Meet a Tree: Black Locust
Black locust has fragrant, drooping white flowers that bloom in late May and June. Photo credit: Jan Samanek, Phytosanitary Administration, Bugwood.org. Each black locust seedpod contains four to eight seeds. Seeds can remain viable in the soil for up to 10 years. Photo credit: Ohio State Weed Lab, The Ohio State University, Bugwood.org. Mature black locust trees have dark brown bark that is deeply furrowed with fl at-topped ridges. Photo credit: Paul Wray, Iowa State University, Bugwood.org. Black locust leaves are made up of 7-21 paired leaflets on both sides of a common stalk. Photo credit: Robert Videki, Doronicum Kft., Bugwood.org.
By Kassandra Tuten, Editor
Black locust (Robinia pseudoacacia) is a fast growing tree in the legume family that can reach up to 75 feet in height. It has deeply furrowed bark with fl at-topped ridges, and seedlings and root sprouts have long thorns and grow rapidly.
Black locust leaves are compound leaves, composed of 7-21 paired leaflets on both sides of a common stalk. Leaflets are oval, 1-2 inches in length, and untoothed. Each leaf is 6-14 inches long and arranged alternately to each other along the twigs. Th ere is one un-paired leaflet at the tip of each leaf and a pair of short, sharp thorns at the base of each leaf. Fragrant, showy, drooping white flowers (which look similar to other pea family flowers) hang from branch tips, appearing in drooping clusters with a yellow blotch on the uppermost petal. Flowers are pollinated by bees and bloom in late May and June. Seedpods are smooth, flat and 2-4 inches long and mature in September, persisting on the trees through the winter. Each pod contains four to eight seeds which are spread by wind, gravity and birds.
Black locust is native to the U.S., but not to the Upper Midwest. Th e native habitats of black locust are temperate, moist to wet forests and wooded slopes. It occurs naturally on the lower slopes of the Appalachian Mountains and has been extensively planted for its nitrogen-fixing qualities in land reclamation, erosion control, for ornamental purposes and for honey production. It is currently spreading into states including Minnesota, which is outside of its historical range, and is a threat to native habitats.
Black locust primarily invades disturbed habitats, degraded woods, thickets and old fi elds, where it crowds out native vegetation and forms single species stands. Th e species spreads mainly through intentional human plantings and disturbance (such as through methods of mechanical control by mowing and burning, which is temporarily effective but can actually stimulate vigorous re-sprouting). The seeds are long-lived, remaining viable in the soil for up to 10 years. Seedlings can sprout and grow fast, enabling this species to colonize disturbed sites very quickly. Once established on a site, the trees reproduce vigorously by root suckering and stump sprouting to form clonal groves linked by a shared root system. This species can change nutrient cycling due to its ability to fix nitrogen in the soil and can potentially create favorable habitat for other non-native species.
Did You Know:
Black locust has been reported in about half of the 87 counties of Minnesota.