The most interesting feature of honey locust trees (Gleditsia triacanthos, Fabaceae) is their impressive woody thorns. Young thorns on branches can grow a few inches long, and are often red. Mature thorns like those on trunks can get perhaps six to eight inches long, and turn brown with age. Here in southern Michigan, these trees are near the extreme northern edge of their native range.
It’s hard to believe these big trees are in the legume family (Fabaceae). Other members of this family include peas, beans, alfalfa, peanuts, and licorice, all relatively small herbaceous plants. One notable feature they have in common is that they bear their seeds in long narrow pods. Honey locust pods can get quite large.
These trees have become popular in recent years for city planting, replacing ravaged inventories of elms (due to Dutch elm disease) and ash (because of the emerald ash borer). They’re relatively tolerant of urban conditions, including flooding, drought, and salinity from winter salt application. Most trees planted in cities have been new thornless varieties, not the native type seen in these photos. The thorns are probably undesirable since they could injure people or puncture tires if broken limbs ended up on streets.
From what I understand, the impressive thorns may have been a defensive adaptation against herbivory by Pleistocene megafauna. Although no large mammals have been grazing on this plant’s foliage in 10,000 years, they’ve still retained their thorns.