Robinia pseudoacacia

Native to Pennsylvania, West Virginia, Illinois, Missouri, Oklahoma, Georgia, and Kentucky. Introduced and naturalized in Minnesota.

Invasive

Moist. Forest borders and openings. Shade intolerant.

Mid-May to mid-July

White

30′ to 60′

This is a medium-sized, fast-growing, short-lived, deciduous tree rising on a single trunk from a shallow, wide-spreading root system. It can be 30′ to 100′ in height and 12″ to 40″in diameter at breast height, though in Minnesota mature trees are usually no more than 60′ tall and 6″in diameter. It reproduces by seed and spreads aggressively by suckers. It often forms thickets.

The trunk is often crooked. The crown is narrow, open, and irregular. The branches are spreading to ascending, short, crooked, and brittle.

The bark on young trees is smooth and gray or brown. On mature trees the bark is gray or brown, thick, and separated into long, forking ridges and deep furrows.

First-year twigs are light green and slightly hairy. Second-year twigs are slender, brittle, brown or reddish-brown, and hairless, with scattered lenticels. There is a pair of spines at each node. The spines are broad-based, sharp, and ½″ to 1″ long. The leaf scars are triangular to 3-lobed. They have 3 bundle scars and often 3 irregular cracks.

There are no terminal buds. Lateral buds are tiny, hairy, and not easy to see. Clusters of 3 or more buds are concealed in the leaf scar beneath the base of the leaf.

The leaves are alternate, deciduous, and 3″ to 8″ long. They are pinnately divided into 7 to 19 leaflets. They are on ⅜″ to 1⅜″ long, sparsely to moderately hairy leaf stalks.

The leaflets are elliptical to oblong, ¾″to 2⅜″ long, and ⅜″ to 13 ⁄16″ wide. They are rounded at the base and rounded or blunt at the tip. The leaf tip is occasionally slightly notched and has a short, sharp, abrupt point. The upper surface is dark green, not shiny. The lower surface is pale green and is usually sparsely covered with short, appressed hairs. The margins are untoothed.

The inflorescence is a drooping, unbranched cluster (raceme) rising from upper leaf axils of current-year branches. Each raceme is 2″ to 5″ long and has 8 to 30 flowers.

Individual flowers are ¾″ to 1″ long and white. They are on on 3 ⁄16″ to ⅜″ long, hairy stalks. The 5 green, finely hairy sepals are fused for most of their length into a bell-shaped, ⅛″ to 5 ⁄16″ long tube (calyx), then separated into 5 shallow lobes.

The 5 petals are butterfly-like, as is typical of plants in the Pea family. They are organized into a banner petal, two wing petals, and two fused keel petals. The banner is upright and bent backward along both sides. The wings are straight and project forward. The keel is curved upward. There are 10 stamens and a single style. The flowers are strongly scented.

The fruit is a drooping, narrowly oblong, flat, hairless seedpod containing 2 to 10 seeds. It is 13 ⁄16″ to 4⅓″ long, ⅜″ to 1″ wide. It is green at first, turning brown as it ripens. The pod matures early August to early September.

Records are not kept for nonnative species.