Senna septemtrionalis (Smooth Senna)

Scientific name

Senna septemtrionalis (Viv.) H.S.Irwin & Barneby

Synonyms

Cassia laevigata Willd.; Cassia septemtrionalis Viv.; Cassia floribunda sensu Brenan non Cav.; Cassia laevigata Willd.; Cassia elegansKunth.

Common names

Smooth senna, yellow shower, arsenic bush, buttercup bush, dooleyweed, laburnum, esenetoi (Maasai), omochegechege (Ekegusii), chul senetwet / senetwet apchimbek (Kipsigis), esenetoi (Maasai)

Family

Fabaceae (Leguminosae): sub-family Caesalpinioideae

Origin

Mexico and Central America.

Naturalised distribution (global)

Locations within which Senna septemtrionalis is naturalised include southern Africa and some oceanic islands with warm climates.

Introduced, naturalised or invasive in East Africa

In the region, this species is naturalised at low elevations in dry, disturbed areas, but occasionally at higher elevations in pastures and even margins of wet forest. It is a common garden escape in Nairobi area. It has been recorded in Kenya in northern Kenya, Rift Valley, Nairobi, Western and Nyanza, Masai and Coast floral regions.

Habitat

Senna septemtrionalis is a common garden ornamental plant that also invades; forest margins, savanna, riverbanks, roadsides, waste ground and plantations.

Description

Senna septemtrionalis is a shrub or tree, 1-5 m or 6.5m tall.

The stem is rough, but hairless.

Pinnate leaves, smooth, bright green, paler beneath. The leaflets 3-4 (5) pairs, the distal ones larger, broadly ovate (egg-shaped in outline with broad end at base) to lanceolate, 3.5-10.5 cm long, 1.1-3.5 cm wide. Leaf apex acuminate or caudate, base obliquely rounded or tapered (cuneate). The glands are located on the leaf rachis, between the leaflets and not on the petiole.

Flowers in axillary racemes (1.5-8) cm long.

The pedicels (12-25) mm long. Bracts sub-membranous, linear, lanceolate, or subulate, (1.5-) 2-4.5 mm long, caducous as pedicels begin to elongate; calyx lobes yellowish green, yellowish brown, or completely yellow, the outer ones relatively firm, ovate (egg-shaped in outline with broad end at base)-elliptic, 4-6.5 mm long, the inner ones submembranous, oblong-obovate (egg-shaped in outline but with the narrower end at the base) or suborbicular, 6.5-10 mm long; flower petals bright yellow and sometimes have brown veins - the standard obovate to obovate-flabellate, deeply emarginate, the others obovate, the longest petal 12-16 mm long; staminodes 3, obovate or suborbicular, (1.7-) 2-2.6 mm long; filaments of 4 median stamens 1.3-2.2 mm long, those of 2 abaxial stamens dilated, ribbon-like, 7-10.5 mm long, that of abaxial central stamen 2-4 mm long.

Fruit is a green pod that matures brown. Pods ascending on a stiff pedicel, chartaceous, cylindrical or obtusely quadrangular, 6-10.5 cm long, 0.8-1.1 cm wide, cavity moderately pulpy, divided into 2 parallel rows of cells. Seeds oriented with broad face to the septum, olive or brown, compressed-obovoid, 3.6-4.9 mm long, constricted at the hilum, smooth or minutely pitted, without an areole" more details are in (Wagner et al., 1999; pp. 701-702).

Reproduction and dispersal

Reproduces by seed. This plant spreads by reseeding itself, but seeds may be contaminants of soil and in garden waste.

Similar species

Senna septemtrionalis is very similar to S. occidentalis (coffee senna), S. hirsuta (hairy senna) and S. obtusifolia (sicklepod). These species can be distinguished by the following differences:

  • 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 mostly hairless (glabrous).
  • 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. 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. 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 glabrous.

Economic and other uses

Senna septemtrionalis is a medicinal plant among some communities in Kenya. It is also a popular garden plant.

Environmental and other impacts

Senna septemtrionalis can invade forest margins, savanna, riverbanks, roadsides, waste ground and plantations where it can establish and suppress the regeneration of desirable species.

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.

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.

The editors could not find any specific information on the control of this species.

Legislation

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

References

Beentje, H.J. (1994). Kenya trees, shrubs and Lianas. National Museums of Kenya, Nairobi, Kenya, 722 pp.

Henderson, L. (2001).  Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa.  Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.

Wikipedia contributors. "Senna septemtrionalis." Wikipedia, The Free Encyclopedia. Accessed January 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