Black Locust (Robinia pseudoacacia)

Family:

Bean Family (Fabaceae)

Other Names:

false acacia, locust tree.

Origin and Distribution:

Black locust was spread outside its native range, which included southern Appalachia and the Ozarks, by early settlers who planted it as an ornamental and for erosion control. Now, it occurs as a weed throughout eastern and central U.S. and in Canada. Black locust was first introduced into the southern portion of Ohio, but is now distributed throughout the entire state. This species grows in a variety of habitats including roadsides, fence rows, fields, farmyards, woods, riverbanks, and waste places. It does not tolerate shade or poor drainage and grows better in light-textured soils such as loam, but it will grow in clay soils.

Plant Description:

Black locust is a perennial shrub or medium-sized tree. Each compound leaf has 7 to 25 leaflets arranged such that one leaflet is located at the tip and the rest are in pairs. At the base of young leaves are two spines, one located on each side of the leaf stalk (petiole) where it attaches to the stem. Other characterisitic features of this member of the bean family include fragrant white flowers in long, drooping clusters and 4-inch-long, brown seedpods. Reproduction is by seeds and shoots that sprout readily from roots of established plants.

  • Root System:

    Roots tend to remain near the soil surface and spread laterally.

  • Seedlings and Shoots:

    Shoots of young plants are zigzagged, smooth, green and become red-brown and spiny with age.

  • Stems:

    Trunks of mature plants are dark brown (almost black) and deeply furrowed. Trees grow about 80 feet tall and average between 2 to 4 feet in diameter. A pair of stiff spines are located at the base of each leaf stalk (petiole) on younger branches. These spines persist for several years, and can grow 1/4 to 1 inch long.

  • Leaves:

    Leaves are alternate (1 leaf per node) and 8 to 14 inches long. Each leaf consists of 7 to 25 leaflets with a single leaflet located at the tip and the rest in pairs, so total number of leaflets is always odd. Leaflets are toothless, oval, and 1 to 2 inches long.

  • Flowers:

    Flowers are white, medium-sized, and grow in long, drooping clusters ranging from 4 to 8 inches long. They resemble sweet pea flowers in shape and smell.

  • Fruits and Seeds:

    The red to orange-brown pods are flat, hairless, 2 to 6 inches long, and contain 2 to 10 kidney-shaped, brown seeds.

Similar Species:

Prickly ash (Xanthoxylum spp.) has reddish buds, leaflets with teeth, and thorny petioles whereas black locust buds are hairy and white, its leaflets are smooth, and its petioles lack spines. The appearance of honey locust (Gleditsia triacanthos) is similar to that of black locust except their spines are unpaired and branched.

Biology:

Black locust flowers from May to June. Pods appear in September and remain until the following spring. As with most members of the bean family, black locusts fix nitrogen. This trait along with the ability to grow from root sprouts results in a rapidly spreading, competitive species. When cut, shoots readily sprout from stumps. Therefore, shoots must be repeatedly cut or herbicide brushed on the freshly cut stump to kill the tree.

Toxicity:

Black locust is poisonous to all animals if ingested. Although fatal cases are rare, recovery may take several days or even weeks. There have been reports of children poisoned by chewing the inner bark or eating seeds. However, most reported cases involve horses that became ill after eating young shoots or chewing bark. Cows, chickens, and sheep have also been poisoned. The toxic young shoots appear to be desired by livestock, even if there is plenty of other forage available. It has been suggested that flowers are toxic as well. In some cases, it may be advisable to fence off black locust trees to eliminate access of grazing livestock to shoots, bark, and seeds. Care should be taken after black locust trees are cut to remove any shoots that are likely to sprout from the stumps.

Facts and Folklore:

  • Although black locust is toxic to humans and livestock, it is thought that birds, rabbits, and deer can safely eat the seeds.

  • The wood of black locust is very hard and durable. It was commonly used for fence posts that resisted decay and also in ship building.

  • Black locust was one of the first trees to be introduced from America into England.

  • The tree has been used in reforestation and to control soil erosion because of its rapid, spreading growth and nitrogen-fixing ability.

  • Flowers are used to make perfume.