Garden Variety: Spotting different traits in 2 types of locust trees
Black locust trees are captivating in mid-May when their large, fragrant white blossoms open and make the medium to large trees suddenly apparent along the edges of wooded areas and in small groves. Black locust is sometimes confused with honey locust because of the name, but the two trees have many different attributes.
Black locust, Robinia pseudoacacia, is native to the Appalachian and Ozark regions of the U.S., making it is very adaptable to eastern Kansas soils and weather. However, it is rarely used as a landscape tree, probably because of its thorns, pods and susceptibility to heart rot and locust borer. Honey locust, Gleditsia triacanthos, is more popular than black locust despite sharing the same maladies and lacking the fragrant showy flowers of black locust. Most likely it surpassed black locust in popularity because of the earlier development of thornless cultivars.
Thorns and pods are the biggest similarities between black locust and honey locust, although there are still differences in the appearances of the structures.
Black locust thorns (technically spines) are small and rose-like, appearing on branches. Honey locust thorns are often stacked upon each other on both branches and trunks of the species, grow to several inches long and appear quite intimidating.
Pods of black locust are 2 to 4 inches long, flat, brown and papery when dry. Honey locust pods are 7 to 18 inches long, twisted or curled, brown and leathery. Both pods have hard brown seeds or “beans,” since both species are members of the Fabaceae or bean family.
Honey locust is native to the lower Midwest, including the eastern half of Kansas and the Appalachian region. Thornless, seedless varieties are popular in the landscape, and the species is known for the dappled shade provided by the tree’s tiny leaflets.
Black locust is probably underutilized and honey locust a little overutilized in the landscape. Newer cultivars of black locust are bringing it into greater favor.
Improved varieties of black locust worth trying are Frisia, Purple Robe, Pyramidalis (Fastigiata), Tortuosa and Twisty Baby (Lace Lady). Frisia has golden-chartreuse leaves and is extremely drought tolerant. Purple Robe has rosy-pink flowers instead of white and is advertised as thornless, although thorns often appear as the trees mature. Pyramidalis (Fastigiata) is shaped as the name implies, like a pyramid. Tortuosa and Twisty Baby (Lace Lady) are both small, slow-growing, contorted varieties that provide a great focal point in the garden.
Popular cultivated varieties of honey locust are all selections of variety inermis, meaning they are thornless or nearly thornless. Some of the most popular are Imperial, Moraine, Shademaster, Skyline and Sunburst. Imperial is the smallest, maturing at about 30 feet tall. Moraine is an older cultivar that shows good resistance to webworms. Shademaster, Skyline and Sunburst are similar in size and shape, but Sunbursts’ foliage emerges bright yellow and transitions to green.
Black locust and honey locust are both susceptible to the native locust borer. Locust borer feeding under the bark of trees interferes with sap flow and weakens trees. Borer damage compounds other environmental stresses such as drought and makes trees more susceptible to cankers, heart rot, and other insect and disease pests.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.