Black Walnut

The black walnut produces a chemical called juglone throughout the tree, especially in the fruit husks, roots, leaves, and buds. Juglone from the fruit husk was cultivated by Native Americans as a black dye for clothes and inks. In the root system, the tree uses toxic juglone to kill nearby plants, which helps reduce competition for resources and allows the black walnut to dominate its habitat. Juglone’s toxicity makes it nearly impossible to grow certain plants, such as ornamental azaleas, rhododendrons, and peonies, as well as economically important blueberries and various flowering nightshades.

A member of a larger group of trees known as angiosperms, plants that produce flowers and fruits containing seeds, the black walnut is monoecious, producing both male and female flowers on the same plant. These unremarkable yellow-green flowers arrive late in Spring. Male flowers are drooping catkins, which are produced by many tree species. Female flowers are short, terminal spikes that are fertilized by wind-dispersed pollen from male flowers. After fertilization, the flowers die, and a fruit that contains a seed with the potential to develop into a new tree develops.

The black walnut produces a class of fruits known as drupes. Common drupes are cherries, peaches, and plums. Drupes consist of an outer husk surrounding a hard inner shell, and within that shell lies a seed. Furrowed within the inner shell is an edible nut.

Young trees begin producing fruit around four to six years old but take up to 20 years to produce a large crop. Black walnut drupes mature in September and October, falling from the trees. The outer yellow-green husk rots and blackens as the fruits decay, exposing a darkly colored shell. These shells are extremely durable, and squirrels are among the few animals with teeth strong enough to crack them and eat the edible nut.

The black walnut drupe is one of the last hand-picked crops in North America. One of its main cultivators, the Hammons Product Company, hand harvests it from wild trees in Missouri. Black walnuts stand out from the most frequently consumed walnuts because of their bolder, earthier flavor.

*This resource is a product of Dr. Sara Wright’s Plant Diversity course in the Biological and Biomedical Sciences Department.