10. Quercus phellos L. (willow oak)

Pl. 413 n, o; Map 1848

Plants trees to 30 m tall. Bark medium gray, divided into persistent ridges, the inner bark light orange. Twigs 1–2 mm wide, dark brown or gray, glabrous or rarely with scattered, branched, spreading hairs. Buds 2–4 mm long, dark brown, glabrous, the scales hairy along the margins. Petioles 1–6 mm long. Leaf blade 6–12 cm long, 1–3 cm wide, rounded or acute at the base, unlobed, entire, with only 1 apical bristle 0.5–2.0 mm long, the secondary veins all turning aside before reaching the margin; the upper surface usually rather shiny, glabrous, the undersurface green, glabrous, or rarely with inconspicuous, unbranched, appressed hairs and inconspicuous hairs on the midrib, smooth to the touch, sometimes with small tufts of 5–15-rayed, often stalked hairs in the axils of the major veins or sometimes all along the lower midrib. Acorn cups 4–6 mm long, 13–16 mm wide, covering 20–40% of the nut, saucer-shaped to shallowly bowl-shaped, the inner surface smooth, densely hairy, the outer surface with the scales tending to be distinctly convex-thickened at the base, pubescent. Nuts 8–11 mm long, 10–13 mm wide, depressed-globose or broadly ellipsoid, without concentric grooves around the tip. April–May.

Scattered in the Mississippi Lowlands Division and adjacent portions of the Ozarks and Ozark Border (eastern [mostly southeastern] U.S. west to Missouri and Texas). Bottomland forests, swamps, banks of streams and rivers, and margins of oxbows and sloughs; also fencerows, ditches, and roadsides.

Juvenile leaves of Q. phellos or leaves from vigorous sprouts may be lobed, and they resemble those of the hybrid, Q. nigra × Q. phellos. In addition to this hybrid, Q. phellos has been documented to hybridize with five other oak species in Missouri. Because such hybrids involve a cross between a species having narrow, entire leaves, and a second parent with broader, lobed leaves, they may be more easily distinguishable than are hybrids in which both parental taxa have similar patterns of leaf division.