Black Locust

Robinia pseudoacacia

Invader Images

    • Black Locust flowers have five petals, and are arranged in a spike
      Black Locust flowers have five petals, and are arranged in a spike
    • Leaves are pinnately compound, with small oval leaflets
      Leaves are pinnately compound, with small oval leaflets (pictured is a single leaf with multiple leaflets)
    • Black locust bark and thorns
      Black locust bark and thorns

Common Look-alikes

    • Honey locust (also non-native to the Northeast) has three-pointed thorns
      Honey locust (also non-native to the Northeast) has three-pointed thorns

Identification

Appearance

Deciduous tree to 20m, rough and furrowed bark on trunk and larger branches.

Foliage

Leaves are thin, bright green with a paler/whitish underside, 15cm long, compound in structure (made up of two or more leaflets attached to the leaf stalk, similar to ferns/some wattles) with 10-20 leaflets in opposite pairs and a single terminal leaflet larger than the others.

Seeds

Black, hard-coated, in a flat, bean-like pod, 3-8cm long.

Flowers

Clusters of sweetly perfumed pure white pea flowers in spring.

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. And when using herbicides, always follow the instructions on the label.

Citations

Photo Credit

Black Locust, 5341035, Jan Samanek, Phytosanitary Administration

Black Locust leaf, 0008365, Paul Wray, Iowa State University

Black Locust bark, 5476650, Chris Evans, University of Illinois

Information Credit

1. Molonglo Catchment Group, Black Locust

2. National Park Service, Black Locust

3. GoBotany, Black Locust