The term “locust tree” is a common name applied primarily to two distinct genera of deciduous trees native to North America: the Black Locust (Robinia pseudoacacia) and the Honey Locust (Gleditsia triacanthos). These trees belong to the legume family (Fabaceae), which also includes peas and beans. Both species have been widely cultivated globally for their fast growth, hardiness, and varied uses. Their resilience and adaptability allow them to thrive in a wide range of climates and soil conditions.
Identifying Physical Characteristics
Trees commonly called locusts share several distinctive botanical features that help in their general identification. The leaves of both the Black and Honey Locust are pinnately compound, consisting of multiple small leaflets arranged along a central stalk. This structure gives the canopy a light, filtered quality that allows dappled sunlight to reach the ground below.
The flowers are another shared trait, often appearing in showy, fragrant clusters during late spring or early summer. These blooms resemble the structure of a pea flower. Following the flowers, both species produce fruit in the form of a legume, commonly known as a seed pod. These pods contain the seeds and are often noticeable on the branches well into the winter months.
Differentiating the Main Locust Species
Distinguishing between the two main locust species often comes down to examining their thorns, bark, and seed pods. The Black Locust (Robinia pseudoacacia) features smaller, paired spines, often described as rose-like, usually found near the base of the leaves on young shoots and branches. Its bark is dark gray-brown and deeply furrowed, often displaying a criss-cross or diamond pattern on mature trunks. The seed pods are relatively short, measuring about two to four inches long, flat, and brown, often persisting on the tree through the winter.
The Honey Locust (Gleditsia triacanthos), by contrast, is known for its much larger, intimidating thorns, which can grow up to several inches long and are often branched and clustered on the trunk and main limbs of wild varieties. Its bark is generally dark gray and develops long, curling plates as the tree matures. The most striking difference lies in the seed pods, which are significantly longer, measuring between seven and eighteen inches, and are often twisted or curled with a leathery texture.
Safety Concerns and Environmental Impact
The Black Locust poses specific safety concerns due to its toxicity. All parts of the Robinia pseudoacacia tree, including the bark, leaves, seeds, and pods, contain the gastrointestinal neurotoxin robin, which can be hazardous to humans and highly toxic to livestock, particularly horses. Ingestion can cause severe symptoms, and even a puncture wound from a black locust thorn or splinter can introduce the toxin.
Both locust species exhibit aggressive spreading characteristics, which can lead to environmental concerns. The Black Locust is known for its vigorous root suckering, where new shoots sprout from the lateral roots, allowing it to quickly form dense, clonal thickets that can crowd out native vegetation. This aggressive growth, combined with its ability to fix atmospheric nitrogen in the soil, can alter soil chemistry and promote the establishment of other non-native species. The Honey Locust also re-sprouts vigorously if cut and can form dense stands, and the large, sharp thorns on wild types present a physical hazard to people and animals.
Commercial and Horticultural Uses
The wood of the Black Locust is highly valued for its durability and rot resistance, a quality attributed to high concentrations of natural extractives in the heartwood. This density and stability make it a superior alternative to chemically treated lumber for outdoor applications. Historically, this wood was used for applications requiring longevity in contact with the ground, such as fence posts, railroad ties, and mine timbers.
Both species have significant horticultural value. Thornless and fruitless cultivars of the Honey Locust are popular in urban landscaping because their open canopy provides light, dappled shade. Their tolerance for pollution, salt, and compacted soil makes them suitable for street planting. The Black Locust is also cultivated for its fast growth, making it an excellent choice for windbreaks, erosion control, and soil improvement due to its nitrogen-fixing capabilities. Its fragrant flowers are a significant source of nectar for honeybees, producing a highly prized, light-colored honey.