Arsenic bush

Senna septemtrionalis
Shrub
Alternate
Compound
Yellow
Green

S. septemtrionalis is a leafy shrub or small tree which abundantly produces seeds that are easily dispersed by humans (machinery and vehicles), birds and animals, and by water. Once established in new areas, this species matures quickly, negatively impacting native flora.

Common names 
Also known as: Smooth senna, yellow shower, buttercup bush, dooleyweed, laburnum,
Family 
Caesalpiniaceae
Deciduous 
No
Flowering time 
Flowering occurs mainly during spring and autumn.
Native/Exotic 
Exotic
Origin 
Mexico and Central America.
Notifiable 
No
State declaration 
Nil
Council declaration 
Class R – Reduce populations
Known distribution 

Widely naturalised in eastern Australia (i.e. in eastern Queensland and the coastal districts of New South Wales). Also naturalised on Lord Howe Island and Norfolk Island, and possibly naturalised in Victoria.

Habitat 

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

Habit 

Usually a shrub growing 1-3 m tall, but occasionally a small tree reaching up to 5 m or more in height.

Impact and control methods 

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

Stem and leaves 

The once-compound (i.e. pinnate) leaves are hairless (i.e. glabrous) and alternately arranged along the stems. These leaves (6-10.5 cm long) are borne on stalks (i.e. petioles) 15-35 mm long and have 3-5 pairs of leaflets. There is a small cone-shaped structure (i.e. erect conical gland) between the lowest three or four pairs of leaflets. The leaflets (3.5-9 cm long and 15-35 mm wide) are elongated to egg-shaped in outline (i.e. lanceolate to narrowly ovate) with entire margins and pointed tips (i.e. acuminate apices). They have glossy green upper surfaces and paler green undersides.

Flowers and fruits 

The flower clusters (2.5-8 cm long) are borne on stalks (i.e. peduncles) 3-4 cm long and usually contain 4-10 flowers. Individual flowers are borne on stalks (i.e. pedicels) 10-13 mm long that elongate to 25 mm in fruit. Each flower has five yellowish-green, yellowish-brown, or pale yellow sepals (4-10 mm long) and five bright yellow or golden yellow petals (12-16 mm long). The uppermost petal (i.e. standard) is slightly larger than the others and has a notched tip. The flowers also have six or seven stamens, two of which are larger than the others, and three partially-formed stamens (i.e. staminodes). Flowering occurs mainly during spring and autumn.

The cylindrical pods (5-10 cm long and 7-12 mm wide) turn from green to brown or straw-coloured as they mature. They are usually borne in an upright position on stiff stalks. The olive or brown seeds (3.5-5 mm long) are somewhat flattened with smooth or minutely pitted surfaces.

Reproduction and dispersal 

This plant reproduces mainly by seed, which are dispersed by water or in mud sticking to animals, humans, machinery and vehicles. They may also be spread as a contaminant of agricultural produce.