By Katie Stannard
Black locust (Robinia pseudoacacia), also called false acacia or yellow locust, is one of the most difficult invasive trees to eradicate. Belonging to the Fabaceae or pea family, it is endemic to the southeastern United States, particularly parts of the Appalachians and Ozarks. Planted outside of its native range for hardwood lumber and fence posts, erosion control, reclaiming of mine soil, and as nectar for honeybees, it was noted in Michigan’s jack pine barrens by 1888, and in Washtenaw County by 1862. Because black locust fixes nitrogen in the soil and spreads clonally by suckers, it can form dense colonies that overshade and outcompete native species.
Intolerant of shade and soils with a high water table, black locust can be found in dry, poor soils, disturbed sites and sunny open woods, savannas, and prairies. The leaves, bark, seeds, and stems contain toxic chemicals that cause neurological and gastrointestinal effects. Horses are especially susceptible to these chemicals, which can be fatal.
Jeff Plakke, manager of landscapes at Matthaei-Nichols, describes another key negative of black locust. “It has the added disruptive trait of being a nitrogen fixer, which sounds great if you’re raising vegetables, but not in a natural community where nitrogen is usually low,” Plakke explains. “It tips the balance for the whole plant community by making abundant what once was a limiting essential nutrient. All the other weeds get a boost and the natives that are adapted to making due with less nitrogen are outcompeted.” Even after black locusts are removed, the remaining soil can still exhibit altered soil chemistry. Some studies suggest that black locust can release toxic substances that suppress the growth of other plants (allelopathy).
Black locust is a fast growing tree that matures to 50-100 feet tall, described as having a “narrow crown and open, irregular form with contorted branches.” Leaves are alternate, pinnately compound, up to 14 inches long, with 7-21 oval-shaped 1-2” inch bluish-green leaflets on the upper side, lighter green on the underside, turning yellow in fall.
Though initially smooth and brown when young, bark thickens with age turning tan to gray-brown with deep, flat-topped, scaly furrows. Two distinguishing features are flowers and thorns. Fragrant, pea-like white blooms with five irregular petals appear in May-June on long clusters. Half-inch paired thorns form on twigs at the base of leaves. Flowering blooms lead to 3-4 inch brown seed pods which look like dried, flat pea pods containing 4-8 seeds; pods persist through the winter. With an extremely durable, impermeable outer coating, seeds last for years in the seed bank.
Since black locust reproduces by the double-whammy of prolific seed production and excessive root sprouts (vegetative reproduction by underground roots), it’s very difficult to control. Removal of sprouts through cutting and bulldozing generates new growth. A single stem can regenerate the entire clone, sometimes even more rapidly. In one study, sprouts that had been cut regrew and blossomed 50% faster and bigger than those that hadn’t been cut. A grazing study in North Carolina using goats and cattle was effective in controlling black locust height and then eliminating the tree altogether after four seasons. Though such a strategy is tricky, as black locust in quantity can be toxic to cattle.
Herbicidal use is generally necessary for control, once a site has been thoroughly evaluated for the presence of native species, how close it is to wetlands or other water sources, effectiveness, and residual effects of indicated herbicides (i.e., does it linger in the soil). Some herbicide application strategies for black locust include foliar, cut stump, basal bark, hack and squirt, frilling (downward cuts made into bark around tree which are then treated with herbicide), and drill and fill (1 inch drilled holes for every inch diameter of tree). With those methods, extreme caution is necessary, as it is easy to exceed the recommended volume of herbicide used. Black locust requires annual and long term monitoring as it may resprout several years after herbicidal treatment appears to have eradicated it.
About the author: In a normal year, Katie Stannard would be coordinating and onboarding the summer interns at Matthaei-Nichols and helping the visitor engagement team run the visitor center at Matthaei. This year—along with a months-long closure and absent student interns—Katie’s been helping out in a big way with our social media and with blog posts, including this series about staff happenings behind the scenes during these extraordinary times.
Katie received her Master Gardener certification in 1999, and has years of experience with heirloom flower bulbs and small business operations. She is passionate about encouraging kids of all ages to “go outside!” and explore the natural world. Her current gardening interests include children’s gardens, native plants, and everlasting yet nonintrusive perennials. Katie volunteers for her kids’ school, for Carleton College as an alumni volunteer, and in her neighborhood to organize community events. She holds an M.A. from Carnegie Mellon University in literary and cultural theory, and a B.A. from Carleton College in English.