The terminal leaflet is often suppressed so that the leaf ends in 2 leaflets. Male flowers are borne in catkins and female flowers in clusters. The walnut fruit is a round nut in a hard, corrugated shell. The terminal bud is ovoid, blunt and slightly hairy.
Lateral buds are smaller and often suppressed.
Black walnut bark is dark gray or black and furrowed with deep, interconnecting ridges that become blocky. The root system is deep, wide-spreading, usually with a tap root. A toxic substance, juglone, is leached from the roots and fallen leaves, inhibiting the growth of many plants, including young walnut seedlings. Aphid, walnut caterpillar and walnut scale are the known insect pests of walnut trees. The wood is heavy, hard, strong, easily worked and has an attractive grain.