Black Locust (Robinia pseudoacacia)

Robina-pseudoacacia-2013-05-18-Beechview-01This tree is ubiquitous in southwestern Pennsylvania, so it may come as some surprise to Pittsburghers that we live at the northern end of a native range that is actually very small, mostly in the Appalachians and foothills. The Black Locust has been much planted elsewhere, however, and may easily naturalize itself. It is in many ways an ideal urban tree: it grows fast, tolerates city conditions with no complaints, and has showy clusters of white pea flowers after most of the other flowering trees have stopped blooming. It does, however, have one serious flaw. Mature specimens are brittle, and can easily drop large branches in storms, crushing cars or bringing down power lines. When your power goes out in a thunderstorm, there’s a very good chance you have a Black Locust to blame.

The dangling chains of white pea-shaped flowers are unique among our native trees. Leaves are pinnately compound, with smooth-edged elliptical leaflets. The leaves often turn yellow and begin to drop in August, long before any other trees are even thinking about autumn. Mature trees have rough and shaggy bark, and often show scars of broken branches.

Gray describes the genus and the species (which he spells “Pseudo-Acacia”):

ROBÍNIA L. LOCUST. Calyx short, 5-toothed, slightly 2-lipped. Standard large and rounded, turned back, scarcely longer than the wings and keel. Stamens diadelphous. Pod linear, flat, several-seeded, at length 2-valved. — Trees or shrubs, often with spines for stipules. Leaves odd-pinnate, the ovate or oblong leaflets stipellate. Flowers showy, in hanging axillary racemes. (Named for John Robin, herbalist to Henry IV. of France, and his son Vespasian Robin, who first cultivated the Locust-tree in Europe.)

R. Pseùdo-Acàcia L. (common L., False Acacia.) Branches glabrous or glabrate; racemes slender, loose; flowers white, fragrant; pod smooth.— Along the mts., Pa. to Ga., and in the Ozark Mts. of Mo., Ark., and Okla.; commonly cultivated as an ornamental tree, and for its valuable timber, and naturalized in many places. May, June.

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