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Green manure growing guide
Sunn hemp is a fast, tall tropical legume grown for fibre and as a green manure that adds nitrogen and organic matter in about two months. Sow it directly into warm soil after the risk of frost has passed.
Coat the seed with a cowpea-type rhizobium inoculant of the 'EL' type just before sowing. Sunn hemp nodulates readily with native cowpea-type rhizobia, so inoculation matters most on land where cowpea-group legumes have not been grown.
Not needed. Sunn hemp seed normally germinates well without treatment.
Not needed. This is a warm-season tropical crop.
Not needed. Sow into moist soil or irrigate after sowing.
Sow into warm soil after the last frost; the plant needs a hot summer and is damaged below about -2 °C (28 °F). In India it is grown as a summer (pre-monsoon) crop.
Drill 1.3-2.5 cm (1/2-1 in) deep; Tropical Forages gives 1-2 cm. If broadcasting, cover the seed with at least 0.6 cm (1/4 in) of soil.
A well-prepared, weed-free seedbed. Sunn hemp grows on poor soils but responds to fertility; it does best on light, well-drained soil at pH 5-7.5 and dislikes salt, heavy clay and prolonged waterlogging.
Moist soil at sowing; once established the crop is drought tolerant. In dry climates a 60-day green manure needs pre-irrigation and at least one more watering.
No light is needed under the soil. Grow in an open, sunny field.
Emergence is quick in warm, moist soil, and fast early growth smothers many weeds.
Green manure: drill 34-56 kg/ha (30-50 lb/acre) or broadcast 45-67 kg/ha (40-60 lb/acre), using the higher rate for short crops or heavy weed pressure. Fibre: about 66 kg/ha in rows, or up to 100-240 kg/ha broadcast. Do not transplant.
Growth is rapid: over 1.2 m (4 ft) in 60 days and over 1.8 m (6 ft) in about 90 days under good conditions. Sunn hemp is a short-day plant, so long summer days keep it leafy and delay flowering.
Incorporate at bud to early flowering, about 60 days after sowing; Tropical Forages reports 150-165 kg/ha of nitrogen contributed in 60 days. Cutting at 30-45 days gives less nitrogen and organic matter.
For fibre, harvest at flowering, 60-90 days after sowing; the fibre comes from the bark of the stems.
Sunn hemp is a poor host of root-knot nematodes, and ploughing in the crop has lowered their numbers before susceptible vegetables.
Seed crops take about five months; harvest when 50-75% of the pods rattle and dry the seed to below 10% moisture.
Sow sunn hemp into warm soil for a fast green manure or fibre crop.
At bud to early flowering, about 60 days after sowing. Left longer, the stems turn fibrous and are hard to incorporate.
Not the seed: raw seed is toxic, and horses and pigs should not be fed sunn hemp at all. Ruminants can be given forage cut before flowering and sun-dried.
Usually very little. It is a short-day plant and flowers poorly under long summer days, which is why seed is produced in the tropics.