Flower:

Showy, hanging racemes in leaf axils and tips of 1-year-old branches, each cluster with up to 15 pale to bright rose pink pea-like flowers. The flowers are about 1 inch long, the broad upper petal (standard) erect with a spot of 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, purplish to green with 5 triangular lobes that are long-tapering to a pointed tip. The calyx and flower stalks are all moderately to densely covered in a mix of short white hairs and longer, spreading, red bristles. Bristles have a small, sticky gland at the tip.

Leaves and stems:

Leaves are alternate, once compound, 6 to 12 inches long with 7 to 13 leaflets, the lowest pair usually smallest. Leaflets are elliptic to egg-shaped to nearly round, 1 to 2½ inches long, mostly rounded at the base, rounded and tip with a minute sharp point at the apex. Edges are toothless, the upper surface smooth, lower surface with sparse white hairs on the surface and major veins and scattered red prickles along the midrib.

Young shoots and leaf stalks are green and covered in a mix of short white hairs and longer, spreading, glandular red bristles. A the base of leaf and leaflet stalks are narrow appendages (stipules), those on the leaflet stalks smaller that those of the compound leaf.

Branches become darker gray-brown with pale lenticels (pores) the second year, the red bristles turning brown and stiff, eventually dropping off except for a pair of larger, stiff prickles at each node though they, too, eventually drop off. Older bark is gray-brown with scattered shallow furrows. Stems are single or multiple from the base and can form dense thickets from root suckers.

Fruit:

Fruit is a flat pod 2 to 3 inches long, green turning brown at maturity, and densely covered in red prickles. Pods contain 3 to 5 seeds.

Notes:

Native to the southeastern US, Bristly Locust's showy flowers and bristly stems and fruits likely made it an attractive addition to the garden trade. Unfortunately it escapes cultivation, can produce dense colonies through root suckering and can be difficult to manage without resorting to toxic chemicals. There are few records of it in the Bell Herbarium but we are seeing it more and more in the wild; additional Minnesota populations are currently being tracked at EDDMapS. It is readily identified at virtually any stage of growth from the compound leaves with red-prickly stalks and young branches, the prickles persisting through the second year, though turning brown. There are several varieties of Robinia hispida, all of which are native to various parts of the southeastern US.  The vars are poorly documented but only var. hispida has been officially recorded in Minnesota.