Black Walnut

The black walnut naturally produces a chemical called juglone all throughout the tree, especially in the fruit husks, roots, leaves, and buds. The juglone from the fruit husk was cultivated by the Native Americans as a black dye for clothes and inks. In the belowground root system, the juglone is used as a toxic chemical to kill nearby plants. This adaptation 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 solanaceous crops.

The black walnut tree is a member of a larger group of trees known as the angiosperms, which are plants that produce flowers and fruits that contain seeds. It is a monoecious tree, meaning that it produces both male and female flowers on the same plant. These unremarkable yellow-green flowers arrive late in Spring, and the arrival of leaves shortly follows them. Male flowers are drooping catkins, which are produced by many tree species, including the many oak species you will find on campus and near this walnut tree. Female flowers are short, terminal spikes that are fertilized by wind-dispersed pollen from male flowers. After fertilization, the flowers die, and a fruit develops that contains a seed with the potential to implant and develop into a new tree. 

The black walnut produces a unique class of fruits known as drupes. Many of the fruits that we eat everyday are considered drupes such as cherries, peaches, and plums. Drupes consist of an outer husk surrounding a hard, inner shell; within this hard, inner shell lies a seed. Furrowed within the inner shell of the black walnut drupe is an edible nut. Young trees begin producing fruit around 4-6 years old, but it may take nearly 20 years for a tree to produce a large crop. Black walnut drupes mature in September and October, falling from the trees. If you are visiting during the fall months, look down on the ground for large, round yellow fruits. As the fruits decay, the outer yellow-green husk rots and blackens, exposing a darkly colored shell. These shells are extremely durable; squirrels are one of the only animals that have strong enough teeth to crack the shell and eat the edible nut.

The black walnut drupe is one of the last hand-picked crops in North America today. One of the main cultivators of black walnuts – Hammons Product Company – hand-harvests black walnuts from wild trees in Missouri every fall. Black walnuts can be enjoyed raw, and they are also commonly used in baked goods and desserts. Black walnuts stand out from the most frequently consumed walnut, the English walnut, due to their bolder, earthier flavor. 

Thank you for taking the time to learn about this beautiful tree in the heart of Rowan’s campus. Closely related trees to the black walnut include the butter pecan, pecan, and English walnut. I encourage you to explore other native North American trees on Rowan’s campus such as the sycamore, sweetgum, and dogwood. We are excited to share our love of these important members of our community with you! 

These resources are a product of Dr. Sara Wright’s Plant Diversity course in the Biological and Biomedical Sciences Department. My name is Michael Giunta, and I will earn my Bachelor of Science degree in Biological Sciences in 2023. I plan to pursue a career in health professions. You can read more about me and the Black Walnut tree below.