Black Walnut Fact Sheet

Black walnut (Juglans nigra) is one of the most valuable trees in the United States. Furniture and cabinet manufacturers use its wood to create veneers for wooden surfaces, and its edible nuts add flavor to pastries and other baked goods. The tree's leaves and bark, which have astringent and laxative properties, are used in herbal remedies.

Identification

Black walnut trees grow between 50 and 75 feet tall on average, with an approximately equal spread. They have open, oval-shaped crowns and single trunks. Their grayish-brown or grayish-black bark is deeply ridged and furrowed. Alternating, 5-inch-long leaflets grow on pinnately compound leaves; the dark-green foliage turns yellow during the fall. In the spring, female flowers produce flowering spikes, while the males produce cylinder-shaped, scaly buds called catkins. Edible nuts follow the flowers; the inner nut is protected by a hard brown shell surrounded by a fleshy green covering. The green covering turns black and opens around September or October, dropping the nut to the ground when it is ready for harvest.

Cultivation

Black walnut trees grow in U.S. Department of Agriculture hardiness zones 4 through 9. They thrive in deep, well-drained, nutrient-rich, moist soil and full sunlight. Black walnuts have a slow-to-moderate growth rate and generally do not transplant well because they have a deep tap root. They are moderately drought-tolerant, but do not grow well in soggy or routinely-flooded soil. Black walnuts will not thrive in shady sites, and the University of Connecticut warns that they tend to be messy in landscape settings because they drop their fruits on the ground.

Diseases and Problems

Powdery mildew, a fungal infection, creates a powdery white fungal coating on black walnut leaves. Anthracnose disease creates brownish spots on the leaves and lowers fruit production, and diseased trees often lose their leaves prematurely. Bacterial blight damages the foliage and ruins the nuts, while canker diseases create sunken areas of diseased bark and cause branch dieback or tree death. Black walnut trees are also susceptible to caterpillar infestations. Fall webworms create unsightly nests near the ends of branches and defoliate trees, while mites and scales suck juice from the stems and foliage. Other pests include walnut husk maggots, curculios, twig girdlers and walnut lace bugs.

Plant Toxicity

Black walnut trees produce a substance called juglone that is toxic to many other plant species. Tomatoes, rhododendrons, azaleas, apples and potatoes are highly susceptible to its effects. The chemical inhibits respiration in plants and deprives them of metabolic energy. Sensitive plants turn yellow and eventually wilt and die if planted within an 80-foot range of the tree's roots. Purdue University Cooperative Extension Service advises planting gardens a safe distance away from black walnut trees, or else planting juglone-tolerant plants such as carrots, squash, redbud, viburnum or hyacinth, among other species.

References

• University of Minnesota Extension; Growing Black Walnut; Melvin J. Baughman and Carl Vogt; 2002
• Purdue University Center for New Crops and Plant Products; Juglans Nigra L.; James A. Duke; 1983
• University of Connecticut Plant Database: Juglans Nigra - Black Walnut
• North Carolina State University: Juglans Nigra
• University of Florida Extension; Juglans Nigra - Black Walnut; Edward F. Gilman and Dennis G. Watson; 2006
• Ohio State University Extension; Black Walnut Toxicity to Plants, Humans and Horses; Richard C. Funt and Jane Martin

Resources

• Purdue University: Black Walnut Toxicity; Michael N. Dana and B. Rosie Lerner; February 1994