Sterculia urens Roxb.

Sterculia urens is a medium-sized, deciduous tree to 15 m in height, usually with a clean, crooked, short bole up to 2 m DBH; branches large, spreading; bark thick, greyish-white or reddish, smooth, shining with a thin, white transparent outer coat, peeling off in papery flakes. Leaves on long petioles, crowded at the ends of branches, palmately 5-lobed, 20-30 cm diameter; tomentose beneath, glabrous above, entire, acuminate; stipules caducous. Flowers greenish yellow, small, in terminal panicles; follicles 4-6, ovoid-oblong, about 2.5 cm diameter, coriaceous, red, covered with stinging hairs. Fruit consists of 5 sessile, radiating, ovate-lanceolate hard, coriaceous carpels, 7.5 cm long, red when ripe, covered outside with many stiff bristles. Seeds 6 mm long, oblong, dark chestnut-brown, 3-6 per carpel. The generic name is based on the Latin name ‘stercus’, meaning ‘manure’, which refers to the smell of the flowers and leaves of some species. The specific name means stinging in reference to the hairs on flowers.

The tree bears a mixture of bisexual and male flowers, the latter are normally numerous. The tree is leafless in winter; flowers in January-March and fruits ripen in April-May, young leaves appearing about the same time.

The tree usually occurs on stony or rocky soils, derived from quartzite, gneiss and schists.

Karaya usually occurs in dry, tropical deciduous forests and is often associated with Boswellia serrata. The tree occupies hilltops, exposed ridges, rocky crevices, eroded slopes and similar habitats. It is a xerophytic species, very resistant to drought and will grow on the poorest of dry stony soils. It is gregarious on ferruginous soils.

S. urens is cultivated in central and northern India for its gum.

Propagation is by seed, which are sown in April. Germination takes 10-15 days and gives nearly 100 % seedlings that reach 15-20 cm height by July.

S. urens is a strong light-demander but seedlings and saplings tolerate shading and are prone to fire damage. The tree does not stand competition and should be protected from browsing animals. It coppices when young.

Defoliation by Sylepta balteata and Oglasa separata has been observed in nurseries, young plantations and forest trees. Cercoseptoria sterculiae, Alternaria macrospora var. sterculiae, Myrothecium roridum, Macrophomina phaseolina, Phoma jolyana and P. macrostoma have been recorded.

Seed storage behaviour is orthodox. Viability is maintained for more than 3 years in hermetic storage at room temperature with 13 % moisture content. There are about 5 300 seeds/kg.

Reclamation: It is useful for reclaiming bare, rocky land.

Food: Trees exude gum karaya used in foodstuffs as emulsifiers, stabilizers and thickeners. Seeds are eaten after roasting. Seeds and young tender roots are eaten in times of famine. Fuel: The tree is used as a fuelwood. Fibre: The bark can be stripped off easily and yields a useful fibre suitable for making coarse cloth and bud ropes. Timber: Sap wood is pale greyish-white, heartwood distinct, red; heavy to very heavy, very strong and very hard, but poor in splitting and retention of shape. It planes and turns to a smooth finish and may also be suitable after seasoning and the adoption of suitable joining techniques for door and window frames, furniture and joinery. It is considered suitable for use as posts, beams, rafters and tool handles. Gum or resin: The tree yields gum karaya from the pith and cortex. The main constituent of the resin is a phlobatannin, containing 3 phenolic OH groups. A large part of the Karaya is used in the pharmaceutical Industry as a bulk laxative and as a denture adhesive in which the finely powdered gum is dusted on the dental plate and swells when it touches the moist surface of the gums. This gives a comfortable and tight fit of the plate.