Ask a Master Gardener: The Big Garden Cover Up

French marigolds planted as a cover crop can help reduce root knot nematold damage. Photo by John Olive.

By: John Olive, Retired Director, AU Ornamental Horticultural Research Center, Mobile Gardener, www.mobilecountymastergardeners.org

Q: What is a cover crop and should I plant one in my garden?

A: A cover crop is a planting that is primarily grown to benefit a future crop and generally not for harvest.  It is mowed down and turned into the soil. Farmers often plant a cover crop to protect the soil from erosion and to improve the soil by adding nutrients and increasing  organic matter. Home gardens can also benefit from cover crop plantings for many reasons.  In the home garden, a cover crop can help suppress weeds, reduce disease damage, add nutrients and organic matter, and can even provide ornamental beauty to the vegetable garden. For a home garden, the most common cover crops include ryegrass, crimson clover, brassicas such as mustard greens and rape greens, and marigolds.  There are many options for cover crop use and this is just a brief overview of some cover crops and the reason for using them.

Ryegrass and grain crops such as wheat and oats are often planted as cover crops in the fall and can be utilized by the home gardener to increase organic matter. They are easy to grow and quickly cover the soil and produce abundant top growth as well as root growth that will decompose and add important organic matter to the soil. They also fill in well and reduce weed competition. In addition to the soil improvement benefits, ryegrass can quickly cover, creating a visually pleasing green carpet.

Crimson clover is another excellent fall planted cover crop for home gardens.  Like ryegrass, it is quick to establish and cover with a nice green carpet. It has the added benefit of being a legume and producing nitrogen and if clover is not turned in until late spring, the gardener and insect pollinators will enjoy a profusion of maroon flowers.

Cruciferous or brassica plants such as mustard greens (Brassica juncea) and rape greens (Brassica napus ‘Dwarf Essex’) are often planted as a cover crop to suppress nematodes and reduce nematode damage in gardens that have high nematode populations. These greens contain compounds that, when chopped and tilled into the soil, decompose and release a biofumigant that is toxic to soil nematodes.   Brassicas are planted in the fall and in addition to providing organic matter to the soil, they can also be harvested for cooking.  

Marigolds are a cover crop that is planted specifically to reduce nematode damage in beds with high nematode populations. They are reported to reduce root knot nematode populations by releasing natural toxins and by not allowing nematodes to reproduce in their roots. The French marigolds (Tagetes patula) are reported to be the best for this use with the cultivar ‘Tangerine’ being specifically mentioned in the literature. They can be planted in the early fall or in the spring and tilled in later.

Planting cover crops can provide benefits to the garden and can make the winter garden more aesthetically pleasing.  Some can provide greens for cooking, but the easiest cover crop may be one you don’t have to plant. Leaves may be the best choice for a cover crop that produces a generous amount of organic matter and suppresses weeds.  Collect leaves in the fall when they are abundant and readily available.  Layer leaves one to two feet deep, or more, and they will decompose by spring when they can be tilled in, or the spring crop can be planted directly into the leaf mold. In a dry fall, wetting down the leaves helps to speed their decomposition. An added benefit of a good leaf pile is that it will create good earthworm habitat.

Cover crops can provide valuable organic matter to your garden and can have other benefits, such as adding nitrogen and providing nematode control. They can also provide aesthetic and ornamental beauty. Many seed catalogs advertise cover crops.  Many seed catalogs advertise cover crops for all regions. Make sure any cover crop you plant is appropriate for The Deep South.

Crucifers such as mustard greens or 'Essex'rape greens are a good cover crop. Photo by John Olive.

A marigold cover crop may reduce nematode damage. Photo by John Olive.