Black walnut grows in association with a variety of hardwood tree species. These include most species of the eastern oaks, the ashes, yellow poplar, beech, sugar and silver maple, basswood, river birch, most hickory species, hackberry and sugarberry, black cherry, cottonwood, the elms, black gum, sweet gum (not related), and to a lesser extent, many smaller trees and shrubs.

A closely related tree, the butternut, or white walnut, Juglans cinerea, is becoming very rare because of a fungal disease called "butternut canker". Young butternut trees are difficult to distinguish from young walnut trees. As a rule of thumb, the terminal bud of a butternut is more elongated rather than rounded and the terminal leaflet is present on butternut, while it is either absent, dwarfed, or occurs as 2 small leaflets in black walnut. A mature butternut has distinct elongated light gray plates on the bark. The fruit is also spherical in shape, while black walnut is rounded.

Identification of Butternuts and Butternut Hybrids (pdf from Purdue Extension)

Conservation and Management of Butternut Trees (pdf from Purdue Extension)