Locust trees, a common sight across many landscapes, can sometimes present a challenge for identification due to their varied forms and similar appearances to other species. Accurate recognition of these trees, which belong to the pea family (Fabaceae), is useful for various interests. This guide provides clear methods for distinguishing locust trees based on their unique physical attributes.
Leaf and Bark Characteristics
Locust trees feature distinct leaf structures that are a primary identification cue. Their leaves are compound, meaning multiple smaller leaflets attach to a central stem, forming one larger leaf. Black locust leaves typically consist of 7 to 23 oval or rounded leaflets with smooth edges, often appearing a bluish-green on top and lighter underneath. Honey locusts, by contrast, can have either pinnately compound leaves with small, bright green leaflets, or bipinnately compound leaves, where the main leaf stem branches again into smaller stems, each bearing leaflets. Honey locust leaflets are generally smaller and can have a glossy sheen, while black locust leaflets often have a more matte or silky texture.
The bark of locust trees also offers reliable identification markers, changing in appearance as the tree matures. Black locust bark is characterized by deep furrows and prominent ridges that often resemble intertwined ropes, appearing dark reddish-brown to black. Honey locust bark, while also furrowed, tends to be shaggy with ridges and can range from dark brown to dark gray. Older honey locust bark often breaks into long strips with raised sides.
Flowers, Pods, and Thorns
Locust trees produce distinctive flowers, pods, and thorns that are important for identification, particularly during specific seasons. Black locust trees are known for their fragrant, pea-like flowers, which are typically creamy-white and arranged in drooping clusters that can be 5 to 8 inches long. They bloom from April to June. Honey locust flowers are less conspicuous, usually small, greenish, or cream-colored, and also form in clusters, blooming in late spring.
The fruit of locust trees develops into characteristic seed pods. Black locust pods are flat, smooth, and brown, resembling pea pods, typically 2 to 4 inches long, and often persist on the tree into winter. Honey locusts produce much longer, flat, and often twisted seed pods, ranging from 6 to 18 inches in length, which start green and mature to a reddish-brown color. These larger pods can spiral.
Both black locust and honey locust trees can possess thorns, although their characteristics differ. Black locust thorns are sharp, paired spines, typically 0.25 to 1.25 inches long, found at the base of leaf attachments, especially on younger growth. Honey locust thorns are generally larger, sharper, and often branched, appearing on stems, branches, and sometimes the trunk. They can be red-brown, up to 12 inches long, and grow in clusters.
Key Locust Tree Species
Distinguishing between the two most common locust species, Black Locust (Robinia pseudoacacia) and Honey Locust (Gleditsia triacanthos), involves observing several key features. Black locust leaves are pinnately compound (7-23 oval, bluish-green, matte leaflets), while honey locust leaves can be pinnately or bipinnately compound (smaller, brighter green, often glossy leaflets).
Their bark and thorns also differ. Black locust bark is deeply furrowed with rope-like ridges (dark gray to black), while honey locust bark is shaggy (dark brown or gray). Black locust thorns are shorter, paired, and on younger growth; honey locust thorns are longer, often branched, and found in clusters on older branches and trunks.
Flowers and pods provide further distinction. Black locust trees produce fragrant, white to yellowish-green flowers in drooping clusters; honey locust flowers are less prominent, greenish or cream-colored. Black locust pods are shorter (2-4 inches), flat, and brown; honey locust pods are longer (6-18 inches), often twisted, and reddish-brown.
Differentiating from Other Trees
Identifying locust trees accurately often requires distinguishing them from other species that share some similar features, such as compound leaves. Ash trees (Fraxinus species) have compound leaves like locusts, but a key difference lies in their branching pattern: ash trees have opposite branching, where leaves and branches grow directly across from each other, unlike the alternate branching of locusts. Additionally, ash leaflets are typically more pointed than the rounded or oval leaflets of black locusts.
Sumac (Rhus species), particularly Staghorn Sumac, also features pinnately compound leaves, but its leaflets have serrated edges, unlike the smooth margins found on most locust leaflets. Sumac fruit typically forms upright, fuzzy, red berry clusters, which is distinctly different from the flat, bean-like pods of locust trees. The bark of sumac is often smoother or velvety on younger stems, lacking the deep furrows of mature locust bark.
Box Elder (Acer negundo) is another tree with compound leaves, but its leaves are usually oppositely arranged and often have only three to five leaflets, which are typically lobed or coarsely toothed, unlike the numerous, unlobed leaflets of locusts. Box Elder bark is also different, often gray-brown with shallow furrows and narrow ridges, not as deeply furrowed or shaggy as locust bark. Focusing on these specific differences in leaf arrangement, leaflet shape, fruit, and bark helps ensure correct identification.