Black Locust

Common Name: Black Locust
Scientific Name: Robinia pseudoacacia L.

Legal Status

Restricted

Propagation and sale of this plant are prohibited in Minnesota. Transportation is only allowed when in compliance with Minnesota Statute 18.82. Although Restricted Noxious Weeds are not required to be controlled or eradicated by law, landowners are strongly encouraged to manage these invasive plants on their properties in order to reduce spread into new areas. 

Background

Black locust is native to North America, but it is not native to Minnesota or the Upper Midwest. It is native to the Appalachian region and the Ozarks, where it is also considered a weed. It has been widely planted as an ornamental, for land reclamation, for erosion control, and for honey production. Black locust is spreading into new areas and is now a threat in states outside its historical range.

Description

• Black locust is a fast-growing tree in the legume family. Adult plants can reach 40 - 90 feet in height.
• The leaves are alternate, pinnately compound with 7 - 21 oval leaflets.
• Seedlings are easily identified by long pairs of thorns. A pair of short, sharp thorns occurs where each leaf attaches to the twig.
• Young saplings have smooth, green bark. Mature trees have dark brown bark that is deeply furrowed with flat-topped ridges.
• Flowers are showy, fragrant, and white, appearing in drooping clusters with a yellow blotch on the uppermost petal. Flowers are pollinated by bees and typically bloom in May and June.
• Fruit pods are smooth, flat, 2 - 4 inches long, and contain 4 to 8 seeds.
• Fibrous roots may be shallow or deep depending on soil type. Roots are wide spreading, highly adventitious, suckering especially after being cut or damaged, and creating dense thickets of new growth.

Habitat

The native habitats of black locust are temperate, moist to wet forests and wooded slopes. It will invade disturbed sites such as old fields, cleared areas, and degraded woods and thickets.

Means of spread and distribution

Black locust spreads mainly through intentional human plantings and disturbance. The seeds are long-lived, remaining viable in the soil for up to 10 years, although germination rates are generally low. Seeds may spread moderate distances by wind, gravity, and possibly birds. The seedlings can sprout and grow fast, allowing the species to colonize disturbed sites very quickly. Once established on a site, black locust reproduces vigorously by root suckering and stump sprouting, forming clonal groves linked by a shared root system.

Black locust occurs throughout the eastern United States, from the East Coast to the Great Plains. In Minnesota, it has been reported in about half of the state's 87 counties.

Impact

Black locust can outcompete and exclude other tree species as well as native prairie and savanna plants. It can form a monoculture on disturbed sites.

Being it is a legume, black locust can fix nitrogen in the soil, which may alter local soil characteristics. Increased soil nitrogen can create favorable conditions for other colonizing nonnative plant species.

Prevention and management

• Once established, black locust is difficult to control. For all management options, infestation sites must be monitored and treated repeatedly until the seedbanks and root reserves are depleted. Sprouts and stumps that appear to have died can resprout even years down the road, so annual monitoring should be conducted and follow-up treatments administered.
• Mowing and burning alone can stimulate suckering and clone production. They are only recommended in combination with herbicide treatment. Black locust seeds can be stimulated to germinate in response to fire as well.
• Large infestations can be managed with cut-stem herbicide applications. Herbicide applied to stems or cut stumps spread into the root system and provide better control. In large, dense infestations with little else growing, foliar applications of herbicide can be effective. When using herbicide treatments, always check with your local University of Minnesota Extension Agent, co-op, or certified landscape care expert for assistance and recommendations. There are several businesses throughout the state with certified herbicide applicators that can be hired to perform chemical applications.

Toxicity

Black locust leaves, stem, bark, and seeds contain robin, a toxic compound. Ingestion results in gastrointestinal and neurological dysfunctions which are particularly acute in horses and can be fatal.