Bur oak is a medium to very large tree with a broad, spreading, rounded crown, a massive trunk, and low, large, spreading branches.
Leaves are alternate, simple, 6–12 inches long, 3–6 inches wide, spatula-shaped, and broadest near the middle; margin with 5–9 lobes, notches shallow on the outer half but deeply cleft near the base, the notch of the two largest lobes almost reaching the central vein; lobe tips rounded; upper surface dark green; lower surface downy and pale.
Bark is thick, gray-brown, and deeply grooved at maturity; ridges long, flat-topped.
Twigs are light brown, hairy, becoming darker and smooth with age; twigs often develop corky ridges after the first year.
Flowers April–May, in catkins.
Fruits September–October, acorns solitary or paired. Nut brown, rounded to broadest near the base, ¾–2 inches long; cup deep, hairy, enclosing ½–¾ of the nut, the scales along the edge producing a fringed or ragged mossy-looking border. Acorns edible, ripening in autumn of the first year.
Similar species: This is our only native oak to develop acorns with such an unusual fringed border along the cup. But there is non-native oak that also produces fringed or shaggy-cupped acorns; it is often planted as a street tree or in other landscape plantings and can escape into the wild: sawtooth oak (Q. acutissima), a native of Asia. Its leaves are quite different, being entire, unlobed, 4–8 inches long, with a toothed margin, with each tooth bearing a bristle at the tip (something like the leaves of an American chestnut). Its acorn cup scales are almost all fringed or shaggy, while only the scales along the rim of the cup are elongated in bur oak.