Black locust (Robinia pseudoacacia) is native to the U.S., but not to the Upper Midwest. It is currently spreading into states like Minnesota that are outside of its historical range. Black locust primarily invades disturbed habitats, degraded woods, thickets and old fields, where it crowds out native vegetation and forms single species stands.
Caution: Black locust leaves, stem, bark and seeds contain the toxic compounds robinin and robitin. Eating these toxins causes gastrointestinal and neurological dysfunctions, which are particularly bad in horses and can be fatal.
Black locust is a fast growing tree that can reach up to 75 feet in height. It has with deeply furrowed bark with flat-topped ridges. Seedlings and root sprouts have long thorns and grow rapidly.
Leaves and stem
Black locust leaves are made up to 7-21 paired leaflets on both sides of a common stalk (pinnately compound). Leaflets are oval, 1-2 inches in length, and untoothed. Each leaf is six to 14 inches long and arranged alternate to each other along the twigs. There is one un-paired leaflet at the tip of each leaf and a pair of short, sharp thorns at the base of each leaf where it attaches to the twig. Seedlings have smooth, green bark, while mature trees have dark brown bark that is deeply furrowed with flat-topped ridges.
Fragrant, drooping white flowers arranged in eight-inch clusters hang from branch tips. Flower bloom in late May and June. Black locust is part of the pea family and its flowers look similar to other pea-family flowers.
Seedpods are smooth, flat and 2-4 inches long. They mature in September and persist on the trees through the winter. Each pod contains four to eight seeds. Seeds are spread by wind, gravity and possibly birds. Seeds can remain viable in the soil for up to 10 years.
Black locust has an extensive fibrous root system that can be shallow or deep, depending on the soil. It spreads vegetatively through root suckering and runners, especially after being cut or damaged.
Black locust primarily invades disturbed habitats, degraded woods, thickets and old fields. It reproduces vigorously by root suckering and stump sprouting, forming dense thickets with a common connecting root system.
Origin and spread
Black locust is native to the U.S., but not to Minnesota or the Upper Midwest. It occurs naturally on the lower slopes of the Appalachian Mountains, where it is known as a weedy tree. It has been extensively planted for its nitrogen-fixing qualities in land reclamation, erosion control, and its hard wood. It is currently spreading into states like Minnesota that are outside of its historical range. For the current distribution of black locust, visit EDDMapS.
Don't be fooled by these look-alikes
- Kentucky coffeetree, Gymnocladus dioicus (native) - leaves are made up of 40-60 pointed leaflets, arranged on branching stalks (bi-pinnately compound), instead of black locust's 7-21 leaflets along only a single stalk; coffeetree seed pods are much larger (six to ten inches)
- Honey locust, Gleditsia tricanthos (native) - leaves are made up of 18 to 28 small leaflets with fine teeth arranged on forked stalks (bi-pinnately compound); seed pod is much larger (10 – 18 inches)
- Black walnut, Juglans nigra (native) - seven to nine pointed-tip leaflets on each leaf; round nut (not seed pod)
- Tree of heaven, Ailanthus altissima (invasive): some of the leaflets have small teeth or glands near where they attach to the stalk, one seed per seed pod.
- Regulatory classification
This species is a Minnesota Department of Agriculture Restricted Noxious Weed meaning it is illegal to import, sell, or transport.
- Threat to Minnesota
- Black locust crowds out native vegetation in prairies, oak savannas and upland forests, forming single species stands.
- 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.
- What you should do
One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.
- REMOVE plants, animals and mud from boots, gear, pets and vehicles.
- CLEAN your gear before entering and leaving the recreation site.
- STAY on designated roads and trails.
- PLANT non-invasive species.
- Native substitutes
- Control methods
Mechanical control can be done by mowing and burning, but mechanical control is only temporarily effective because of black locusts' ability to resprout and spread vegetatively. Such damage to stems can actually stimulate vigorous re-sprouting.
Herbicide control can be done by cutting the stem and applying glyphosate or triclopyr to the cut stump. Oil-based triclopyr ester products can also be sprayed along the base of an un-cut stem, coating all sides of the lower 12-18 inches of the main stem.These are systemic herbicides that are taken up by plants and move within the plant, which can kill leaves, stems, and roots. Foliar spray of glyphosate or clopyralid can also be effective on single-species stands, where there are not desirable plants nearby that could be damaged.
- Black locust identification training module (Midwest Invasive Species Information Network)
- Identification and management of black locust (Minnesota Department of Agriculture)
- Identification and management of black locust (Woody Invasives of the Great Lakes Collaborative)
- Black locust management recommendations (Midwest Invasive Plant Network Plant Control Database)
- Management of unwanted trees and shrubs (University of Minnesota Extension)
- Identification and management of Minnesota Noxious Weeds (MN Department of Transportation)