Black Locust

Robinia pseudoacacia

Images of this species:

Common look-alikes:

Identification

Appearance

A deciduous tree that, while native to parts of the United States, has spread to and become invasive in other parts of the country. Trees grow from 40-100 feet in height. Trees grow upright in forests, but develop an open growth form in more open areas. The bark of black locust is light brown, rough, and becomes very furrowed with age.

Foliage

Leaves are pinnately compound with 7-21 small, round leaflets per leaf. Leaflets are 1.5 inches long. A pair of long, stipular spines is found at the base of most leaves.

Seeds

Black, hard-coated, in a flat, bean-like pod, 2-4 inches long.

Flowers

Flowering occurs in the spring, when showy, fragrant, white to yellow flowers develop in 8 inches long clusters.

Biology

Ecological Threat

Once introduced to an area, black locust expands readily into areas where their shade reduces competition from other (sun-loving) plants. Dense clones of locust create shaded islands with little ground vegetation. Lack of ground fuel limits the use of fire in natural disturbance regimes. The large, fragrant blossoms of black locust compete with native plants for pollinating bees.

Origin

Southeastern US and Appalachian Region

Habitat

Along streams, rivers, fields, meadows, disturbed areas, forest edges

Life Cycle

Fragrant white flowers appear in drooping clusters in May and June and have a yellow blotch on the uppermost petal. Fruit pods are smooth, 2 to 4 inches long, and contain 4 to 8 seeds. Two other locusts native to the Appalachians, Robinia viscosa (with pink flowers), and Robinia hispida (with rose-purple flowers), are used in cultivation and may share black locust’s invasive tendencies.

 

Management Options

This species is considered a watch list species. 

Mechanical Management

Mowing and burning are only effective in reducing the further spread of young shoots from a clone or parent tree. To kill a clone, cutting alone is ineffective.

Chemical Management

Herbicides applied to the stems or cut stumps spread into the root system and provide better control. From mid-June to August hand sprayer application of 6.25% glyphosate solution (15:1 water:glyphosate) to stumps cut near the ground has been used by the Minnesota Department of Natural Resources, Region V State Parks Resource Management Office. Resprouting and suckering from dense clones may require follow up treatment after a few years. Because plants that appear to have been killed can resprout even several years after treatment with herbicide, annual monitoring should be conducted and follow-up treatments made as needed.

  • Be careful not to damage or kill nearby native plants when conducting management work.

  • Always read and follow pesticide label directions. Application of pesticides may require a certification from the Agency of Agriculture, Food and Markets. The Agency website provides information on what applicator certification is needed.  

Citations

Photo Credit

5341035, 5341034, 5341039,5341032, Jan Samanek, Phytosanitary Administration, Bugwood.org

0008365, 0008027, Paul Wray, Iowa State University, Bugwood.org

5476650, Chris Evans, University of Illinois, Bugwood.org

Information Credit

1. Molonglo Catchment Group, Black Locust

2. National Park Service, Black Locust

3. GoBotany, Black Locust