Common buckthorn is a many-branched, shrubby tree with a spreading, irregular crown.
Leaves are simple, some appearing alternate but most are opposite, egg-shaped, abruptly pointed (sometimes rounded) at the tip, finely toothed. Both upper and lower surfaces are smooth. Leaves have 3–5 pairs of veins branching from the midvein that curve along the edges of the leaf. Leaves appear early in spring and are retained late into autumn.
Bark is grayish brown with white narrow lines (lenticels), and is rough in texture at maturity. Trunk diameter to 10 inches.
Twigs are elongated, dark buds are closely pressed against the twig. The terminal bud is often modified into a spine.
Flowers April–June; male and female flowers usually produced on separate plants. Flowers have 4 small, yellowish-green petals and appear in clusters.
Fruits August–September, often remaining until December; round, black, small (up to ¼ inch in diameter), juicy, typically contain 3–4 seeds.
Similar species: Five species in genus Rhamnus are recorded to live wild in Missouri.