Black walnut is a large tree with a straight trunk and rounded, open crown. The nuts, spicy odor, large feather-compound leaves, and chambered pith in the twigs help identify it.
Leaves are alternate, compound, 1–2 feet long, with 11–23 leaflets. Leaflets 3–5 inches long, 1–2 inches wide, broadest below the middle, the end leaflet smaller than side ones or absent; margin toothed; upper surface yellow-green; lower surface paler, hairy.
Bark is grayish-brown or black, grooves deep, ridges broad with sharp or rounded edges, roughly forming diamond-shaped patterns, chocolate-colored when cut.
Twigs are stout, rigid, brown to gray-brown, hairy; end bud about ½ inch long; pith light brown, chambered when cut lengthwise.
Flowers April–May. Male flowers in catkins, female flowers in a short spike on the same tree.
Fruits September–October, usually single or in pairs. A green, rounded husk, 1½–2½ inches across, covers the round, hard, bony, dark brown or black nut. The kernel is oily, sweet, and edible.
Similar species: Butternut, or white walnut (Juglans cinerea), is scattered and declining in the eastern two-thirds of Missouri, mostly in low and moist soils. It has rather cylindrical fruits, and the nut inside has 4 lengthwise ribs; leaf scars have the upper edge straight (not notched), bordered by a well-defined velvety ridge. The mild-tasting English (or Persian) walnut is the species J. regia. It is native to Eurasia and when cultivated in Missouri does not escape. The state of California grows nearly all of the US commercial supply of English walnuts. Walnuts are in the same family as hickories and pecans.