The term “locust tree” generally refers to two distinct North American species: the Black Locust (Robinia pseudoacacia) and the Honey Locust (Gleditsia triacanthos). These trees are often noted for their rapid growth and ability to thrive in poor soils, but they possess significantly different characteristics that make accurate identification important. The Black Locust, in particular, contains toxins in its bark, leaves, and seeds, which is a significant concern for livestock and pet owners. Distinguishing between these two common trees requires close attention to permanent structures and seasonal features.
Understanding the Black Locust and Honey Locust Distinction
While both species belong to the legume family (Fabaceae) and share the trait of having compound leaves, they diverge at the genus level, which accounts for their different features. The Black Locust (Robinia pseudoacacia) is typically a smaller tree, often reaching heights of 30 to 50 feet, and is known for its tendency to spread aggressively through root suckers. In contrast, the Honey Locust (Gleditsia triacanthos) is a larger canopy tree that can grow to 60 to 80 feet tall, with a more open, spreading crown. The most important distinction involves toxicity, as the Black Locust contains poisonous elements throughout its structure, while the Honey Locust produces seed pods with a sweet, edible pulp. The differences in their thorn structure and seed pods provide the clearest framework for identification.
Permanent Structural Identifiers: Bark, Thorns, and Trunk
The mature bark of the Black Locust is characterized by deeply furrowed, dark gray-brown bark that often appears rope-like, with long, running ridges and a tendency to show an orange undertone when scuffed. Conversely, the Honey Locust bark is generally smoother on younger trees, maturing into a gray-brown color with scaly or plate-like sections. These sections do not exhibit the same deep, ropey furrows as the Black Locust.
Thorn structures are a key marker, though some cultivated varieties of Honey Locust are thornless. Black Locust thorns are typically short, sharp spines, often appearing in pairs at the base of the leaves or buds, and are most prominent on young shoots and suckers. These spines are relatively small, usually less than an inch in length, and resemble rose thorns. The thorns of the native Honey Locust, however, are large, formidable, and multi-branched, often exceeding four inches in length. These sharp thorns usually grow directly on the trunk and older branches in dense, clustered arrangements, creating a striking and unique pattern.
Seasonal Identifying Features: Leaves, Flowers, and Seed Pods
The leaves of both species are pinnately compound, meaning multiple small leaflets are arranged along a central stalk. Black Locust leaves are typically odd-pinnate, with a single terminal leaflet, and have a matte, dull green appearance. Their leaflets measure about one to one and a half inches long. Honey Locust leaves can be either singly or doubly compound on the same tree. Their smaller leaflets, about half an inch long, exhibit a glossy sheen and a finer texture.
The flowers and seed pods offer the most visible seasonal differences. Black Locust produces highly fragrant, pea-like white flowers in drooping clusters, reminiscent of wisteria, which typically bloom in late spring. Honey Locust flowers are much less conspicuous, appearing as small, greenish-yellow clusters that do not possess the strong fragrance of its counterpart.
The seed pods are also distinct. Black Locust produces thin, flat, brown pods that are relatively short, measuring about three to four inches in length. Honey Locust pods are much longer and more distinctive, often growing between 6 and 18 inches long. They are dark brown, leathery, and often twisted or spiraled.
Avoiding Misidentification: Locust Look-Alikes
Several other native trees share the compound leaf structure with locusts, leading to common misidentification, particularly when the trees are young. Trees like Ash (Fraxinus spp.), Walnut (Juglans spp.), and Sumac (Rhus spp.) all have leaves with numerous leaflets arranged along a central stem. One key to ruling out these look-alikes is the presence of thorns, which are absent on all three of these other species.
Differences also lie in the leaf scar and bud arrangement, especially during the winter. Both Black and Honey Locust have alternate leaf arrangements, while Ash trees have opposite branching, a much rarer trait in North American trees. Black Walnut is distinguished by its chambered pith—a hollow, segmented core inside the twig—and its crushed leaves and fruit husks emit a distinct, nutty odor. Sumac leaflets, unlike the typically smooth-edged locust leaflets, usually have serrated or toothed margins, and the tree often produces upright clusters of red, fuzzy fruit.