The Notholithocarpus (prev. Lithocarpus) densiflorus leaves are alternate, 7–15 cm (2.8–5.9 in), with toothed margins and a hard, leathery texture, and persist for three to four years. At first they are covered in dense orange-brown scurfy hairs on both sides, but those on the upper surface soon wear off; those on the under surface persist longer but eventually wear off too.
The seed is an acorn 2–3 cm (0.79–1.18 in) long and 2 cm in diameter, very similar to an oak acorn, but with a very hard, woody nut shell more like a hazel nut. The nut sits in a cup during its 18-month maturation; the outside surface of the cup is rough with short spines. The nuts are produced in clusters of a few together on a single stem. The nut kernel is very bitter, and is inedible for people without extensive leaching, although squirrels eat them.
Tanoak is one of the species most seriously affected by "sudden oak death" (Phytophthora ramorum), with high mortality reported over much of the species' range.
Notholithocarpus densiflorus var. echinoides
in Berkeley California
Members of populations in interior California (in the northern Sierra Nevada) and the Klamath Mountains into southwest Oregon are smaller, rarely exceeding 3 m (9.8 ft) in height and often shrubby, with smaller leaves, 4–7 cm (1.6–2.8 in) long; these are separated as "dwarf tanoak", Notholithocarpus densiflorus var. echinoides. The variety intergrades with the type in northwest California and southwest Oregon. Tanoak does grow on serpentine soils as a shrub.