Walk by the Cannon River these days and you are sure to find (or trip over, like I do) green and browning spheres, the size of golf balls. These are the fruit of the native black walnut (juglans nigra).

Edible and highly nutritious, black walnuts have been consumed by humans for centuries. The trees, which grow in moist soils, begin to drop their fruit in early fall. The soft outer husk rots and then sheds, revealing a furrowed inner shell. Crack it with a hammer and the black walnut tumbles out, a zestier version of the more widely consumed English walnut. 

Yet black walnuts are known for more than their scrumptious nuts. The trees are often given as an example of allelopathy: plants producing chemical compounds which inhibit other plants’ growth. Almost all parts of the black walnut tree contain hydrojuglone, which becomes toxic juglone when exposed to air. A host of species, including different native pines, olives, tomatoes and soybeans, struggle to grow in soils with juglone. This presents a competitive advantage to young trees competing for limited resources.

306 million black walnuts grow throughout the eastern United States, but forests entirely composed of the species are uncommon. Here at Carleton, we boast an unusually high black walnut population; the northeast facing Arb and the flooding of the Cannon provide a particularly suitable habitat. Often growing to be 60ft tall — and sometimes taller — their shade-intolerant canopies provide shelter for the Arb’s riverside paths each summer.

And now? The leaves of the black walnut have begun to turn yellow and glide to the ground. Preparing for a cold winter, a winter at the northernmost part of its range, the tree is buckling down, transitioning to the next season.