Cover crops are grasses, legumes or small grains grown between regular crop production for the purpose of protecting and improving the soil. Green manures are plants that are sown specifically to improve fertility.They are not harvested for food, and are not allowed to flower. Any plant can be grown as a green manure although some are much better than others. As you can see these terms are interchangeable. I'll tell you what to use and when to plant them.
Vegetable gardening involves many practices that can compromise soil health. Vegetable gardeners want to overcome this situation in order to maintain soil health. Some of the reasons for which gardeners use cover crops include the following:
Protecting soil from rain or runoff
Improving soil aggregate stability
Reducing surface crusting
Adding active organic matter to soil
Scavenging soil nitrogen
Suppressing soil diseases and pests
The following is a list of goals and what you would look for in a cover crop.
Rapidly establish smothering crops to suppress weeds
Protect soil from rain or runoff with broad leaves to intercept rain. Robust root system to hold surface soil.
Improve soil stability: Actively establish glues and fungus food from roots and surface fungi that produce aggregate glues
Reduce surface crusting with shallow, fibrous root systems
Add active organic matter to soil: High biomass with mixture of quickly and slowly-decomposing parts
Break hardpan: Deep roots that swell during growth
Fix nitrogen: Legumes with high nitrogen content and active fixation in gardens.
Scavenge soil nitrogen: Active growth in fall and good nitrogen storage over winter.
Suppress soil diseases and pests and support beneficial soil microbes, Produce suppressive compounds
Types of Cover Crops
Selection of a cover crop depends on when it can be planted and what you want to accomplish. There are many cover crop species. Legume cover crops fix atmospheric nitrogen into a form plants and microorganisms can use. Non-legume species recycle existing soil nitrogen and other nutrients and can reduce leaching losses.
A combination of two or more types of cover crops may be beneficial for quick establishment and improved nutrient availability. Johnny's Selected Seeds has a cover crop combo mix that I've been using for several years. It provides several types of seeds to give you a variety of plants.
Cover Crops in the Spring For maximum soil and moisture conservation and tilth improvement benefits from a cover crop, delay as long as possible killing or tilling it under in the spring.
Cover Crops in the Orchard or Fruit Patch
Dutch White Clover and New Zealand White Clover are excellent choices for use in the orchard or small fruit beds. They grow relatively low and choke out weeds. They also handle foot traffic very well. They also supply some nitrogen and help decrease temperature fluctuations in the soil. Clover does very well when cut repeatedly while weeds do not. It is an excellent choice for the walkways and paths in your garden; this keeps the soil protected and the weeds down.
New Zealand White Clover
Dutch White Clover
One note regarding clover, if allowed to get too thick and high; voles like to nest in it during the winter. To eliminate this give it one last cutting just before winter.
Use quick growing cover crops
Many of us grow what is usually known as cool season crops; lettuce, carrots, beets etc. We sow these early in the spring, harvest them and don't usually replant them until late summer or early fall depending on you location. Why let that fallow soil bake under the hot summer sun? Plant a quick growing cover crop such as Buckwheat in the areas where you had your cold season crops. It will enrich the soil, reduce or eliminate weeds. When ready to pant in the fall, till it under and plant away.
This practice works well under taller plants such as tomatoes. After setting out your transplants, keep them well tilled to control weeds for a few weeks. Go ahead and stake or cage your plants. Then sow Hairy Vetch around your plants. It will suppress weeds and add nitrogen to the soil. When your tomatoes are finished, pull the vines and leave the vetch in the garden. It also is great at attracting beneficial insects.
In order to protect the fertility of your soil, beds should be rested every five years. I know this is not possible for those of us you utilize every inch of space that we have. If possible give it a try, sow and area in cover crops and give the soil a chance to rejuvenate itself. Many of us are trying to get away from chemicals in the garden and cover crops are an excellent way to enrich the soil and reduce weeds. Why not give it a try?
Recommended Reading: Managing cover Crops profitability U.S. Department of Agriculture ISBN 1-888626-04-6 Building soils for better crops by Fred Magdoff and Harold van ES ISBN 888626-05-4
Johnny's Selected Seeds
Earthworks Urban Garden
Johnnys Selected Seed
Colorado State University Extension
About Paul Rodman
Paul Rodman has been gardening for over 45 years. He is an Advanced Master Gardener, and American Rose Society Consulting Rosarian. He is President Emertius of the Western Wayne County Master Gardener Association in Wayne County, Michigan. He currently serves as the greenhouse chairman of this group. Rodman has amassed over 5500 volunteer hours in the Master Gardener program.
Rodman is the garden columnist for The News Herald newspaper, in Southgate, Michigan. He has also written for the Organic Gardening.com web site.
He is a certified Master Canner and has taught classes on Home Food Preserving for 7 years.
He has lectured on various gardening topics throughout southeastern Michigan.
His favorite pastime is teaching children about gardening. For the past several years he has conducted classes for second grade students teaching them about subjects ranging from vermi-composting to propagation.