Zanthoxylum oahuense, a member of the rue family (Rutaceae), is a small tree, 3 to 6 m (10 to 20 ft) tall, with gray to dark brown bark that is thin and smooth to slightly furrowed. New growth is finely hairy, becoming glabrate (without hairs). Young branches are subtended by a pair of bud scales. Leaflets are in sets of three, leathery, triangular, with the lateral pair usually asymmetrical, 7 to 16 cm (2.8 to 6 in) long, 4.5 to 9 cm (1.8 to 3.5 in) wide. Leaf veins form a reticulate pattern. Leaf margins are shallow-toothed to smooth, usually sickle-shaped, with the base flat or notched. Leaf stalks are twice jointed and 35 to 72 mm (1.4 to 2.8 in) long. Plants are dioecious (male and female flowers on separate plants), rarely monoecious. Unisexual flowers are in groups of 15 to 40 arising between the leaf base and stem, and the terminal flowers bloom first. The inflorescences are usually 8 to 12 cm (3 to 4.7 in) long and finely hairy throughout. Petals are pale greenish yellow, sometimes tinged with red, 4 to 7 mm (0.15 to 0.3 in) long and glabrous. The dry, dehiscent (opening when mature) fruit is obovoid, 9 to 13 mm (0.35 to 0.5 in) long, wrinkled and pitted with one black seed 8 to 12 mm (0.3 to 0.47 in) in diameter.