a one-size-fit-all decision and a little planning is needed to maximum the benefits.

In general, cover crops are divided into two major categories: legumes (pea family) and nonlegumes (grasses and grain crops). Legumes include such plants as peas, Fava beans, clovers and vetches and are generally grown for their ability to capture or “fix” nitrogen from the air and make it available to plants. In general, legumes are slow to establish and are not recommended as a weed control. Nonlegume crops are small grains and grasses such as cereal rye, wheat, oats and barley. Nonlegume crops are generally planted to protect the soil from erosion, add organic matter to the soil, and suppress weeds. They do not have the capacity to add additional nitrogen but will scavenge nutrients such as nitrogen and phosphorus and prevent them from leaching out of soil. When the green material is tilled into the soil, the green manure is broken down and nitrogen, phosphorus and other trace elements become available for use by subsequent plants. Cereal rye is quick growing and is highly recommended for controlling weeds.

Selecting and Planning a Cover Crop

One of the essentials for a good cover crop is timing which also requires some planning. Plan your cover crop in advance by selecting the best species for your gardening conditions and obtain seed early so you can be ready to plant at the proper time. Virginia Cooperative Extension Publication 426-344 provides not only a list of recommended cover crops for Virginia, but also information on planting recommendations per 100 square feet, when to plant, when to turn under, the various attributes of each cover crop, and miscellaneous information on soil requirements, hardiness, etc. The New England Vegetable Guide provides a brief description and lists attributes of the various cover crop plants. Gardeners plant and harvest different crops at different times. Planning ahead makes it easier to stagger the planting and termination of cover crops with the planting and harvesting of vegetables in the garden.

Decide which vegetables crops you want to plant in the spring so you can plan your winter crops accordingly. If you intend to plant early vegetables, such as cabbage, broccoli, spinach, and beets, plant a cereal cover crop such as rye or oats the preceding fall. If you intend to plant vegetables in April or early May, a legume crop may be a winter crop option, as it will have sufficient time to grow so as to provide a nitrogen benefit. Legumes will provide even more nitrogen if the vegetable crop is not planted until late May or June. Also, if you are planting a cover crop in late October, you’ll need to select a cover crop that germinates at low soil temperatures such as cereal rye or Fava beans.

Ceral Rye planted between garlic beds.

Ceral Rye planted between garlic beds.

If you are not planning on planting fall or winter vegetables, consider seeding your entire garden in a cover crop. Also, cover crops may be sown in between “rows” of overwintering vegetables such as garlic.

A combination winter cover crop planting of legume (hairy vetch) and cereal rye can be sown in the fall to provide both a source of nitrogen and weed control.

If you are interested in incorporating cover crops in your garden year around, “The Joys of Cover Cropping Part 2: Cover Cropping Strategies and Species” by Harry Ussery provides recommendations for year-round (fall, winter, spring and summer) strategies.

Planting Cover Crops

To prepare your cover crop bed, remove all old plant material, debris and large stones, and rake the planting area smooth. Seed can be hand-broadcasted over the intended area at the proper rated suggested by Virginia Cooperative Extensive Publication 426-344. Good seed-to-soil contact improves germination. When seeds get wet, the seed coat splits and the germinating seed dries out which quickly destroys the seedling. Good soil contact ensures that the seed will not be exposed to the drying effects of sun and wind and it provides the seed with the continued moisture needed to complete the germination and emergence process. After the seed has been broadcast, lightly rake and tamp the seeded area with the rake head and water the cover crop with a fine water mist.

It is best to rake or lightly till larger seeds into the soil to ensure good germination. Crops with large seeds such as pea, vetch, oat, and cereal rye may be planted to a depth of 1-1½ inches.

Terminating the Cover Crop

One of the big advantages for planting a cover crop is that they are generally very easy to get established. However, that ease can also promote a weed problem when cover crops are left to go to seed. Buckwheat and vetches generally reseed themselves with little difficulty so the gardener needs to be vigilant in mowing or pulling the crop before it goes to seed.

In conclusion, not planting a cover crop is a missed opportunity for improving your garden. Planting a cover crop is like saving pennies–one penny or one cover crop is not going to be that noticeable. However, just like compounding interest, several years of using cover crop strategies in your garden can greatly affect the quality and health of the soil over time. Regardless of garden size, cover crops provide an easy, economical way to improve soil and guard against erosion. In addition to the benefit of improved production from improved soil, a garden that is filled with green in mid-winter is much more appealing to the eye than a bed of winter weeds or bare soil!

Thanks for taking the time to stop by The Garden Shed and we hope to see you again next month!

Sited Sources:

Brady, Nyle C., Weil, Ray R., The Nature and Properties of Soils, 14th ed. Pearson Prentice Hall, 2008 pp 685-687

Virginia Cooperative Extension, Publication 426-344, “Cover Crops                                     

Extension, “How Cover Crops Suppress Weeds”,                                                  

Sustainable Agriculture Research &Education (SARE), “Brassicas and Mustards,                    

Virginia Cooperative Extension, “Building Healthy Soil”, Publication 426-711,                       

The New England Vegetable Management Guide: “Cover Crops”,                                            

Clemson Cooperative Extension Publication HGIC1252, “Cover Crops”,

The University of Vermont, “Winter Rye: A Reliable Cover Crop”,

North Carolina Cooperative Extension, Improve Your Garden Soil with Cover Crops,

Pen State Extension, “Reducing Soil Born Diseases with Cover Crops

University of Illinois, Focus on Plant Problems, “Verticillium Wilt”,            

Ohio State University Extension, Fact Sheet HYG-3122-96, Fusarium and Verticillium Wilts of Tomato, Potato, Pepper and Eggplant,

Cornell gardening resources, “Improve your Soil with Cover Crops”, Ecogardening Factsheet #9 spring 1993

Virginia Cooperative Extension, Publication CSES-121NP, “Virginia Cover Crops, Fact Sheet No.2: Cover Crop Performance Evaluation in Field and Control Studies”,

Ussery, Harvey, “The Joys of Cover Cropping Part: 2 Cover Cropping Strategies and Species”.



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