Senna obtusifolia (Sicklepod)

Scientific name

Senna obtusifolia (L.) H.S.Irwin & Barneby

Synonyms

Cassia obtusifolia L.; Cassia tora L. (misapplied); Cassia tora L. var. obtusifoliaL.

Common names

Sicklepod, Chinese senna, cheporon (Pokot), emang, emany (Turkana)

Family

Fabaceae (Leguminosae): sub-family Caesalpinioideae

Origin

This species is thought to be native to southern and eastern USA, Mexico and tropical America.

Naturalised distribution (global)

Locations within which Senna obtusifolia is naturalised include south-eastern and several Pacific islands.

Introduced, naturalised or invasive in East Africa

Senna obtusifolia is invasive in parts of Kenya and Uganda and naturalised in parts of Tanzania (A.B.R. Witt pers. obs.). It is found mainly in the coastal and central parts of Kenya. In Tanzania, it is found in the Morogoro floral region.

Habitat

Senna obtusifolia is a weed of disturbed sites, waste areas, roadsides, riparian zones (banks of watercourses), floodplains, drainage channels, open woodlands, fallow land, crops and pastures in wetter tropical and subtropical environments. It usually grows as a pasture weed but is sometimes found along roads and in disturbed areas in rain forest.

Description

Senna obtusifolia is a short-lived (annual or biennial) shrub growing to up to 2.5 m tall, but usually less than 2 m in height. The lower stems often sprawl along the ground in open areas.

Plants produce numerous, branched, sprawling stems that are 1.5-2 m long. These stems are usually softly hairy (pubescent) when young, but become mostly hairless (glabrescent) with age. The foliage has a slightly rank odour.

The compound (pinnate) leaves are alternately arranged along the stems and are borne on relatively short stalks (petioles) 15-20 mm long. They have two or three pairs of leaflets (17-65 mm long and 10-40 mm wide), with those further from the leaf stalk usually being larger. The leaflets are egg-shaped in outline with the narrower end attached to the stalk (obovate) and have rounded tips (obtuse apices). Their surfaces may be either hairless (glabrous) or sparsely hairy (pubescent) and the entire margins are usually edged with tiny hairs (cilia). There is a small elongated structure (gland) 1-3 mm long located on the main leaf axis (rachis) between the lowest pair of leaflets (occasionally also between the second pair of leaflets as well).

The yellow flowers (10-15 mm across) are borne on stalks (pedicels) 7-28 mm long. These flowers usually occur in pairs in the leaf forks (axils) and are mostly located near the tips of the branches. They have five green sepals (5.5-9.5 mm long) and five yellow or pale yellow petals (8-15 mm long). Each flower also has seven fertile stamens with anthers (3-5 mm long) that have a short narrow projection (beak) on one end.

The fruit is a slender, are strongly curved downwards (sickle-shaped), pod (6-18 cm long and 2-6 mm wide) that is almost round (cylindrical) in cross-section (sometimes slightly flattened or four-angled) and curved downwards and resembles a sickle in shape. The pods turn brownish-green as they mature and are slightly indented between each of the numerous seeds (faintly septate). The seeds (3-6 mm long) are dark brown in colour, shiny in appearance, and either diamond-shaped (rhomboid) or irregular in shape.

Reproduction and dispersal

This species reproduces by seed only. This plant spreads by reseeding itself. Seeds are dispersed by water and animals that eat the fruit (e.g. cattle). They may also be spread as a contaminant of agricultural produce (fodder and pasture seeds) or in mud sticking to animals, footwear, machinery and vehicles.

Similar species

Senna obtusifolia is very similar to Senna hirsuta (hairy senna), Senna occidentalis (coffee senna), Senna septemtrionalis (smooth senna), which are also naturalised in the region.

