Walnut Family

Species in the walnut family are woody plants that develop as trees, and mostly grow in angiosperm-dominated forests in temperate and subtropical climates. The range of most species is the Northern Hemisphere, although a few species penetrate to the Andes of South America and the southwest Pacific.

The wood of trees in the walnut family is strong and resilient and is highly prized as lumber. The twigs have a chambered pith which is visible in a longitudinal cross-section.

Species in the walnut family have prominent, hairy buds and seasonally deciduous, compound leaves which are shed in the autumn. The flowers are small and greenish. The staminate flowers occur in catkins, while pistil-late flowers occur individually. The ripe fruit of the hickories and walnuts is properly termed a drupe in which their large, hard-coated seed is encased in a leathery case. The fruits of walnuts and hickories are edible.

An interesting characteristic of the black walnut and some related species is their ability to apparently poison the soil in their immediate vicinity. Few plants are able to grow beneath a large walnut tree, an observation attributed to the presence of a toxic alkaloid known as juglone. Even walnut seedlings cannot normally grow beneath a parent tree. The production of phytotoxic chemicals by plants for use in a type of chemical warfare with other competing plants is a form of allelopathy. Black walnut is often cited by ecologists as an archetypal, allelopathic species.