American hazelnut is a thicket-forming, spreading shrub to 10 feet high.
Leaves are simple, alternate, 3–6 inches long, 2–4½ inches wide; egg-shaped to oval, the tip pointed, the base broadly rounded or heart-shaped, the margin finely double-toothed, thin; the upper surface dark green, somewhat rough and smooth or nearly so; the lower surface paler with matted hairs to more or less hairy; leaf blade with 6–9 veins on each side of the central vein, sometimes branched; leaf stalk ¼–1⅛ inch long, often with reddish, gland-tipped hairs, or long white hairs. The leaves can be very colorful in autumn, varying from orange to brick red or purplish red, or with combinations of rose, orange, yellow, and pale green.
Bark is brown to grayish brown, fairly smooth, with the thin outer layer slightly grooved; the wood is nearly white, medium hard, and fine-grained.
Twigs are slender, angled upward, reddish brown at first with long, spreading, usually gland-tipped hairs, gray and smooth later.
Flowers late February–April. Male (staminate) and female (pistillate) flowers are separate but on the same plant. Staminate flowers in catkins (dense, elongated flower clusters); these are mostly solitary, 3¬–4 inches long, cylindrical, brown, petals lacking; stamens 4. Pistillate catkins are much shorter, clustered at the ends of short branches on the previous year’s growth (as are the staminate catkins), petals lacking, stigmas red.
Fruits mature July–August. They are nuts, in clusters of 1–3 (sometimes to 5), encased in large bracts (modified leaves). The bracts are more or less flat, unlobed, leafy to paper in texture, completely enclosing the nut and falling with it; each bract is ¾–1¼ inch long, projects far beyond the nut, is strongly veined, more or less felty-hairy, with the outer edge sharply toothed. The nuts are ⅜–⅝ inch long, egg- to globe-shaped, usually wider than long, the shell thick and hard; light brown, sweet, and edible.
Similar species: Other Missouri members of the birch family are in different genera. They include European alder (an introduced species) and common alder (Alnus glutinosa and A. serrulata); river birch (Betula nigra); hornbeam (musclewood) (Carpinus caroliniana); and hop hornbeam (ironwood) (Ostrya virginiana). Of these, common alder is probably most similar to hazel when it comes to leaf shape and texture, but the fruits of alder are quite different, resembling miniature pinecones.
Beaked hazel (Corylus cornuta) is closely related, but it does not occur naturally in Missouri. It grows in states farther northeast and east.