  • S. obtusifolia is a relatively small slender shrub (usually 0.5-2 m tall) that has leaves with a few (2-3) pairs of leaflets. The relatively large leaflets (17-65 mm long) are relatively broad (15-40 mm wide) and have rounded tips (obtuse apices). Its flowers are borne in pairs in the leaf forks and the very elongated (6-18 cm long) pods are almost rounded in cross-section (cylindrical) and very narrow (2-6 mm wide). These pods are sickle-shaped and are mostly hairless (glabrous).
  • S. hirsuta is a relatively small slender shrub (usually 0.5-2 m tall) that has leaves with several (2-6) pairs of leaflets. These relatively large leaflets (40-105 mm long) are relatively broad (20-40 mm wide) and have pointed tips (acute apices). Its flowers are borne in small clusters in the leaf forks and its very elongated (10-18 cm long) pods are usually somewhat flattened and very narrow (4-6 mm wide). These pods are slightly-curved downwards and are densely covered in long white hairs (pubescent).
  • S. occidentalis is a relatively small slender shrub (usually 0.5-2 m tall) that has leaves with several (3-7) pairs of leaflets. The relatively large leaflets (30-100 mm long) are relatively broad (20-40 mm wide) and have acute apices. Its flowers are borne in small clusters in the leaf forks and its very elongated (7.5-13 cm long) pods are rounded (cylindrical) or slightly flattened and relatively thick (6-11 mm wide). These pods are straight or slightly-curved upwards and are glabrous.
  • S. septemtrionalis is a moderately-sized shrub (1-3 m tall) that has leaves with several (3-5) pairs of leaflets. The relatively large leaflets (45-70 mm long) are relatively broad (15-35 mm wide) and have acute apices. Its flowers are borne in relatively loose somewhat elongated many-flowered clusters (racemes) and its elongated (6-10.5 cm long) pods are cylindrical and relatively thick (10-15 mm wide). These pods are straight and glabrous.

Economic and other uses

Senna obtusifolia can be used as a medicinal plant, a green manure for poles, hedges and for fuelwood.

Environmental and other impacts

Senna obtusifolia is regarded as an environmental weed in Kenya. It also affects other parts of the world including northern Queensland, the Northern Territory and northern Western Australia.

S. obtusifolia has been listed as a noxious weed in South Africa.

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

Senna obtusifolia is very difficult to control with cultivating usually spreading rather than controlling this weed. Hand pulling is difficult because of its long curved taproot. Slashing reduces vigour and if combined with other management measures such as restricted grazing and soil fertility augmentation, can bring this plant under control. A variety of herbicides can be used to control S. obtusifolia. Their success depends upon environmental conditions. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

A great deal of work has gone into finding agents for biological control of S. obtusifolia. In the USA. Alternaria cassiae, formulated as a mycoherbicide, has given greater than 96% control of S. obtusifolia and increased the yields of soybean. Other agents have been investigated for possible biocontrol potential.

Legislation

Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.

References

CABI Invasive Species Compendium online data sheet. Senna obtusifolia (sicklepod). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

Germplasm Resources Information Network (GRIN). www.ars-grin.gov/npgs/index.html. National Germplasm Resources Laboratory, National Genetic Resources Program, Agricultural Research Service (ARS), United States Department of Agriculture (USDA), Beltsville, Maryland, USA. Accessed March 2011.

Global Compendium of Weeds. www.hear.org/gcw. Hawaiian Ecosystems at Risk Project. Accessed March 2011.

Lazarides, M., Cowley, K. and Hohnen, P. (1997). CSIRO Handbook of Australian Weeds. CSIRO Publishing, Collingwood, Victoria.

Pacific Island Ecosystems at Risk (PIER). Senna obtusifolia (L.) H.S.Irwin & Barneby, Fabaceae (Leguminosae): plant threats to Pacific ecosystems. www.hear.org/pier/species/senna_obtusifolia.htm.. Institute of Pacific Islands Forestry, Hawaii, USA. Accessed March 2011.

Editors

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: eafrinet@africaonline.co.ke