General: Beech Family (Fagaceae). Native trees often reaching 20–30 m tall, less commonly up to 50 m; bark dark gray or black, shallowly furrowed into broad hard scaly ridges, inner bark reddish to pink; generally developing a strong taproot and network of deep, spreading laterals. Leaves are deciduous, alternate, elliptic, 10–25 cm long and 8–15 cm wide, divided less than halfway to midvein into 7–11 shallow wavy lobes with a few irregular bristle-tipped teeth, sinuses usually extending less than 1/2 distance to midrib, glabrous and dull green above, light dull green below with tufts of hairs in vein angles. Male and female flowers are borne in separate catkins on the same tree (the species monoecious), the staminate catkins in leaf axils of the previous year's growth, the pistillate in 2–many-flowered spikes in the leaf axils. Acorns maturing in the second year, about 15–30 cm long, with a broad usually shallow cup, borne singly or in clusters of 2–5. The common name is in reference to the red fall foliage color, red petioles, and reddish interior wood. This is a different species from “southern red oak” (Q. falcata).
Northern red oak is a member of the red oak subgroup (subg. Erythrobalanus = sect. Lobatae). It hybridizes with related species, including scarlet oak (Q. coccinea), northern pin oak (Q. ellipsoidalis), shingle oak (Q. imbricata), scrub oak (Q. ilicifolia), blackjack oak (Q. marilandica), swamp oak (Q. palustris), willow oak (Q. phellos), Shumard oak (Q. shumardii), and black oak (Q. velutina).
Variation within the species: There are different interpretations of variation patterns among trees of northern red oak. A single species without formally variants is sometimes recognized, or two varieties may be recognized.
Quercus rubra var. ambigua (A. Gray) Fernald
SY= Q. borealis Michx. f.
SY= Q. rubra var. borealis (Michx. f.) Farw.
Quercus rubra var. rubra
SY= Q. maxima (Marsh.) Ashe
SY= Q. borealis var. maxima (Marsh.) Ashe
Var. rubra has a shallow cup, to 3 cm wide, enclosing 1/4–1/5 of the nut. Var. ambigua has a deeper cup, to 2 cm wide, enclosing 1/3 of the nut. McDougal and Parks (1984, 1986) found evidence of correspondence between morphological types and flavonoid chemotypes but the evolutionary status and geographic distribution of these have not been worked out in detail.