Plant Profile: Yellowwood
As part of our Horticulture Program, Madison Square Park Conservancy keeps phenological data on five species of trees located in the Park. Phenology is the study of cyclical events in the life cycle of plants and animals. For our horticulturists, this commonly comes in the form of noting when our plants break bud in the spring, when they flower, and when they set seed. This data lets you see the rhythm of the seasons acting on plants, and lets you see how natural cycles have changed over time—tracking is a vital tool in understanding the effects of climate change. The American Yellowwood, Cladrastis kentukea is one of the five plants studied.
Yellowwood is a rare, attractive tree native to the southeastern U.S., growing in clusters throughout Kentucky, Tennessee, and Arkansas. It is a large-sized tree, generally growing between 30 and 75 feet tall and spreading from 40 to 80 feet wide. The oldest tree of this species usually lasts about 100 years, but is considered short-lived and slow growing, putting on between 9 and 12 feet of growth in a decade. American Yellowwoods are hardy across the conterminous U.S. with the exception of the hottest parts of the south. C. kentukea grows well in average soils, preferring full sun—the more established trees can tolerate drought.
Yellowwoods have fragile branches that require structural pruning at a young age. Addressing poor structural conditions help reduce damage caused by ice, wind, and snow that can significantly reduce the lifespan of the tree. Any pruning done to the tree should ideally be done in the summer as Yellowwoods bleed sap profusely from cuts made in the winter and spring months. This species of trees, does not suffer from many serious diseases, with the exception of one major illness being verticillium wilt and sun scalding on the bark.
Yellowwoods are members of the pea family, Fabaceae, a relationship which is most apparent in the flowers and the leaves. They have showy, rather fragrant flowers that resemble the flowers of many other legumes. The flowers are often white and are produced in draping racemes similar to those of wisteria and are pollinated by bees. Cladrastis does not flower until it reaches heights between 12 and 18 feet, which usually means they will not flower for at least the first decade. Flowers bloom in spring, usually around May, and are followed by the characteristic fruit of the legume family: the legume. In Yellowwoods, the legume is thin and dry, generally around 4.5 inches long, and matures from September to October. It is important to note that the bark is very thin and damages easily, so care must be taken not to injure these trees when work is done on or around them.
Yellowwoods are showy plants throughout the growing seasons. Their new spring leaves are a light green that eventually darkens to a saturated emerald once summer arrives; in autumn, Yellowwood leaves turn a bright yellow. The generic name, Cladrastis, is based on the Greek words for “brittle branch,” in reference to its easily broken branches and twigs. The specific name, Kentukea, means “from Kentucky,” referencing the plant’s native range. The common name “Yellowwood” is a reference the intensely yellow color of its wood when first cut. This color led to the tree’s use for furniture, small novelties, and gun stocks. A yellow dye obtained from the roots was used by the early settlers of the Appalachian region.