Southern red oak, or Spanish oak, is a large tree with a long, straight trunk, open, rounded crown, and spreading branches.
Leaves are alternate, simple, 7–9 inches long, with a rounded base; variable in shape; with 3–5 bristle-tipped lobes, the first (lowest) pair of lobes are usually the largest and longest, often sickle-shaped, with the notch of the lobes wide and extending nearly to the midvein. Some leaves, particularly those growing in deeply shaded parts of the tree, may have the largest side lobes above the midpoint, with an unloabed tip that is no larger than the lateral lobes. Upper surface dark green, shiny; lower surface paler with light brown to grayish-white matted hairs. Leaves often droop; turn reddish-brown in fall. The 3-lobed leaves are distinctive.
Bark is grayish-black, broken into deep grooves, becoming ridged and rough-plated near the base, not scaly. Inner bark only slightly yellow.
Twigs are stout, reddish-brown, hairy at first, smooth later.
Flowers April–May, in catkins.
Fruits September–October, acorns single or in pairs, brown, faintly striped, round, ½ inch long; cup covering about one-third of the nut, thin, scales flattened, with a reddish-brown dark border, hairy. Ripening in autumn of the second year.
Similar species: The cherrybark oak, or swamp Spanish oak (Q. pagoda) is closely related and has sometimes been considered a subspecies of southern red oak. In Missouri it occurs naturally only in our southeasternmost counties. Its leaves usually have 3 or 4 lobes per side, evenly spaced, with no extra smaller lobes (not 1 or 2 large lobes per side, plus up to 2 smaller lobes); also, there is no narrowly rectangular, unlobed, strap-shaped section at the leaf tip, as there is in southern red oak. The evenly spaced, symmetrical lobes of cherrybark oak look something like the silhouette of a Chinese pagoda (hence the scientific name).