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Senna occidentalis (L.) Link 1831
pronounced: SEN-huh ocks-ih-den-TAH-liss
(Fabaceae — the morning glory family)
subfamily: Caesalpinioideae – the cassia subfamily
synonym: Cassia occidentalis L. 1753
pronounced: KASS-ee-uh ocks-ih-den-TAH-liss
common names: Coffee Senna, Arsenic Bush, Stink Weed
Sennais the Latin form of the Arabic word sana, for species that have leaves and pods with cathartic and laxative properties; occidentalis is also Latin, ‘of the west’.
The plant is native to the tropical and subtropical regions of the Americas, and places where it has become naturalized include southern and eastern USA, eastern Africa, and Australia. In Australia, it is found in north-eastern Western Australia, Arnhem Land, Cape York Peninsula, the Gulf Country, and down the Queensland coast. It usually grows as a weed of agricultural land, but is also found as a weed of roadsides, waste areas and disturbed sites, often in semi-arid regions. On Magnetic Island, I have seen it as a roadside weed in Picnic Bay, and as a weed of the planted areas in the car park of the ferry terminal at Nelly Bay.
This is a slender upright and short-lived (annual or biennial) shrub, usually between 50 and 150 cm in height, and is distinguished by the foetid odour that arises when any of the plant is crushed. The plant has no spines or prickles.
The compound leaves consist of 3 – 7 pairs of lanceolate to ovate leaflets, 2 – 10 by 2 – 3 cm, with acute apices. There is a conspicuous dark-coloured gland near the base of the leaf stalk. The leaflets are hairless, except for scattered to common hairs on the margins.
The flowers, 2 – 3 cm in diameter, have 5 bright yellow petals, and are borne in small racemes in the upper leaf axils, or terminally. The are usually 2 – 6 flowers per raceme. Each flower has 10 stamens or staminodes, and 6 or 7 fertile anthers.
The fruit is a pod, somewhat flattened, straight or slightly falcate, up to about 13 cm long and 1 cm wide, and containing up to about 24 seeds (occasionally to 50), the pods opening at maturity. The seeds are traverse in the pods, and are ovate to oblong, flattened, olive to dark brown, up to about 5 mm long.
The plant reproduces entirely by seed; seeds are dispersed by water or in mud sticking to animal, humans, farm machinery and vehicles. They may also be spread as a contaminant of agricultural produce. It is regarded as an environmental weed in Queensland, the Northern Territory and Western Australia, and in 7 states in the USA. The plant quickly invades over-grazed pastures.
All parts of the plant are suspected of being poisonous to stock, but it is rarely eaten by them. It is reckoned to cause scouring, but evidence of this appears to be anecdotal, rather than conclusive. The seeds have been used as a coffee substitute.
Photographs taken at Picnic Bay 2010
Page last updated 11th February 2017