Yellowwood is a deciduous medium size tree, growing to 50 feet in height and 1-1/2 to 2 feet in diameter, with a short trunk and a broad rounded crown of spreading branches.
The bark is thin, gray and very smooth becoming wrinkled with some age. Bark frequently has a collection of moss or lichen.
Twigs are smooth, shiny reddish-brown with many light colored lenticels. They are a bit zigzag in shape, somewhat brittle and the leaf scars almost encircle the twig. Buds are shaped like cones and have fuzzy hair.
Leaves are alternate, pinnately compound, 8 to 12 inches long, with 5 to 11 (7 to 9 most common) elliptical leaflets that are 2-1/2 to 4 inches long. Leaflets are short stalked, arranged nearly in pairs, smooth margins, shiny green on top, paler below. The main leaf stalk is 3 to 6 inches long and is swollen at the base so that it almost encloses the twig, thus producing large leaf scars. Fall color is yellow.
Flowers: Unlike many trees, Yellowwood has perfect flowers that appear in drooping clusters near the tips of twigs after the leaves. They are pea-like, 1 to 1-1/4 inches long, creamy-white with 5 petals, each on a long green stalk and somewhat fragrant. One petal forms the upright banner which reflexes backward and is notched at the top and a yellow patch at the throat. There are two outward spreading lateral petals and two form a keel enclosing the reproductive parts. The calyx forms a light-green tube with 5 teeth. There are 10 stamens and a single style. Trees do not bloom abundantly every year.
Seed: Flowers mature to a yellow-brown flat pod, 2 to 3-3/4 inches long, that contains 2 to 6 bean-like seeds about 8 mm long. The pods drop in the fall before splitting open.
Habitat: Yellowwood grows in moist soils of valleys. The root system has a taproot with branching laterals. It adapts to a variety of soils and does best as a landscape tree in full sun.
Names: The wood contains a yellow dye which gives color to the heartwood and hence the common name. The genus, Cladrastis, is from 2 Greek words - klados, meaning 'branch' and thraustos, meaning 'fragile, which together mean brittle branch, referring to the brittleness of the twigs. The species kentukea, refers to Kentucky the location of the original type. The author names for the plant classification are as follows: The older scientific names for this tree had it classified in the genus Sophora, and begin with Sophora kentukea by Dumont de Courset in 1811, then Sophora kentukea by Michaux, then Cladrastis lutea, by Koch and finally Cladrastis kentukea by Rudd in 1972. The ‘Dum. Cours.’ is for Georges Louis Marie Dumont de Courset (1746-1824) French botanist who published 6 volumes describing 8,700 species and ‘Rudd’ is for Velva Elaine Rudd (1910-1999), American Botanist, Curator, Dept. of Botany, U.S. National Herbarium, Legume Specialist in the Fabaceae family, who wrote over 70 papers on the subject.
Comparisons: The white pea-like flowers will look similar to Black Locust, Robinia pseudoacacia, but that tree has spines on the twigs and the bark is not smooth.
Above: In the understory Yellowwood forms a large shrub. In an open area, as this specimen above shows, it forms a nicely shaped tree with a broad crown of ascending spreading branches. Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: 1st photo - The terminal and adjacent leaflets of the compound leaf. Note the slight offset from each other of the stalks of the lateral pair. 2nd photo - New leaves opening where the offset is more visible.
Below: 1st photo - Yellowwood has perfect flowers that are pea-like, 1 to 1-1/4 inch long, creamy-white with 5 petals, each on a long green stalk and somewhat fragrant. The banner petal has a yellow patch. 2nd photo - Twigs are smooth, shiny reddish-brown with many light colored lenticels. They are a bit zigzag in shape, somewhat brittle and the leaf scars almost encircle the twig. Buds are shaped like cones and have fuzzy hair.
Below: 1st photo - The leaf stalk is swollen at the base so that it almost encloses the twig, thus producing those large leaf scars. 2nd photo - Flowers mature to a brown flat pod (shown here in the immature state), 2 to 3-3/4inches long, that contains 2 to 6 bean-like seeds. The pods drop in the fall before splitting open.
Below: 1st photo - The forming bean shaped seeds. 2nd photo - Mature seeds, each about 8 mm long.
Below: Flowers appear in drooping clusters near the tips of twigs after the leaves. The branched panicle can be up to 12 inches long. 3rd photo - The green calyx tube has 5 teeth. Inside the 2 keel petals are 10 stamens and a style.
Below: 1st photo - The compound leaf has 5 to 9 leaflets. 2nd photo - Bark is thin, gray and very smooth on young stems becoming wrinkled with some age on older stems (3rd photo) and frequently with moss or lichen making a home.
Notes: Yellowwood is not native to Minnesota. It was added to the Garden in recent years. Its range is just south and east of Minnesota, being known primarily from the southern Appalachians and more sparse in states westward and northward from there.
Uses: Outside of its native area Yellowwood has been cultivated as a landscape tree as it makes a handsome will shaped ornamental. The yellow heartwood has been used as a source of yellow dye. Charles S. Sargent in the 14 volume Silva of North America called it the most beautiful flowering tree of North America. Several cultivars are available from the nursery trade.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"