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.