The bark is thick, dark brown in color, and divided by rather deep fissures into rounded ridges. Black walnut and butternut are allelopathic and produce a compound called juglone. This compound will injure some other plants. Many plants in the rose family and heath family are very sensitive and will be injured or killed if planted beneath the drip line of a black walnut tree. Other plants such as bluegrass grow very well beneath the crowns of black walnut or butternut trees.
Flowers are monoecious.
Male flowers are borne in long catkins. Female flowers are rounded and have conspicuous, red-fringed stigmas. The fruit is borne singly or in pairs and enclosed in a solid green husk which does not split open even after the nut is ripe. The nut itself is black with a very hard, thick, finely ridged shell, enclosing a rich, oily kernel which is edible and highly nutritious. This tree produces the black walnut nuts of commerce, but the fruit is considered a liability in an urban site.