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Black Walnut (Juglans nigra)

Black Walnut Leaves Black Walnut Nuts Black Walnut Bark

Black Walnut, a relatively common tree at the Arboretum, is readily recognized by large, pinnately compound leaves with 11-23 leaflets; dark, ridged to platy bark; and green, turning to yellow, ball-shaped fruits. It is considered a pioneer species, invading fields and other open areas.

Black Walnut, an allelopathic plant, produces a chemical compound, hydrojuglone. When hydrojuglone is oxidized in air or soil, it becomes the toxic chemical juglone, which is produced by the leaves, fruits, and roots. It accumulates in the soil under the tree, inhibiting the growth of many other plants. A few plants, especially grasses, are unaffected by juglone and can grow under the tree’s canopy.

Black Walnut is a highly prized wood. Early settlers used it extensively for construction, but today its primary use is for making furniture and gunstocks. The nuts are used in cooking; the oily husks have been used to make dyes and walnut stain; and the pulverized shells are used in oil drilling, in cleaning jet engines, and for making activated charcoal. The presence of Black Walnut trees at various places around the Arboretum may indicate former home sites. Fruits from these trees may be somewhat larger than normal, perhaps reflecting a selection by the homeowners of more vigorous cultivars.

Black Walnut is susceptible to Thousand Cankers Disease, a fungal disease carried from tree to tree by the Walnut Twig Beetle. This disease has the potential to severely affect Tennessee populations. A ban on transporting firewood within the state is currently in place to try to reduce the impact of this disease.

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