Sunn hemp is often used as a winter cover crop for Southern farming systems. It is adapted to a wide range of soils and increases organic matter and provides nitrogen. The use of sunn hemp is a conservation effort designed to improve the quality of soil. Sunn hemp grows fast and has a short growing season, which makes it ideal to use between the fall harvest and spring planting. Even though it's a tropical plant, when it's grown in the U.S., it performs more like an annual.
Grow sunn hemp in most any type of soil, including nonfertile, sandy soils. It does best, however, in fertile soil. Make sure it's a well-drained area, without standing water.
Expect sunn hemp to grow best when the average air temperature is at least 70 degrees. Growth and development thrives in areas with high temperatures and moderate humidity, whereas it is slowed down by cool weather. If the temperature dips below 28 degrees, the plant may freeze.
Check the pH of the soil with a test kit obtained from a hardware store or nursery. Soil does best with a pH between 5 and 7.5.
Increase the pH by 1.0 point to make it more alkaline by adding 4 ozs. of hydrated lime per square yard if the soil is sandy. For loamy soil, add 8 ozs. To make the soil more acidic and lower the soil pH by 1.0 point, mix in 1.2 ozs. of ground rock sulfur per square yard for sandy soil and 3.6 ozs. for other soils.
Drill seed into the ground using a commercial planter or small spade. The holes should be 1/2 to 1 inch deep. Drop the seeds into the holes. Cover with soil. If you use this method, you can use 30 to 50 lbs. of seed per acre, in 6-inch rows.
Use a broadcasting method if you want to scatter a large amount of seed over a given area. Till the area first to loosen the soil. Walk the area and drop 40 to 60 lbs. of seed per acre. Rake it into the ground and pack it down by walking over it or using a wood board.
Water spring and summer seedlings regularly the first four to six weeks. Water every other day in the early morning for about 30 minutes.
Consider mowing in the middle of summer to remove the very top of the plant. This isn't mandatory but can be beneficial, because mowing promotes branching and the production of additional flowers and leaves.