American Yellowwood

May-June flowering tree

American yellowwood (Cladrastis kentukea), a member of the pea family (Fabaceae), is native to southeastern U.S. All other Cladrastis species are indigenous to Asia. Yellowwood is a beautiful medium-sized (35-45 feet) high tree with a broad crown, narrow profile, and upright branching.

Their pinnate compound leaves are alternately arranged. Each compound leaf is comprised 7-11 leaflets borne on a slender, green, 7-9 inches (20-30-cm) long rachis.

Dainty drooping panicles, almost wisteria-like, drip from the tips of branches in May or June, depending on regional location. The fragrant white-and-yellow pea-flowers are seldom borne on young trees and may take upwards of 10 years to bloom the first time. 

Yellowwood tends to be alternate-year bearing (i.e., bloom heavily one year, with little or no flowering the following year). Clusters of flattened green pea pods soon followed; each pod is 2-3 inch (10 cm) long and turns papery brown in autumn. Fall leaf color is brilliant yellow. The tree's light gray smooth bark, similar in appearance to beech (Fagus spp.), is an attractive four- season asset.

Yellowwood prefers a rich, deep, well-drained soil along with modest summer moisture. Leaves are subject to chlorosis (yellowing) in alkaline soils (pH above 7.2). This tap-rooted tree is drought tolerant within 2 years after planting. Yellowwood is not a good choice as a street tree as its surface roots may eventually push up sidewalks.

Narrow branch angles may result in limb trunked. When young, yellowwood needs to be correctively pruned to encourage wider branch angles and prevent bark inclusions. The cut branches tend to “bleed” sap excessively if pruned in winter.

Pests and diseases on yellowwood are rare, but the tree is susceptible to verticillium wilt. This thin bark tree is unfortunately subject to improper damage by line trimmers, mowers and other equipment, as well as sunscald over winter if young branches are left unshaded.

Yellowwood --"In The On-Year"

American yellowwood (Cladrastis kentukea) is a medium-sized flowering landscape tree. Generally, from early to late May, the beautiful yellowwood tree blooms in the Southern Appalachian region (USDA zone 6-b to 7-a). Twelve to fourteen inch long white pea-shaped flower panicles drape from the tips of tree branches.

I know the location of a dozen yellowwood trees in northeastern TN and 11 of 12 did not bloom in 2009. The 12th tree bloomed sparsely. In 2010 all trees are exceptionally beautiful in full bloom this week and last. So far, plant scientists are at a loss predicting the “on” and “off” annual flowering pattern.

Yellowwood may bloom 2 to 3 consecutive years and not flower again for the 1-2 years. A complex of environmental and physiological factors may be involved. The weather history over the past decade in the Southern Appalachian region has included several abnormally hot, dry summers and mild winter temps. The 2009 summer was unusually cool and moist followed by a longer cold winter.

A second theory, called "biennial bearing", states that if a tree sets an unusually heavy seed load in the summer, few to no flowers are initiated for the following spring.

The flowering trigger for yellowwood tree is not understood. Whether in flower or not, yellowwood makes a fine addition to any landscape.