1. Albizia julibrissin Durazz. (silk tree, mimosa)

Map 11700, Pl. 387 i–l

Plants small trees or large shrubs to 10 m tall, often several-trunked and more or less flat-topped, unarmed, the wood soft, the bark smooth, gray and sometimes somewhat mottled. Twigs often somewhat zigzag, usually several ridged or finely fluted, green to brown or gray with scattered, small, lighter spots or bumps, glabrous, the winter buds small, more or less circular and slightly flattened, with several overlapping scales, the outer pair more or less obscuring the inner scales. Leaves with the petiole 3–6 cm long, the petiole and rachis moderately pubescent with short, curved hairs on the upper surface, with a large, lozenge-shaped gland near the petiole base and sometimes also between the uppermost pair of pinnae. Stipules shed during leaf development, small, linear. Leaf blades 10–22 cm long, with 5–12 pairs of pinnae, each with 13–30 pairs of leaflets. Leaflets 8–13 mm long, 1.5–4.0 mm wide, asymmetrically oblong to slightly crescent (the midvein near the upper margin), angled or short-tapered to a sharply pointed tip, the margins with minute, ascending hairs, the surfaces glabrous or sparsely hairy. Inflorescences conical to hemispheric, headlike clusters, these arranged in terminal racemes, each 2.5–4.0 cm in longest dimension (including the stamens), the stalk 2–5 cm long, with 15–30 or more sessile flowers. Calyces 2–4 mm long, narrowly conical to nearly tubular, fused most of the way to the tip, with bluntly to sharply pointed, broadly triangular lobes, glabrous or sparsely short-hairy. Corollas 7–8 mm long, shallowly lobed, white to pale pink. Stamens numerous (more than 20), the filaments 20–25 mm long, fused toward the base, pink to reddish pink. Fruits 8–20 cm long, 10–20 mm wide, narrowly oblong, tapered to a short, stalklike base, strongly flattened, straight, the margins sometimes slightly undulate and occasionally more strongly narrowed between a few of the seeds, the surfaces slightly constricted between the seeds, lacking prickles, finely hairy when young, usually glabrous or nearly so at maturity, light tan at maturity, indehiscent or slowly dehiscent with age. Seeds 12–18, 7–8 mm long, 4 mm wide, oblong-ovate, flattened, light brown. 2n=26. June–August.

Scattered, mostly south of the Missouri River (native of Asia, widely introduced in the southern U.S. north to New York, Illinois, Utah, and California). Bottomland forests and mesic upland forests; also old fields, pastures, railroads, roadsides, and disturbed areas.

This species is grown as an ornamental for its gracefully spreading branches, delicate leaves, long flowering period, and attractive flower clusters. It is easily propagated from seeds or cuttings and grows rapidly. Unfortunately, these same features have led to its widespread establishment as an invasive exotic in the southern half of the state, particularly in southeastern Missouri. Steyermark (1963) merely mentioned A. julibrissin as a cultivated relative of Acacia but did not know it outside of cultivation. In his survey of legumes of the United States, Isely (1973) mapped it from southwestern and central Missouri as an escape from cultivation without any further details. Thompson (1980) documented the plant from an old pasture in Wayne County in the Mingo National Wildlife Refuge. Today, silk tree is present in most counties in the Ozark, Ozark Border, and Mississippi Lowlands Divisions, although it is still quite undercollected (as attested by the map for the species in the present work).