The main species of locust trees in the United States--the black locust, honey locust and water locust--are all members of the bean family. According to the “National Audubon Society Field Guide to Trees,” the locust trees are useful ornamental and erosion-control species. The leaves of locust trees are of the compound variety, with many individual leaflets making up one leaf.
The black locust grows to heights of from 50 to 80 feet, while water locust tops out at 60 feet and honey locust typically around 80 feet. The locust trees feature twigs with sharp spines and the fragrant flowers that emerge in springtime turn into bean-like pods full of seeds that hang down from the branches. Wildlife will flock to the locust tree to consume these pods in autumn and through the winter.
The rachis, or primary stem of a compound leaf to which the separate leaflets attach, varies in length on the different types of locust trees. The Virginia Department of Forestry states that the rachis of a black locust leaf is from 8 to 14 inches long. The rachis of the honey locust is 7 to 8 inches long and that of the water locust measures from 5 to 8 inches in length.
The size and number of the leaflets that attach to the rachis on locust species also differ. The leaflets, arranged in two rows opposite each other on the rachis, can be as large as the 1- to 2-inch leaves of honey locust and black locust or an inch long, like those of water locust. The black locust has from seven to 19 leaflets, while the honey and water locust trees typically have leaves with a varying number of leaflets, often as many as 20+.
Shape and Color
The locust tree leaflets are oval to elliptical in terms of their shape. Black locust leaves are a dark shade of green-blue on their upper sides and a more pale green-blue hue on the undersides. The leaves change to what the University of Connecticut Plant Database site calls an “unimpressive” yellow-green before falling off. The U.S. Department of Agriculture website states that honey locust leaves are a dark green color that by the end of summer changes to yellow, with the leaves usually falling off the locust tree relatively early. Water locust leaves are also dark green and take on a yellow-green color in autumn.
The leaves of the locust species are susceptible to different leaf diseases and insect pests, which can cause them to fall off the tree prematurely. The Ohio Department of Natural Resources website says that the leaf miner, an insect that subsists on the leaves of trees, can damage a black locust’s leaves and make them look less than attractive by the midpoint of summer.