Sunn Hemp
Sunn Hemp is a relatively new cover crop for New York. Seed availability and cost improved a lot about 2016. This tropical legume was not much tested in northern climates like New York earlier. It has proven to grow very well during the heat of summer. Sunn hemp produces a lot of biomass and can grow over 7 feet tall.
Land Preparation
Smooth seedbed, well-drained but with adequate moisture for germination. Minimum-tillage works well.
Seeding Rate
Drill 15 lb/ac; if terminated early (at 60-70d), 25 lb/ac. Plant ½-3/4 inch deep. Can be planted with 15 lb/ac sudangrass or sorghum-sudangrass.
Seeding Dates
The range of seeding dates has not been determined for New York. Plant into warm soil, with enough warm season left. Mid-June to mid-July should be successful. Later if terminated at flowering.
Seed Suppliers
Integrity Seeds (Global Sunn), Hancock Seed.
Maintenance
Little maintenance necessary. Determine when to terminate based on fiber development in stem.
Control
Terminate either while stalks are still tender (60-70d) or allow to winter kill. Early termination can be with flail mowing or herbicide. Sunn hemp is difficult to mow once the stems become fibrous. They are even tougher than mature sudangrass. The stems decompose considerably over winter.
Tips
Use in a mixture with sudangrass. The sudangrass serves as a nurse crop and stimulates nitrogen fixation. They will both resprout if mowed to ~1 foot once they are 2-3 feet tall. Mowing delays fibrous stems. Use a cowpea/peanut inoculant each year. Don’t try to mow the fibrous stalks after flowering. Amount of nitrogen fixation has not been determined for New York (short season, high fertility) A non-host of root-knot nematode