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Choosing the Right Cover Crop at the Right Time

Choosing the right cover crop at the right time is important because of our hot and rainy summers. Wisely managing bare land after cleaning up vegetable crop residues is a serious consideration. Keeping land fallow during summer with as much as 40- or 50-inches of rainfall definitely results in a substantial loss of water and nutrients through leaching. It is especially critical in the sandy or gravelly soils, such as those found in Miami-Dade County. Weed control is another important concern.

Promising cover crops available in this area

An alternative approach to conserve soil and water, and suppress field weeds is to grow summer cover crops, such as sunn hemp and sorghum sudangrass, which can grow fast and cover the land quickly and densely (Figures 1 and 2). Both of these cover crops can scavenge leftover soil nutrients applied on the previous season’s vegetables and accumulate them in plant tissues.

Figure 1. Sunn hemp plants seeded after a month

These cover crops can produce large amounts of biomass, suppress weeds by shading them out, and improve soil fertility as a green manure after being incorporated into the soil. In addition, some cover crops, especially sunn hemp, can suppress some soil pests, especially root-knot nematodes.

Figure 2. A field with sorghum sudangrass

Choosing the right cover crop for the right vegetable crop

To control soil-born pests, please keep crop rotation in mind. For instance, if you plan to grow non-legume vegetables, such as squash, tomato, or sweet corn, etc., as the first crop in the coming growing season, sunn hemp is an ideal choice. If you plan to grow legumes, such as beans, sorghum sudangrass is a better choice.

Growing cover crops at the right time 

Both sunn hemp and sorghum sudangrass are usually seeded from the middle of May to early June, with the seeds germinating in 3-5 days. Within about a month, the land will be covered completely, and after 2 months, both sunn hemp and sorghum sudangrass can reach 6 ft tall, which can be cut back (Figure 3) for regrowth or flail mowed and incorporated into the soil.

Figure 3. Cutting back of sunn hemp for regrowth

Seed cost of cover crops

In recent years, seed costs are reasonably affordable. The price can change any time but based on the previous experience, sunn hemp seed is about $1.50 per pound, sorghum sudangrass costs $0.69 per pound (please check the current price). With seeding rates at 25-30 lb per acre for sunn hemp or sorghum sudangrass, it costs about $40-$45 per acre for sunn hemp, and $17-$20 per acre for sorghum sudangrass.

For more information, please refer to EDIS publications:

Wang, Q., Li, Y., Hanlon, E. etc. Cover crop benefits for south Florida commercial vegetable producers. UF/IFAS EDIS SL 242. http://edis.ifas.ufl.edu/pdffiles/SS/SS46100.pdf

Wang, Q., Li, Y., Klassen, W., and Hanson, E. Sunn hemp – a promising cover crop in Florida. UF/IFAS EDIS SL 306. http://edis.ifas.ufl.edu/pdffiles/TR/TR00300.pdf