Robinia pseudoacacia L.

Black Locust

© SRTurner

Family - Fabaceae/Faboideae

Habit - Trees to 15 m or more tall, root suckering and often colonial.

Stems - Woody, single, to 15 m. Twigs sparsely and minutely hairy when young, not bristly, becoming glabrous or nearly so with age. Paired spines present on younger branches.

	New growth twig and pulvini.
© SRTurner

	Bud and spines.
© DETenaglia

Leaves - Alternate, odd-pinnate, typically with 11-19 leaflets, the petioles 2-3 cm long, the rachis 10-25 cm long, sparsely to moderately pubescent with minute, soft, curved hairs. Stipules initially 3-4 mm long, linear, and soft, but often becoming modified into slender to stout spines 5-10 mm long. Leaflets 3-7 cm long, 1.5-3.0 cm wide, ovate to elliptic, the surfaces glabrous or nearly so.

	Leaf adaxial.
© SRTurner

	Leaflet abaxial.
© SRTurner

	Pressed leaf.
© DETenaglia

Inflorescence - Drooping Axillary racemes to 15 cm long, typically with 10-35 flowers, the stalk and axis moderately to densely pubescent with minute curved and often velvety hairs, not bristly. Flowers with the stalk 5-10 mm long.

	Inflorescence.
© SRTurner

Flowers - Calyces with the tube 3-5 mm long, densely short-hairy, the lobes 1-3 mm long, angled or short-tapered to a usually sharply pointed tip. Corollas papilionaceous, white, the banner 20-25 mm long, with a yellow spot toward the base, the wings 15-22 mm long, 4-6 mm wide, the keel 15-22 mm long, 4-7 mm wide. Stamens diadelphous, tube to 1.4 cm long. Filaments 14-18 mm long. Anthers orange, 1 mm long. Ovary 9-12 mm long, the style 7-9 mm long, upcurving near apex.

	Calyx.
© SRTurner

	Calyx and corolla, ventral view.
© SRTurner

	Intact flower.
© SRTurner

	Flower, with wing petals removed.
© SRTurner

	Flower, with keel petal removed to expose functional organs.
© SRTurner

	Corollas.
© SRTurner

Fruits - Legumes 5-11 cm long, 10-14 mm wide, strongly flattened, glabrous at maturity, 3-10-seeded. Seeds 5-6 mm long, 3.0-3.5 mm wide, kidney-shaped, the surface dark brown, often mottled with purplish brown, smooth.

	Fruit.
© DETenaglia

Flowering - May - June.

Habitat - Forests, streambanks, pond margins, bases and tops of bluffs, pastures, fencerows, mine tailings, roadsides, and disturbed areas.

Origin - Native to the U.S.

Lookalikes - Gleditsia triacanthos.

Other info. - This tree is somewhat scattered in Missouri, common in eastern and southwestern counties of the state. It is found across most of the continental U.S. It is considered native in the eastern U.S., introduced elsewhere. Its native range is somewhat obscured due to being extensively spread by Native Americans and early settlers. The flowering tree is unmistakable, with hanging inflorescences of large, white flowers of the legume pattern.

Inflorescences of black locust are showy and very fragrant, and the tree has often been planted as an ornamental shade tree. Care must be used in garden situations, as it can spread aggressively by root suckers. It can grow in poor soils and has been used to revegetate mine barrens. The tree can become quite large and live for centuries if not attacked by insects. The wood is hard, durable, and resistant to decay, and has been made into fence posts, railroad ties, ladders, tool handles, clubs, boats, and even houses.

Black locust is toxic to all types of livestock. Poisoning is usually not fatal but recovery can be protracted. Since the young shoots are often grazed preferentially, the trees should not border areas used for pasturage. The toxicity is due to a lectin glycoprotein, which is especially prevalent in new growth, bark, and seeds.

Photographs taken in Columbia, MO., 2-21-04 and 5-11-04 (DETenaglia); also near Labadie, Franklin County, MO, 5-3-2015 and 4-29-2023, and at Meramec State Park, Franklin County, MO, 5-7-2019 (SRTurner).