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Shrub or tree to 20 m, 50 cm dbh; habit extremely variable, though the crown is typically narrow and open. Bark rough, dark greyish brown with rust-coloured longitudinal fissures. Branchlets smooth and greyish brown, terete and covered in white hairs or glabrous. Leaves bipinnate, green though lower surface paler, (10–)12–25 × (5–)7–15 cm; pinnae (four to) six to eight (to nine) pairs, pinnae rachis with one to two (to three) nectaries; pinnules 13–21 pairs, linear to oblong or elliptic, 0.9–1.6(–2.1) × 0.2–0.4 cm, sessile and asymmetrical, apex acute to acuminate, margin ciliate; petiole 1.3–3.4 cm long, green or yellowish green and with a single nectary; rachis (5–)7–15.5 cm long with one to two (to four) small, concave nectaries, apex extending beyond terminal pinnae as a mucro 0.2–0.5 cm long; stipules ovate or lanceolate, 0.3–0.4 cm long with a pair of asymmetrical wings, persistent though shrinking. Inflorescences capitate, 1.2–2.1 cm diameter, bearing 100–180 flowers; these in fascicles of two to six in the axils of new shoots only; peduncle 1.5–2 cm long with a whorl of dentate bracts near the apex. Flowers densely-packed, 5-merous, sepals and petals pale green, rather insignificant; stamens with white filaments. Legumes (3–)5–20 per capitulum; pendulous, linear-oblong, compressed, 11–19 × 1.5–2.1 cm, papery and dehiscent. Hughes 1998. Distribution The exact range of L. leucocephala is difficult to determine because of human activity, though it is probably native to Mexico and Central America. Habitat As a pantropical weed, it tolerates a range of conditions, but is particularly often found in open (often coastal or riverine) habitats, semi-natural, disturbed, degraded habitats and urban sites. USDA Hardiness Zone 9–10. Conservation status Not evaluated. Illustration NT447. Taxonomic note Three subspecies of L. leucocephala are recognised by Hughes (1998). Subsp. leucocephala (‘Common’ or ‘Hawaiian’ type) was transported to the Philippines by the Spanish before 1815 and is now pantropical. It is more often shrubby than tree-like. Subsp. glabrata (Rose) Zárate (‘Giant’ or ‘Salvador’ type) is typically arborescent and was widely introduced across the tropics in the 1970s and 1980s (Hughes 2005). Subsp. ixtahuacana C.E. Hughes is not known outside its native range (Hughes 1998).
Leucaena leucocephala is an extremely important tree in the semi-arid tropics and subtropics, where its vigorous growth even in difficult circumstances makes it a valuable source of a range of forage products and firewood (Mabberley 1997a). Its ubiquity in these areas and its temptingly obvious pods make it an easy target for casual collectors on holiday, and it is probable that its cultivation is often attempted – indeed, Susyn Andrews (pers. comm. 2008) reports that it is quite frequently submitted to the Enquiry Unit at Kew for identification. It could therefore be included here as a speculative species. In the Hoyt Arboretum, however, it is well established, a group of trees there being now about 4.5 m tall – limited somewhat in development by the dry summers of Portland (S. Hogan, pers. comm. 2007). Provenance does matter, as other stocks have frozen back to the ground at Cistus Nursery, on low-lying Sauvie Island. Summer warmth to ripen the wood and a winter that is not too severe are probably essential requirements for its success. As a tree seen in the tropics it is apt to be rather gaunt, but the slightly pewtered bipinnate leaves and white flower pompoms are not unattractive. As with most legumes, the seeds should be soaked in very hot water before sowing.