Cover crops for Florida vegetable gardens

Story and Photos by Bill Pitts

Cover cropping is an ancient agricultural technique for improving the soil. The practice is regaining popularity not only in farming, but also in home gardens because it is sustainable – and because it works. This is especially true in Florida, where certain cover crops can reduce root-knot nematodes.

In the spring, French marigolds and some varieties of cowpeas make great cover crops.

In home gardens, cover crops are best sown thickly, grown until lush but still tender, and then turned under, so that as the plants decompose, they add both organic matter and nitrogen to the soil. 

One of the best cover crops for Florida is sunn hemp (Crotalaria juncea). Sunn hemp loves our heat and humidity and is a fast grower. It adds lots of organic matter and nitrogen to the soil and it has been shown to control root-knot nematodes. Farmers in Florida often rotate cash crops, such as tomatoes, with sunn hemp for these reasons. There is no reason why home gardeners shouldn’t take advantage of sunn hemp as well.

Planting cover crops is one of the best methods for improving our sandy soils, and certainly one of the most underutilized in home gardens.

The best time to plant a sunn hemp cover crop is summer when it is difficult to grow any but the most heat-tolerant vegetables. Sow the large seeds thickly, as you would a ryegrass lawn. Press the them into the soil and keep the beds moist. The seeds should germinate in a week or so. Water until established, and the sunn hemp will take care of itself – and your garden – for the next couple of months. 

A thick stand of sunn hemp will outcompete most weeds. It will prevent erosion from wind and torrential summer rains. It needs no mulch, no fertilizer, no pesticides, and it is drought tolerant. As it grows, it improves the soil in various ways. If you want great lettuce in the fall, peas in the winter, and tomatoes in the spring, then plant sunn hemp in the summer. 

A thick stand of sunn hemp will improve the soil and help control root-knot nematodes.

Sunn hemp will grow fast and lush. When the plants reach 3-4 feet tall, it is time to turn them in. You don’t have to be exact with timing, but it is important to turn sunn hemp in before it flowers. Do not wait until the plants are past the bud stage or the stems will be woody, less beneficial to the soil, and much more difficult to work with. The flowers on sunn hemp aren’t much to look at anyway. A good general rule for cover cropping in the home garden is this: turn them in while they are young. 

If you don’t have a tiller, don’t worry. There are other ways to “turn in.” You can dig sunn hemp in with a shovel (this is not difficult because the stems are soft). If you are a no-till gardener who prefers to leave the soil undisturbed, mow the sunn hemp down or pull it up (it comes up easily, by the handful) and spread it out as mulch. For better results, add more mulch on top of it. You can also use the sunn hemp as composting material. Any of these methods will improve the soil and help to control root-knot nematodes. 

Another cover crop that works well in Florida is sorghum sudangrass (Sorghum x drummondii). It does all the good things that sunn hemp does, including controlling nematodes. Sorghum sudangrass is not as easy to pull up by hand as sunn hemp, but you can mow it as you would a lawn until it is time to turn in or mulch over it. In winter, cereal rye (Secale cereale) is another great cover crop that can be treated in much the same way. 

In the springtime, my favorite cover crop is French marigold (Tagetes patula) especially the big, sprawling, old-fashioned heirloom varieties. I treat marigolds as bedding plants. They don’t add as much organic matter to the soil as the other cover crops I have mentioned, but they help control root-knot nematodes, they attract beneficial insects, and they are highly ornamental.

Buckwheat is a great cover crop in much of the country, but in Florida it is unfortunately a nematode magnet.

Some cover crops that work well in other parts of the country are not as effective in Florida. Buckwheat (Fagopyrum esculentum), for example, is touted in many garden books as a great cover crop and bee plant, but in Florida it is a nematode magnet. I found out the hard way that you can actually increase root-knot nematode populations by planting buckwheat. The same can be said of some varieties of cowpeas, though other types show resistance. Seeds for sorghum, sudangrass, and sunn hemp may be a bit more difficult to find than some other cover crops, but they are worth the trouble in Florida. 

Planting cover crops is one of the best methods for improving our sandy soils, and certainly one of the most underutilized in home gardens. That said, it is important to have realistic expectations. A planting of sunn hemp will not convert Florida sand into fertile loam in a single season, or even several seasons, but when incorporated into the gardening routine, cover cropping can be just as beneficial as mulching and composting. Don’t expect a small patch of the cover crops I have mentioned to instantaneously drive all root-knot nematodes from the entire vegetable garden, but they will reduce root-knot nematodes in the spot where they have been planted. 

French marigolds not only help control root-knot nematodes, they attract lots of beneficial insects.

You can use cover crops in many different ways. If you want to give your vegetable garden and yourself a break for a few months, plant all the beds solid with the appropriate cover crop. Or you can plant half the garden in cover crops, half in vegetables or herbs, and rotate. In late spring tomato vines can get scraggly, but you may be reluctant to pull them up until the last fruits ripen. Go ahead and plant a cover crop around them. You can cut the tomato vines out later while the cover crop is growing.  

My approach has been to plant a cover crop at every opportunity. Even if I am planting only a few square feet, and even if it has time to grow only a foot before I turn it in, it is still improving and protecting the soil, and I would rather look at a patch of green than bare earth or mulch. 

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