Cladrastis kentukea (Dum. Cours.) Rudd (yellowwood, American yellowwood)

C. lutea (Michx) K. Koch

C. lutea f. tomentosa Steyerm.

Sophora kentukea Dum. Cours.

Virgilia lutea Michx.

Pl. 391 j–m; Map 1723

Plants medium to large trees to 20 m tall, the main trunk commonly forking. Bark smooth, gray, often somewhat mottled with lighter gray, the branches more or less 2-ranked, somewhat zigzag, with alternate branching, unarmed. Buds small, enclosed in the hollow base of the subtending leaf’s petiole (exposed after the leaves are shed), irregularly ovoid to obovoid, lacking scales, densely woolly. Twigs slender, initially green and sparsely to moderately short-hairy, becoming reddish brown and glabrous or nearly so at maturity, with scattered, pale, more or less circular lenticels. Leaves alternate, the petiole (10–)20–50 mm long, expanded and hollow at the base. Stipules absent; stipels absent. Leaf blades appearing odd-pinnately compound (but sometimes with an even number of leaflets), the rachis 5–18 cm long, glabrous, with 5–9(–11) leaflets, these alternate to less commonly subopposite on the rachis. Leaflets 3–14 cm long, 2–9 cm wide, broadly ovate or oval to elliptic or obovate, the terminal leaflet largest and broadly elliptic to broadly oval, rounded, angled, or tapered at the base, angled or tapered to a bluntly or sharply pointed tip, the margins entire, the upper surfaces glabrous, the undersurface glabrous or sparsely to moderately pubescent along the main veins with short, curved hairs, pinnately veined. Inflorescences terminal, open panicles 10–40 cm long, pendant, the stalk (2–)4–8 cm long. Bracts minute, shed early; bractlets absent. Flowers fragrant, with stalks 1–2 cm long, these sparsely to densely short-hairy. Calyces 5–8 mm long, cylindric to narrowly bell-shaped above a short hypanthium (this conic, usually darker colored, and somewhat asymmetric), densely pubescent with short, woolly hairs, the lobes much shorter than the tube, more or less similar (the lowermost slightly shorter than the others, the upper pair sometimes slightly fused basally), semicircular to broadly oblong, rounded at the tips. Corollas white, the banner 12–20 mm long, the expanded portion broadly obovate to nearly circular above the short, stalklike base, reflexed, usually shallowly notched at the tip, the inner surface with a yellow area below the midpoint, this usually with fine red spots, wings 12–18 mm long, 5–6 mm wide, oblong-lanceolate above the short, stalklike base, straight, the keel 11–18 mm long, 5–6 mm wide, oblong-lanceolate above the short, stalklike base, straight, folded and overlapping, but not fused. Stamens 10, the filaments fused only at the very base, 14–16 mm long, curved, graded in size, the anthers all similar, small, attached near the midpoint, yellow. Ovary 4–7 mm long, linear, densely short-hairy, the style 4–5 mm long, curved, glabrous, the stigma minute, terminal. Fruits legumes, (4–)7–10 cm long, 7–12 mm wide, linear to narrowly oblong, flattened, straight, sometimes shallowly indented between the widely spaced seeds, sometimes becoming loosely spirally twisted, glabrous at maturity, tapered to a short, stalklike base, tapered to a sharply pointed, sometimes short-beaked tip, the valves thin, papery, usually indehiscent (occasionally dehiscing tardily from the tip), with (1–)4–7 seeds. Seeds 7–8 mm long, 3–4 mm wide, oblong to somewhat asymmetrically kidney-shaped, flattened, the surface smooth, olive to dark brown, usually with a darker line around the circular attachment point. 2n=28. May–June.

Uncommon in the southwestern portion of the Ozark Division, disjunct in Wayne County; introduced in Franklin County (Indiana and possibly Ohio south to South Carolina, west to Missouri and Oklahoma). Mesic upland forests, talus slopes, and bottoms and ledges of bluffs; usually on calcareous substrates.

Although most common in Tennessee, C. kentukea is not abundant anywhere. Yellowwood is identified by its graceful trunks with smooth bark (similar to that of the beech, Fagus grandifolia Ehrh., Fagaceae), odd-pinnately compound leaves with petioles that cover the buds, loose panicles of white flowers with separate petals and stamens, and relatively thin, few-seeded legumes.

The scientific name of yellowwood has undergone a change in recent decades (K. R. Robertson, 1977; Sand, 1992; Spongberg and Ma, 1997). For many years, it was called C. lutea, until Rudd (1971) discovered an overlooked earlier publication in which the taxon was named Sophora kentukea. Thus, the oldest correct name in the genus Cladrastis became C. kentukea.

Yellowwood is valued as an ornamental tree, although it is not common in cultivation because it is relatively slow-growing and does not flower until it is 10–20 years old. It is easy to propagate by cuttings or seed. A yellow dye was obtained from the wood and used for coloring homespun fabrics. It is seldom used for this purpose today, especially since this species is considered rare or imperilled in much of its range.