11. Quercus rubra L. (northern red oak)

Quercus rubra var. ambigua (F. Michx.) Fernald

Quercus borealis F. Michx.

Quercus borealis var. maxima (Marsh.) Ashe

Pl. 415 j, k; Map 1849

Plants trees to 30 m tall. Bark medium gray, divided into persistent ridges, the inner bark pinkish. Twigs 1.5–4.0 mm wide, usually dark purplish brown, sometimes dark brown or grayish, glabrous. Buds 3–9 mm long, dark brown or reddish brown, glabrous or the upper scales pubescent. Petioles 18–58 mm long. Leaf blade 11.5–20.0 cm long, 9–16 cm wide, truncate or broadly obtuse at the base, divided 25–90% of the width, the lobes (3)4 or 5 per side, evenly spaced, the largest lobes usually at or above the midpoint; well-developed lobes oblong or lanceolate, seldom slightly broadened outward, 12–40 mm wide, obtuse to slenderly tapered apically, usually toothed, often with a secondary lobe on the lower (seldom also the upper) margin, each with 1–7(–11) bristles 2–6 mm long (the whole blade with 16–53 marginal bristles), the strongest secondary veins reaching the margin at the tips of the lobes and ending in bristles, others reaching toward sinuses and turning aside before reaching the margin; the upper surface dull, glabrous, the undersurface green, glabrous, smooth to the touch, the vein axils with small tufts of 5–9-rayed, stalked hairs, or glabrous. Acorn cups 5–13 mm long, 15–30 mm wide, covering 10–30% of the nut, saucer-shaped, the inner surface smooth, usually sparsely pubescent, at least near the nut scar, the outer surface with the scales mostly distinctly convex-thickened at the base, sparsely pubescent. Nuts 19–26 mm long, 13–23 mm wide, ovoid to cylindrical, without concentric grooves around the tip. 2n=24. April–May.

Common throughout the state (eastern U.S. west to Minnesota, Nebraska, and Oklahoma; Canada). Mesic to dry upland forests, bases and tops of bluffs, banks of streams and rivers, margins of sinkhole ponds, and edges of glades, upland prairies, and loess hill prairies; also pastures and roadsides.

Steyermark’s (1963) key to oak species stated that leaves of Q. rubra are never lobed for much more than half of their width. This is not accurate, even on specimens named by Steyermark himself, and has led to many subsequent misdeterminations. Hybrids involving Q. rubra can be difficult to distinguish from those involving other parents with similar leaves. Nevertheless, hybrids between Q. rubra and five other oak species have been collected in Missouri.