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Black walnut,
Juglans nigra

Biology Department NSF

Black walnut, Juglans nigra, is a tree that can grow up to 150 feet in height. The bark is black, thick and deeply furrowed. When scraped with a knife, it reveals a chocolate-covered sub-surface. The twigs are stout, greenish or orange-brown in color, and may be hairy or smooth. The pith (inner portion of the twig) is dark brown in color and is partitioned (chambered) when sliced with a knife. The leaves are alternate (occurring at different points along the twig), and pinnately compound. When crushed, the leaves have a distinct "walnut" smell. The flowers are borne separately, but on the same tree. They appear as yellow-green catkins (think of birch trees), and appear when the leaves are partly grown. The staminate (males), are most abundant and are thicker when compared to the pistillates (females), which are fewer in number and smaller. The flowers have no petals. The fruits (walnuts) occur singly or in groups of 2 or 3. They are spherical in shape and 2 inches or larger in diameter. The outer husk is yellow-green, but quickly darken and turn black when they fall to the ground. The inner nut has a very hard shell, is dark brown in color, deeply ridged and has a sweet tasting edible nut.

The natural range of black walnut is western Massachusetts and Connecticut, south to the North Carolina coast, southwest through Georgia and westward through the Gulf states to eastern Texas, north through Oklahoma to southern Nebraska, and eastward through the southern regions of the northern Lake states. It is scattered in New York and can be found in extreme southern Canada.

Black walnut leaves, fruits and twigs contain a chemical called "Juglone" which is alleopathic. It is a growth inhibitor to some species of plants such as tomatoes or apple trees, but our focus species, spicebush, is known to be tolerant. Spicebush herbivory has been shown to be reduced when growing near black walnut.


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