Both members of the pea family, black locusts and honeylocusts are native trees found throughout the continental United States, except for Washington and Oregon, where only black locusts are native. These tall, airy trees have a number of differences, some of which affect their suitability as landscape specimens for home gardeners.
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Size and Shape
Black locusts (Robinia pseudoacacia) are medium-sized trees reaching 50 feet tall, although wild specimens sometimes grow as high as 80 feet. The trunk is straight, but individual stems and branches grow in a zig-zag fashion. The crown of the tree is a narrow oblong. The blackish-gray bark gives the tree its common name. Honeylocusts (Gleditsia triacanthos) grow 60 to 100 feet tall and wide, with an open, spreading canopy and a short or arching main trunk. The stems on honeylocusts also zig-zag. The bark is medium to light gray.
Leaves and Thorns
As you would expect from members of the pea family, the leaves on both trees are long and compound, with a central stem and small leaflets. The leaves on black locusts grow 6 to 14 inches long, with seven to 19 leaflets. The leaves are dark bluish-green and change to a weak yellow-green in fall. Black locusts have 1/4- to 1/2-inch-long spines, always in pairs, at the nodes where the leaves emerge from the stems. The leaves on honeylocusts are smaller, rarely growing more than 8 inches long. In addition, the leaves may either be in the same compound form as black locusts, with a central stem holding the leaflets, or bipinnately compound, meaning side stems emerge from the central stem; the leaflets are attached to the side stems. Honeylocust leaves are bright green in summer and clear yellow in fall. The thorns on honeylocusts are quite distinct: large and reddish brown, they branch into three points, although spines on the trunk may have more points. The thorns can be found singly along the stems or in large clumps.
Flowers and Fruits
Both locusts bloom in mid to late spring. The flowers on black locusts are small and white, held in a drooping, 4- to 8-inch-long cluster. The flowers are showy and fragrant, and are followed by flat, brown or black seed pods up to 4 inches long. The greenish-yellow flowers on honeylocusts are sweetly fragrant but not showy, growing only 2 inches long. However, they're followed by long, twisting seed pods that have a strong honey scent when ripe, giving the tree its common name.
Black locusts are rarely used as landscape specimens but are excellent for reforesting cleared land and stabilizing slopes. The roots sucker profusely, so an individual tree can quickly turn into a small thicket of trees. Honeylocusts make excellent lawn shade trees, as the airy foliage allows enough dappled sunlight to allow grass to grow under the trees. Choose only thornless, male varieties (G. triacanthos inermis) for home use. Good cultivars for home landscapes are Shademaster, Imperial and Sunburst.