Sunn Hemp is a day length sensitive crop. Sunn Hemp may be planted any time after there is no danger of frost in the spring and will die in the fall when temperatures reach 28 degrees or less. August 15 is the last recommended planting date in Alabama. Sunn Hemp planted after this time will quickly go into a reproductive flowering stage and will not grow as tall or produce as much Nitrogen.
Sunn Hemp is not an invasive weed. Sunn Hemp will not flower and go to seed until the days start getting shorter (late September here). So the plants do not reproduce mature seed in our area. The only place Sunn Hemp will make seed in the U.S. is Puerto Rico and Hawaii.
Sunn Hemp is excellent for the reclaiming of fallow land and making your worst soil your best. It is perfect for the home gardener, commercial fruit and vegetable farmer, organic farmer, turf farmer and row crop producer. A good rotation in South Alabama would be corn, Sunn Hemp, and then wheat. This would allow the Sunn Hemp to rebuild the soil and supply the Nitrogen for the following wheat, corn, or cotton crop.
Deer and goats thrive on Sunn Hemp which makes it an excellent summer food plot. The leaves are 30 % protein and provide upward browsing for goats and deer and provide food and habitat for other wildlife species.
Research has been conducted in the U.S. since the 1930’s. Even then it was reported for its excellent soil conditioning benefits. Research is presently being conducted in Hawaii, South Carolina, Florida, Auburn, and other Universities. Seed supply here in the U.S. is limited.
This crop may prove very beneficial in ridding land of unwanted weeds. Sunn Hemp does have allelopathic activity on some weeds including palmer amaranth and its very rapid growth almost totally chokes out other weeds. It appears to smother out most if not all other weeds including pig weed, palmer amaranth, and nutsedge.
Sunn Hemp is resistant to root-knot and reniform nematodes. This is another major factor for growing this crop. Root-knot and reniform nematodes are a major pest in South Alabama agriculture. And root-knot nematodes are a pest in almost every home garden in South Alabama. Sunn Hemp would be a tremendous benefit in reducing nematode populations in almost every home garden in our area. Gardeners could plant their spring crop and then in June, July, or August plant the Sunn Hemp.
The recommended seeding rate is 15 pounds per acre. Seed costs are now around $3.00 per pound so costs would be $45 per acre. Seed can be broadcast and covered about 1/4-1” deep or drilled in. Seed is actually coming up from 0-2.5” inches deep in test plots.
Sunn Hemp will grow on poor soil with a pH from 5 to 7.5 so your soil will not need liming. It will grow on sandy or clay soils, just not too hard packed clay. The soil does need to be fairly well drained. It will need zero Nitrogen, but will grow better if ample Phosphorus and Potash are already in or added to the soil.
Sunn Hemp is very drought tolerant. There must be moisture for germination. However, in 2010 several growers reported Sunn Hemp to grow 8’ tall with only one inch of rain after germination. Of course, it will grow better if it receives ample moisture.
The plant grows very fast and in about 60 days will be six feet tall or taller. This is a good time to mow the Sunn Hemp to about 12-18 high and let it ratoon or regrow again. If allowed to get too tall and old the stems will become a little tough and fibrous and will not decompose as rapidly. If the plants are too tough they may cause problems the following year when you are trying to prepare your soil and plant.