Black locust

Robinia pseudoacacia
Family: Fabaceae

Natural Historyblack locust branch
Leaves of black locust
Photo credit: Larry Korhnak, University of Florida

Black locust is a legume, meaning that its root nodes work in concert with bacteria to fix nitrogen from the atmosphere into the soil. These soil nitrates are then usable by it as well as other plants. The black locust has a native range in the United States, yet has been transplanted in many states outside of this range as well as in Europe, southern Africa, and Asia. It is capable of having invasive abilities and has become a pest in some of these areas.

The specific epithet for this species means “false acacia.” Black locust looks very similar to trees in the genus Acacia, which are also in the pea family (Fabaceae).

Habitat & Range

The black locust is native from Pennsylvania to northern Georgia and westward as far as Arkansas and Oklahoma, but has been widely spread in other areas of the world. The tree grows on a variety of sites ranging from moist slopes to dry soils, preferring sandy or rocky soil. It is found in old fields, open areas, streamsides, woods, and along roadsides and fencerows. Black locust is shade intolerant and does very well in disturbed areas.

Wildlife Use

Seeds of the black locust are eaten by northern bobwhite quail, mourning dove, wild turkey, white-tailed deer, eastern cottontail, and squirrels. White-tailed deer also eat the leaves and twigs. Many animals use this tree for cover. Cavities in the tree are good homes for bird and other animals, especially woodpeckers. Additionally, black locust is the host-plant for a couple species of butterfly.

Human Use

The wood of the black locust is extremely hard, durable, and resistant to rot. This makes the tree prized for furniture, flooring, paneling, fence posts and small watercraft. The tree is also known for its honey, and from it is made the famous acacia monofloral honey from France. Black locust honey is also produced in the United States. Because the wood burns slowly, has the highest heat content of any species growing widely in the eastern US, and produces little visible flame or smoke, the tree is also heavily utilized as firewood.


 

Identifying Characteristics

Size/Form: Black locust is a medium tree that reaches heights of 60 to 82 feet. It has an open crown.
Leaves: The leaves are pinnately compound, alternate, and deciduous. They generally have 7 to 9 leaflets on a central stalk 20 to 30 inches long with a terminal leaflet. Leaflets are slightly longer than they are wide, ½ to 2 inches long by ½ to 1 inch wide. Leaflets are oval or elliptical in outline. The lower leaflet surface is lighter and covered with short hairs along the midrib. The leaflet margins are entire. The leaflets turn yellow in autumn. A pair of spines at the base of the leaf vary in size.
Twigs: Twigs zigzag and are somewhat stout and angular. Color is red-brown and numerous lenticels are visible. Paired spines exist at each leaf scar, but may be absent on older or slow-growing twigs.
Bark: The bark is light brown to gray. A young tree has smooth bark but as the tree ages the bark becomes furrowed with dark brown, thick, and interlacing ridges.
Flowers: Flowers are perfect and borne in loose drooping racemes 4 to 5 inches long. Cream-white in color, they are about 1 inch long, and bear fragrant nectar.
Fruit: The fruit is an oblong legume measuring 2 to 4 inches in length and about ½ inch wide. Several flat, smooth, and reddish-brown legumes remain on the tree throughout winter. The inside of the fruit contains small pea-sized, hard and dark seeds.
Similar Trees on the Florida 4-H Forest Ecology Contest List:
Several other plants on our list also have alternate, pinnately-compound leaves.

 

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