Flower:

Showy, hanging racemes 2 to 5 inches long in leaf axils of 1-year-old branches, each with up to 30 white pea-like flowers. The flowers are fragrant, ¾ to 1 inch long, the upper petal (standard) erect and greenish yellow at the base, the lower petal (keel) hiding the 10 stamens and single slender, curved style. The calyx surrounding the base of the flower is tubular, about ¼ inch long, hairy, often reddish or brownish green with 5 triangular lobes that are shorter than the tube.

Leaves and stems:

Leaves are alternate, once compound, 4 to 12 inches long with 7 to 23 leaflets. Leaflets are elliptic to narrowly egg-shaped, ¾ to 2¼ inches long, rounded or tapered at the base and blunt or rounded at the tip, sometimes with a small notch. Edges are toothless, the upper surface smooth or finely hairy, lower surface with short appressed hairs along major veins. Fall color is typically a dirty yellow.

Young shoots are green and hairy with numerous pale lenticels (pores), turning red-brown and smooth. A pair of spines, ½ to ¾ inch long, may persist at the leaf scars or drop off older twigs. Buds are embedded in old leaf scars.

Branches become increasingly darker gray to gray-brown, the bark on the trunk coarse and deeply furrowed with forking ridges. The trunk can reach 24 inches in diameter at breast height (dbh).

Fruit:

Fruit is a small, thin, flat pod 2 to 4 inches long, green turning reddish or purplish brown at maturity.

Pods contain up to 10 hard seeds, mottled brown and black and about 1/6 inch long.

Notes:

Native to the higher elevations of the southeastern US, Black Locust's fast growth rate, tolerance of harsh growing conditions and its tough, rot resistant lumber gave it a high value during early settlement. It is also quite a pretty tree and horticulturists, especially in Europe, have made numerous selections on form and leaf color. In the past two centuries, humans seemed to have planted it everywhere and it is now widely established throughout much of the US, Europe, China and southern Australia. Unfortunately it is not well behaved, spreading by seed and, once established, produces large colonies through root suckering. It can readily exclude native shade trees important to natural ecosystems and due to its root suckers, difficult to manage without resorting to toxic chemicals. Young trees and saplings may be mistaken for Prickly Ash (Zanthoxylum americanum), which has leaflets with pointed tips, at least a few prickles along leaf stalks, and buds that are clearly visible, where Black Locust leaflets are rounded, has no spines or prickles on leaf stalks, and buds are obscured in the leaf scars. Seedlings and new root suckers may also be confused with any number of other members of the Pea family with similar compound leaves, such as Veiny Pea (Lathyrus venosus) or False Indigo (Amorpha fruticosa), all of which would lack spines.