Plant of the Week
Robinia pseudoacacia range map. USDA PLANTS Database.
Close up of flower. Photo by David D. Taylor.
Tree at Middlefart, Denmark; Slotsbanke Hindsgavl (Hinds Gable at Castle Hill) peninsula. Photo by David D. Taylor.
Twig with stipular spines. Photo by David D. Taylor.
Cluster of fruits (legumes). Photo by David D. Taylor.
Bark. Photo by David D. Taylor.
Black Locust (Robinia pseudoacacia)
By David Taylor
Black locust is a member of the Fabaceae (pea) family. The pea family includes species that are large trees, shrubs, herbs and vines. Well known species such as garden peas (Pisum sativum), peanut (Arachis hypogaea), mesquite (Prosopis species), and mimosa (Albizia julibrissin). It also includes spices such as tamarind (Tamarindus indica) and fenugreek (Trigonella corniculata).
This tree may reach 25 meters (82 feet) tall and up to 120 centimeters (47 inches) diameter. The bark on older trees is thick and deeply furrowed. Twigs are generally smooth or finely hairy, but often armed with a pair of sharp spines at each leaf or leaf scar (see photo). Leaves are pinnately compound with 3 to 9 pairs of opposite leaflets plus a terminal leaflet. They are 18 to 35 centimeters (7 to 18 inches) long by 12 to 15 centimeters (4.75 to 6 inches) wide. Leaflets are oval or elliptical and 2 to 5 centimeters (0.75 to 2 inches) long. Flowers are in axillary clusters (racemes) that are 10 to 20 centimeters (4 to 8 inches) long. The white flowers are very fragrant and 2 to 2.5 centimeters (0.75 to 1 inches) long. The upper petal has yellow spot on it. The fruits are dry legumes (pea pod like) which split open to release a few seeds. The fruits are dark brown to almost black in color and 5 to 10 centimeters (2 to 4 inches) long.
Black locust flowers in late April to early June depending on location. The species needs open ground and much sunlight to become established. Trees found in forests established in openings, burned areas, or similar locations. It is frequent on roadsides, in fencerows, old pastures, and disturbed areas. While growing on almost any soil type, base rich soils promote some of the best growth. It is considered native from Pennsylvania through the Appalachians to northern Georgia and westward to Arkansas and Oklahoma. It is documented from every conterminous state in the United States. It also occurs in Ontario to Nova Scotia and British Columbia. It has also been introduced to Europe (ca. 1636), Southern Africa, and Asia, in part as an ornamental and in part for firewood and fence post production.
The species is widely cultivated, but often becomes invasive due to high seed production and prolific root sprouts. The flowers are an important source of nectar/pollen for bees and the honey produced is considered choice. The wood is hard and durable, often used for fence posts. In parts of its range, leaf miners (larvae of the beetle Odontota dorsalis) turn the leaves brown in early summer. As the species fixes nitrogen, it was widely planted on old strip mines to improve soil quality.