Black gum is a tall tree with horizontal branches and a flat-topped crown. Young trees are pyramidal; older trees more oval.
Leaves are alternate, simple, oval-elliptical, and lack teeth. In summer they are shiny dark green above and downy below. Often crowded toward the tips of branches. Early color changers, they turn bright scarlet or purple in late summer, well before the first frost.
Bark is gray to brown or black, deeply grooved, with ridges broken into irregularly shaped blocks with an “alligator hide” appearance.
Twigs are slender, reddish brown, slightly hairy at first, becoming gray and smooth later; some twigs short, pointed; pith white, with chambers.
Flowers April–June, as the leaves unfold. Male and female flowers greenish, in clusters on separate trees; petals 5, small.
Fruits September–October; plumlike, bluish black with a whitish coating, about ½ inch long, egg-shaped, thin-fleshed, with a single seed or pit. Pit flattened, with 10–12 broad, rounded ribs.
Similar species: Water tupelo (N. aquatica) develops a large, swollen base and the leaf margins are often irregularly toothed, the leaf tip abruptly pointed. Fruit is dark purple, thick-skinned, and dotted, in drooping clusters, each fruit about 1 inch long and widest above the middle. It occurs naturally in swamps with bald cypress trees in Missouri's southeastern lowlands and in two sinkhole ponds in the Ozarks.