Sunn Hemp

Flowering Sunn hemp cover crop

Sunn Hemp (Crotalaria Juncea) is a erect summer legume that has become a popular green manure option worldwide. Used as a cover crop, Sunn Hemp can improve soil health, decrease soil erosion, improve water retention, and recycle plant nutrients.

Organic matter

Sunn Hemp is a rapid, fibrous growing plant - often reaching 6ft in height. Part of its popularity results in its ability to produce large amounts of biomass in 60 to 90 days. According to research conducted by Rotar & Joy (1983) in Hawaii, some varieties of Sunn Hemp added up to 7 tons of dry matter per hectare when grown for 60 days in favourable conditions.

Nitrogen fixation

As a legume, Sunn Hemp has the ability to produce large amounts of organic Nitrogen. In the same trials, Rotar & Joy (1983) found the crops to produce up to 165 kg of Nitrogen per hectare. Sunn Hemp becomes most beneficial where this Nitrogen is leveraged in a following cash crop.

Other benefits

Due to Sunn Hemp's fast growth and height, it has the ability to suppress weeds. Sunn hemp is especially suited for weed control in orchards, unlike vining or creeping ground covers, constant maintenance to keep it from covering the trees is not necessary. In such a context, especially where trees may be small, Sunn Hemp can also act as a natural windbreak.

CTAHR research has shown that Sunn Hemp suppresses many plant-parasitic nematodes. Nematode management systems that use the “Tropic Sun" variety as part of a crop rotation program can be effective in controlling nematode infestations. Root-knot and reniform nematodes may infect the roots, but the pests do not reproduce well due to the nematicidal compound.

Planting & Management Tips:

  • Sunn Hemp is considered a summer annual - grown in tropical and subtropical areas. It has been adapted to hot, semi-arid and arid areas, with average annual rainfall as low as 200 mm. Irrigation is necessary for maximum growth and nitrogen fixation. It is also more productive with relatively high humidity.

  • 8 - 12 frost-free weeks are required to establish the crop.

  • Sunn Hemp can grow on a wide range of soils - but performs best in well-drained soils with a pH between 5 - 7.5. The crop can grow on infertile soils, and performs better on poor sandy soils than most crops.

  • In Southern Africa, with varying rainfall patterns, this crop is often selected due to its ability to tolerate drought.

  • To establish a uniform stand, the seed should be broadcast or drilled at a depth of approximately 2cm. Should a drilling method be used, it is important to consider row spacing and population. When Sunn Hemp is planted in narrow rows, mature plants tend to fall over.

  • Sunn Hemp can be mowed at anytime and left on the surface as mulch. Should the plant be mowed to 30cm in height, the plant will regrow a second time.

  • As a green manure crop, it is recommended that the plant be mowed down or plowed in before it reaches full bloom as it become fibrous and therefore more difficult to manage.

Sunn Hemp, in its green manure function, can be mixed with other crops in order to leverage other plant benefits simultaneously. As similarly sized seed, Forage Sorghum, Sunflower, or Pearl Millet could blended with Sunn Hemp - depending on your soil objectives.

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