Black Locust

Robinia pseudoacacia

black_locust_robinia_pseudoacacia.jpg

Illustration of black locust leaves and flowers.
Black locust, Robinia pseudoacacia.
Paul Nelson
Thorny
Family

Fabaceae (beans)

Description

Black locust is a medium-sized tree with an irregular, open crown and some low branches, as well as several large upright branches.

Leaves are alternate, feather-compound, 8–14 inches long, with 7–19 leaflets; leaflets ½–2 inches long, with tiny bracts at the base, the margin entire; upper surface bluish- to dark green, dull, smooth; lower surface paler, smooth except with hairs on the veins.

Bark is reddish-brown to almost black, with deep grooves and long, flat-topped, interlacing ridges.

Twigs are dark brown, stout, zigzag, brittle, smooth, angled with a pair of small spines where the leaves attach.

Flowers in May–June; pea-shaped, showy, white, fragrant, in loose, drooping clusters 4–5 inches long; each flower has five petals, with a yellow blotch on the inside of the uppermost petal.

Fruit in flattened pods 3–5 inches long and about ½ inch wide, smooth, reddish-brown, splitting along both sides; seeds 4–8, spotted, flattened; pods often persist on the tree through winter.

Size

Height: to 70 feet; spread: to 45 feet.

black_locust.jpg

Image of a black locust leaf
Black Locust

black_locust_in_flower_scan_1.jpg

black locust tree in flower
Habitat and conservation

Occurs in dry or rocky upland woods, along streams, and in pastures, thickets, and disturbed sites. A pioneer species, black locust easily invades disturbed sites, often developing clones from sucker sprouts. A tough tree, it is difficult to eradicate once established. The wood is ranked as the seventh hardest of any tree in North America. Widely planted for windbreaks, soil erosion control, and fence posts; also a popular fuelwood.

image of Black Locust Distribution Map
Distribution in Missouri

Statewide.

Status

Common. Because it easily invades disturbed sites, often developing clones from sucker sprouts, this tough tree can be hard to eradicate, and for this reason some consider it a nuisance.

Life cycle

Black locust grows best in humid climates, although it has been introduced in many parts of the world where the climate is much drier. Black locust is a prolific seed producer, but seedlings are not common; few seeds germinate because of the impermeable seed coat. Most natural reproduction is vegetative by means of root suckering and stump sprouting. Root suckers arise spontaneously from the extensive root system of trees as young as 4–5 years old.

Human connections

Planted as an ornamental. Native Americans chewed the root bark to induce vomiting and to reduce toothaches. All parts of the tree are considered toxic, although beekeepers disagree about the honey's toxicity. Some deem it the best of honeys. The exceptionally hard wood has had many uses.

Ecosystem connections

The flowers are a good nectar source for bees. Deer browse the leaves and twigs. Bobwhite and squirrels eat the seeds. This tree is an important colonizer of disturbed sites.