Tree ID: Black locust

The first recorded introduction of black locust was in 1601, when Jean Robin, the herbalist to King Henry IV of France, received seeds and planted them in the royal garden in Paris. This marked the beginning of its cultivation in Europe, where it quickly became popular for ornamental and utilitarian purposes.

Scientific Name: Robinia pseudoacacia

Plant Family: Fabaceae (Legume family)

Clusters of cream colored blossoms with yellow centers. — The black locust is native to North America's east coast but can also be found in Europe and in several Eastern Washington state parks.

Long before the tree appeared in Paris, Indigenous people on this continent used it for medicinal purposes and made crafts and tools from the hard, durable wood.

Early Euro-American settlers used black locust wood to make fence posts, bows, flint making batons, pegs for log cabins, sills for houses, blow gun darts and large mallets. Farmers planted it widely for its ability to enrich the soil with nitrogen (through nodules in its roots), and many of the American Naval ships used in the War of 1812 were made from its wood.

The black locust, which was native to the East coast, hitched a ride to California during the gold rush, where miners cultivated the tree and used the wood to build mining equipment.

A wood round with pronounced rings and fracture. — Black locust wood is extremely hard and strong; it was used to build Naval ships in the War of 1812. Photo: Tim van de Saak.

How to identify them

The black locust bark can be light or dark gray, or grayish-brown. In some cases, the outer bark is gray, with reddish-brown bark in the furrowed diamond-like grooves. Sometimes the trunks are knotty, with protrusions near the base and clusters of leaves growing out of them.

Each leaf is compound, which means it looks like a stem with several leaves growing in neat rows along its length and a single leaf at the tip. The leaflets are almond shaped and lighter green on the undersides. They appear relatively late in spring and turn a clear pale yellow in autumn. The leaflets fold together in wet weather and at night.

The cream-colored blossoms grow in clusters that may resemble geometric patterns when viewed up close. Each flower has a yellow center. Being in the legume family, the black locust drops seedpods that look like pea pods and have legumes inside.

A cream colored hanging cluster of blossoms with yellow centers. — This closeup shows the Black locust blossoms and leaves with leaflets. Photo: H. Zell.

Some of the younger trees also have thorns.

These hardy trees can live to be 100 years old, although 80 to 90 is a typical lifespan. They can reach 90 feet in height and four feet in diameter at the base, though the black locusts in our parks are generally 40 to 60 feet tall.

However, black locusts are prone to beetle infestations. The black locust borer beetle lays eggs in the fall in bark crevices and wounds. In spring, the larvae bore into the tree’s heartwood, and in August they emerge as adult beetles. Over time this decimates the tree.