Black locust (Robinia pseudoacacia) is native to specific regions of the United States. The tree was originally reported in two regions—the Appalachian Mountains from Pennsylvania south to Alabama; and the western region, including parts of Missouri, Arkansas, and Oklahoma. Black locust has been widely planted as an ornamental and escapes cultivation. It is currently naturalized throughout much of the United States, far outside its native range (Figure 1).
Biology and Impacts
Black locust is a deciduous tree, growing from 40 to 60 feet tall and 12 to 30 inches or more in diameter (Figure 2). Young black locust has smooth, brown bark with thorns. As trees age, the bark becomes thick, deeply furrowed, scaly, and dark brown. Leaves are pinnately compound with 7 to 19 leaflets on a central stalk that is 8 to 12 inches long. Flowers are white, fragrant, and occur in showy, drooping clusters about 6 inches long. Black locust fruits are flat legume pods about 4 inches long, containing four to eight dark, bean-like seeds. Seed germination is low; however, it is stimulated by fire and cutting.
Black locust develops extensive lateral root systems that extend 1.5 times the tree height. The tree reproduces and spreads vigorously by sending up new shoots from roots. Root sprouting usually begins when plants are four to five years old. Re-sprouting from roots or stumps is stimulated following physical damage from cutting, fire, or disease.
Dense black locust stands can replace existing native vegetation, reducing plant diversity. The tree is particularly problematic to land managers in pine barren, sand prairie, and black oak savanna plant communities. Other habitats at risk of black locust invasion include prairies, sand dunes, meadows, wetland, and riparian areas.
Black locust also alters soil characteristics through its ability to fix nitrogen. In some situations, black locust facilitates the spread of other nonnative species, such as cheatgrass (Bromus tectorum), by increasing soil nitrogen levels. Other nonnative species, including Tatarian honeysuckle (Lonicera tatarica), raspberry (Rubus spp.), common barberry (Berberis vulgaris), and garlic mustard (Alliaria petiolata) have been associated with black locust clones.
Low-Volume Basal Treatments: Black locust is more difficult to control than most woody plants. Field trials conducted in the eastern United States show that low-volume basal applications of Garlon® 4 Ultra in combination with Milestone® will control black locust. Apply a 25% Garlon 4 Ultra with 2% Milestone LVB mixture (Table 1) to trees that are less than 6 inches in diameter. Thoroughly coat bark around the entire stem, 12 to 15 inches above the ground level. Allow at least two months of growing season conditions to elapse following basal treatments before cutting down black locust. Annual monitoring should be conducted for several years with follow-up treatments made as needed.
Table 1: Low-volume basal tank mix guide for Garlon® 4 Ultra in combination with Milestone® herbicide plus oil-based carrier for black locust control*.
Black locust sprouts can be effectively controlled with high-volume foliar applications of Milestone® herbicide at 7 fluid ounces per acre (fl oz/ac). Data from field trials show excellent control (greater than 96 %) one year following high volume foliar application of Milestone alone or in combination with Garlon 4 Ultra.
With foliar applications, it is important to thoroughly cover all leaves and stems, including the tops of trees, with the herbicide mixture. Application with ground-based equipment is restricted by the height of the target plant. The addition of surfactant is recommended. Review and follow label guidelines for mixing and application.
Other Control Methods
Mechanical control of black locust is largely ineffective because of the plant’s vigorous re-sprouting ability. Cutting generally increases sucker and sprout productivity. Repeated cutting or mowing may reduce cover, but plants will regrow. Seedling trees may be hand-pulled if the entire root is removed.
Attempts to control black locust using fire are not effective because of re-sprouting following fire. Fire may increase black locust seedling germination and establishment.
Brooks, Louanne. Dow AgroSciences (retired) personal communication.
Burch, Pat. DowDuPont (retired) personal communication.
Dow AgroSciences. Unpublished field data.
EDDMapS. 2019. Early Detection & Distribution Mapping System. The University of Georgia — Center for Invasive Species and Ecosystem Health. Available online: www.eddmaps (accessed April 29, 2019).
Evans, Christopher. Extension Forester, University of Illinois, NRES. Personal Communication.
Stone, Katharine R. 2009. Robinia pseudoacacia. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available online: www.fs.fed.us/database/feis/plants/tree/robpse/all.html (accessed April 29, 2019).
USDA, NRCS. 2019. The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA. Available online: plants.usda.gov (accessed April 29, 2019).
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When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed, or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details. State restrictions on the sale and use of Milestone® apply. Consult the label for full details before purchase or use. Always read and follow label directions.
State restrictions on the sale and use of Garlon® 4 Ultra apply. Consult the label for full details before purchase or use. Always read and follow label directions. ©2019 Corteva