Honey Locust
Gleditsia triacanthos
Senna family (Caesalpiniaceae)

Description: This tree is 50-100' tall at maturity, forming a single trunk about 2-3' across and an open plume-like crown that is somewhat flattened at its apex. Trunk bark of mature trees is light gray to gray-black and divided into large flat plates with upturned margins; these plates are slightly scaly and they are separated by shallow furrows. The bark of branches and twigs is more smooth, brown, and hairless, while young shoots are light green and pubescent. Alternate compound leaves occur along the twigs and young shoots. These leaves are evenly pinnate or bipinnate and 6-14" long. Pinnate leaves have 5-11 pairs of simple leaflets, while bipinnate leaves have 4-7 pairs of pinnate leaves that are each divided into 5-11 pairs of simple leaflets. There are no terminal leaflets. The rachis of each compound leaf is light green and pubescent. The leaflets are -1" long and about 1/3 as much across; they are oblong to lanceolate-oblong and slightly crenate along their margins. The upper surface of the leaflets is yellowish green to dark green and hairless, while the lower surface is more pale and either hairless or minutely hairy. The leaflets have very short petiolules (basal stalklets) that are less than 1/8" (3 mm.) long. Along the trunk, there are usually both simple and branched thorns up to 8" long; there are also simple and tripartite thorns along the lower branches. However, there is also a thornless variety (var. inermis) of Honey Locust that is uncommon in the wild, although often cultivated.

The small greenish yellow flowers are produced in racemes about 2-5" long; they are usually male (staminate) or female (pistillate), although sometimes perfect (both staminate & pistillate). Individual male flowers have a calyx with 5 lobes, 4-5 petals, and 3-10 stamens, while individual female flowers have a calyx with 5 lobes and a pistil with a single style. Individual perfect flowers have both a pistil and several stamens. The blooming period occurs from late spring to early summer for about 2 weeks. The flowers have a sweet fragrance. Fertile female flowers are replaced by flattened seedpods that become 6-14" long and 1-1" across at maturity. Mature seedpods are dark brown, hairless, and often hooked or spirally twisted. Each seedpod contains several large seeds that are reniform in shape and about 1/3" (8 mm.) long; they have hard seed coats. The seeds are embedded in a thick sweet pulp. The seedpods fall to the ground unopened during the late fall or winter. The woody root system has a taproot and abundant lateral roots that are widely spreading and deep. The deciduous leaflets turn yellow during the autumn.

This adaptable tree prefers full to partial sunlight and moist to dry-mesic conditions. It will flourish in almost any type of soil (pH range 6.0-8.0) if it is not too acidic. Both temporary flooding and hot dry weather are tolerated. The root system doesn't fix nitrogen in the soil. New trees can be propagated by seeds or vegetatively by cuttings. Growth and development is fairly fast; young trees can produce seedpods in as little as 10 years. Longevity of healthy trees is typically 100-150 years. One of the advantages of Honey Locust as a landscape tree is the light shade that is cast by its open crown; this allows the survival of turfgrass and other plants.

Range & Habitat: The native Honey Locust has been found in almost every county of Illinois; it is common. Habitats include upland woodlands, bottomland woodlands, woodland openings, woodland borders, powerline clearances in wooded areas, savannas, edges of limestone glades, thickets, fence rows, pastures, and roadsides. This tree colonizes disturbed areas that are relatively open; it is intolerant of shade. Because of the thin bark, Honey Locust is vulnerable to wildfires. The thornless variety of this tree is often cultivated as a landscape plant; it often escapes in both urban and suburban areas.

Faunal Associations: The flowers are cross-pollinated primarily by small bees and flies. Both nectar and pollen are available as floral rewards to such visitors. The caterpillars of Epargyreus clarus (Silver-Spotted Skipper) feed on the foliage of Honey Locust. Several moth caterpillars prefer this tree as a host plant: Catocala illecta (Magdalen Underwing), Catocala innubens (The Betrothed), Catocala minuta (Little Underwing), Spiloloma lunilinea (Moon-Lined Moth), Sphingicampa bicolor (Honey Locust Moth), and Sphingicampa bisecta (Bisected Honey Locust Moth). Several leafhopper species also prefer this tree as a host plant: Erythridula aenea, Erythridula brundusa, Erythridula clavata, Erythridula diffusa, Erythridula gleditsia, and Macropsis fumipennis (Honey Locust Leafhopper). Other insect feeders include the treehopper Micrutalis clava, Diaphnocoris chlorionis (Honey Locust Plant Bug) and other plant bugs, Anomoea flavokansiensis and other leaf beetles, the larvae of Agrilus difficilis (Honey Locust Borer) and other wood-boring beetles, the larvae of the seed weevil Amblycerus robiniae, and the larvae of Dasineura gleditchiae (Honey Locust Pod-Gall Midge). See the Insect Table for a more complete listing of the invertebrate species that feed on this tree. Some mammals and birds also use Honey Locust as a source of food. The seedpods with their edible sweet pulp are eaten by cattle, sheep, goats, deer, opossums, tree squirrels, crows, starlings, and Bobwhite quail. It is thought that some extinct megafauna of the ice age, including the American Mastodon, also ate these seedpods and helped to distribute the seeds into new areas. Cattle, deer, rabbits, and groundhogs browse on the foliage of seedlings, saplings, or the lower limbs of trees; rabbits also gnaw on the bark of young trees during the winter.

Photographic Location: Busey Woods and other locations in Urbana, Illinois.

Wild trees are often formidably armed by thorns; it is possible that this functioned as a defense against the American Mastodon and other large megafauna of the last ice age. Today, these thorns discourage squirrels, opossums, raccoons, and humans from climbing this tree. The only other tree that Honey Locust can be confused with in the Midwest is Black Locust (Robinia pseudoacacia). Black Locust has slightly larger leaflets, fewer thorns, shorter seedpods, and more showy white flowers. These flowers have a pea-like floral structure that is typical of species in the Bean family (Fabaceae). The greenish yellow flowers of Honey Locust, in contrast, have a more conventional floral structure.