Senna siamea (Lam.) H.S.Irwin & Barneby

Tree, evergreen, up to 10 m high. Young stems ± longitudinally ridged and angled, densely pubescent with short, straight appressed white hairs, becoming ± glabrous with age. Bark on twigs brown, faintly longitudinally striated. Leaves, when very young, silvery with dense, fine short white appressed hairs, at maturity with petiole and rhachis 9-22 cm long; stipules 2-3 mm long, linear-subulate, finely pubescent, caducous; petiole 1.5-3(-5) cm long including basal pulvinus, petiolar and rhachidal glands lacking; rhachis channelled adaxially; leaflets in 5-8(-12) pairs, elliptic, ovate-elliptic or oblong, 2-6.5 cm long, 1-2.5 cm wide, uppermost pair usually not the largest, bases broadly cuneate to rounded, usually symmetric, apices broadly obtuse or rounded, usually emarginate and minutely mucronate, margins yellow, glabrous, surfaces closely veined, pubescent with fine, short, straight white appressed hairs becoming glabrescent then glabrous with age, texture firm coriaceous; petiolule ±3 mm long. Inflorescences corymbose racemes up to 6.5 cm long elongating to 12 cm with age, in the axils of the upper leaves, or aggregated into panicles 15-20 cm long that terminate lateral branches; peduncles, at flowering, 1-2 cm long, at fruiting stouter, blending into the elongated axis of the raceme; bracts ±7 mm long, linear, slightly incurved, sometimes broadened about the middle to ±1 mm wide, densely pubescent, present at time of flowering, eventually deciduous; pedicels, at flowering, 2-3 cm long, at fruiting stouter, hardly longer, pubescent. Sepals obtuse, thick, leathery, densely pubescent abaxially, persisting on the receptacle even until pods are mature. Petals obovate, slightly stalked, 1-1.5 cm long, ±1 cm wide, deep yellow. Stamens 10, all functional: 3 adaxial small, slightly flattened, reduced, 4 lateral and central-abaxial medium, 2 lateral-abaxial large, filaments ±8 mm long, dehiscence porose. Ovaries ± straight, slightly 4-angled, densely and shortly velutinous; style 4-5 mm long, apically recurved; stigma narrowed, both glabrous. Pods compressed, very shortly stalked, linear, 20-27 cm long, 1-1.3 cm wide, dark brown, many seeded, tardily dehiscent, sutures slightly thickened, valves coriaceous, transversely veined. Seeds laterally compressed, elliptic or orbicular, 6-8 mm long, 6-7 mm wide, testa bright shining brown; areole central on each lateral face, oblong, 3-4 mm long, 1.2-1.5 mm wide, with faint transverse cracks.

A tropical plant. It comes from SE tropical Asia and grows in Ethiopia and East Africa. It suits subhumid and arid zones in the tropics. It grows in the Sahel. It needs moist, well drained soils. In East Africa it grows from sea level to 1,600 m altitude. It grows in areas with a rainfall of 700-1500 mm annual average. It is drought and frost tender. It can grow in arid places. It suits hardiness zones 10-11. In Yunnan.

UsesPlanted as ornamental in parks, gardens and frequently used as wayside tree up to 1000 m altitude. As a rapid growing species it is also appreciated for use as shade trees and windbreak in e.g. coffee plantations. It is sometimes running wild but hardly occurs as truly naturalized. Locally used for medicinal purposes. See K. Heyne Nutt. pl. Indon. 3 1950 747 Burkill Diet. Econ. Prod. Malay Penins. 1935 480.

The plants are grown from seed. The seed need to have the hard seed coat broken before planting. The plants can be cut back and will re-grow. They can be used as a hedge.