August 1, 2014
By Trevor Hylton
Photo by Trevor Hylton
Cover crops have traditionally been used by farmers to improve soil, but they can also benefit the home gardener. Cover crops, also known as green manures, are an alternative way to manage soil fertility, by recycling nutrients and adding organic matter to the soil. When nutrients are needed for the next crop, the cover crop plants are dug into the soil or used as mulch on top of the soil. In other words, the old plants supply nutrients for new plants by composting in place.
Cover crops strengthen soil structure, letting more air into the soil, improving drainage, and maintaining viable living space for beneficial microorganisms.
A good ground cover can discourage weeds from growing by out-competing the weeds for nutrients, space and light. Most cover crops are easy to plant and require only basic care to thrive, but best results are seen when these crops are managed well.
To establish a cover crop, work up the soil gently with a garden rake, broadcast seed evenly over the soil, and then rake it in. Raking establishes good soil-to-seed contact and protects the seed from birds and other nuisance wildlife. Once the crop becomes established you should cut it back before seeds are set. The optimum time to do this is at flowering. You can mow cover crops with a lawn mower or a weed trimmer, depending on how tall the plants are and how heavily the crop is planted. Make sure you wait a day or two after cutting until the leaves and stems dry down, and then turn them into the soil.
If you have several weeks to a month before you plan to plant your next crop, you can simply cut the cover crop, drop it in place, and cover it with mulch—hay, straw or pine straw, and let it decompose on top of the soil. At planting time, simply move the mulch aside and insert seedlings or plant seeds.
Florida’s sandy soils tend to have low fertility as well as low water and nutrient retention capabilities because of their low organic content. Cover crops improve these shortcomings by adding the much needed organic matter.
Although cover crops have many benefits, selection of the right cover crop is very important. Some may be too weedy, woody, and/or tall, which can interfere with cultivation and harvest of future crops. Others may harbor pests and diseases which could attack your main crop.
Cover crops are generally planted after a primary crop for one or more of the following reasons:
- Controlling weeds through competition for available space, light, water, and nutrients.
- Preventing soil erosion by binding loose soil.
- Making available residual nutrients that would be leached in the off-season.
- Recycling and restoring nutrients.
- Reducing selected harmful nematode populations.
- Possibly creating an additional income such as hay production.
- Providing habitat for beneficial insects and birds.
While it is difficult to achieve all of the perceived benefits with one crop, gardeners should select cover crops that offer multiple benefits at once. Growers in this area have a number of options when choosing cover crops. Different cover crops are appropriate for different seasons.
Sunn hemp is best planted between early spring and mid-summer. It is an excellent choice for a summer cover crop for Florida growers because it returns nitrogen to the soil, suppresses weeds and nematodes, improves soil texture and water holding capacity, and reduces erosion in fields otherwise left without plant cover. Sunn hemp forms a symbiotic relationship with soil bacteria that remove nitrogen gas from the atmosphere and transforms the nitrogen to plant-available forms.
Sorghum-Sudan grass grows best when planted between March and June.As its name suggests, this grass is a cross between sorghum and Sudan grass. This hybrid generates large amounts of organic matter and needs little management to grow 5 to 12 feet tall. You can keep it in check by mowing it down to six inches when it reaches a height of three feet
If weed suppression is the main goal, buckwheat is preferable. Buckwheat is also a great soil conditioner. It establishes, blooms, and is ready for incorporation into the soil 35 to 40 days.It does not do well in compacted, droughty, or excessively wet soils.
Cowpeas are best when planted between April and August. They are drought tolerant and do well even in extremely high temperatures as long as soil moisture is adequate. No fertilizer is required, and cowpeas do a good job of drawing up excess nitrogen and phosphorous left in the soil from previous crops. The effect of cowpeas in nematode management is dependent on the variety. Some cowpea cultivars can be damaged by root-knot nematodes.
Fall cover crops include clover and various grains such as rye or oats. A mix of cover crops can also be planted together to obtain additional benefits.
Cover crops and green manures are sometimes disregarded as inefficient and expensive, without due consideration of their many biological benefits. Fertilizers cannot replace the function of organic matter and other management practices. Cover cropping was a large part of past Florida farming practices, so it may be worthwhile to take advantage of the experience and knowledge of older farmers in your region who farm similar soils and have used cover crops successfully.
Trevor Hylton is an Extension Agent with Florida A&M University and University of Florida IFAS Extension in Leon and Wakulla Counties. For gardening questions, email us at Ask-A-Mastergardener@leoncountyfl.gov