About the Black Walnut
The black walnut (Juglans nigra) is one of the most commercially valuable native hardwood trees in North America, and one that Middle Tennessee homeowners frequently encounter on older rural and suburban properties. Its dark chocolate-brown heartwood has been prized for centuries for furniture, veneers, and gunstocks, making a large black walnut one of the few landscape trees with genuine timber value.
In the landscape, black walnut is a handsome, large shade tree with long, pinnately compound leaves that cast light, filtered shade. It produces edible walnuts beloved by squirrels and people alike – though cracking the extremely hard shells takes some effort. Black walnut is native throughout Middle Tennessee and thrives in Nashville's deep, well-drained upland soils.
The most important consideration with black walnut is juglone – a naturally occurring chemical released from its roots, leaves, and nut husks that inhibits or kills many common garden and landscape plants. Understanding juglone is essential before deciding where to plant or how to landscape around an existing walnut.
Identification
- Bark: Dark gray-brown to nearly black, with deep, diamond-shaped furrows creating a rugged, interlocking ridge pattern distinctive from other hardwoods.
- Leaves: Pinnately compound, 12 to 24 inches long with 13 to 23 paired leaflets. Crushed leaves have a distinctive, pungent, spicy-walnut aroma. Leaflets turn yellow in fall.
- Fruit: Round, green husked nuts 1.5 to 2.5 inches in diameter, ripening in September – October. The husk turns black and stains anything it contacts dark brown.
- Form: Tall, relatively open-crowned tree with an irregular, rounded canopy and strong central leader when young, becoming more spreading with age.
Understanding Juglone & Siting Your Walnut
Juglone (5-hydroxy-1,4-naphthoquinone) is produced throughout the black walnut tree and leaches into the soil, inhibiting sensitive species within roughly 50 to 80 feet of the trunk – often beyond the drip line.
Plants sensitive to juglone include tomatoes, peppers, eggplant, potatoes, apples, pears, blueberries, blackberries, rhododendrons, azaleas, mountain laurel, and many ornamental perennials and annuals.
Plants tolerant of juglone include most lawn grasses, black raspberries, pawpaw, elderberry, redbud, and many native wildflowers. When siting a black walnut, keep it well away from vegetable gardens and sensitive ornamentals. Even after a walnut is removed, juglone can persist in decomposing roots and organic matter for several years.
Planting & Care in Nashville
- Soil: Thrives in deep, moist, well-drained, fertile soils. Dislikes compacted, wet, or shallow soils. Nashville's deeper upland soils are well suited.
- Sun: Full sun is essential – at least 6 to 8 hours of direct sun daily. Black walnut will not thrive in shaded sites.
- Pruning timing: Prune in summer (July – August) or late fall after leaf drop. Avoid late winter / early spring – sap bleeds heavily when cut near bud break. All cuts should be clean and minimal.
- Nut litter management: In fall, the combination of large compound leaves and staining husked nuts can create significant cleanup work on driveways and in the lawn. Plan the planting location accordingly.
- Timber value: If you have a large, straight-trunked black walnut that must be removed, contact a local sawmill or lumber buyer first. The wood can be quite valuable – sometimes several hundred to several thousand dollars for an exceptional specimen.
Watch: Black Walnut Pruning & Care Tips
Frequently Asked Questions
What is juglone and which plants does it affect?
Juglone is a naturally occurring allelopathic chemical produced by black walnut trees in their roots, leaves, nut husks, and bark. It inhibits the growth of many susceptible plants within roughly 50 to 80 feet of the trunk. Sensitive plants include tomatoes, peppers, apples, blueberries, rhododendrons, and azaleas. Tolerant plants include most grasses, black raspberries, and pawpaw.
Are black walnuts edible?
Yes – black walnuts are edible and prized for their bold, earthy flavor. Wear gloves when handling the husks (they stain skin and clothing dark brown). The shells are extremely hard and require a specialized cracker or vise. The nutmeat is excellent in baked goods, ice cream, and candies.
How far should I keep a garden from a black walnut?
A safe buffer zone for sensitive vegetable and ornamental gardens is typically 50 to 80 feet from the trunk. Even decomposing walnut roots and organic matter can release juglone into the soil for several years after a tree is removed, so plan accordingly if you intend to plant a garden where a walnut once stood.
Is black walnut wood valuable?
Yes. Black walnut produces some of the most valuable hardwood lumber in North America – the dark chocolate-brown heartwood is prized for furniture, cabinetry, gunstocks, and veneers. Before removing a large, straight-trunked black walnut, contact a local sawmill or lumber buyer to determine whether the wood has timber value.
When is the best time to prune a black walnut?
Prune black walnut in summer (July – August) or in late fall after leaf drop. Avoid late winter and early spring pruning, as black walnut bleeds sap heavily when cut near bud break. While sap bleeding is not directly harmful to the tree, summer or dormant-season pruning is cleaner and easier to manage.
Black Walnut Work Needed?
Whether you need pruning, removal, or a health assessment for your black walnut, Smith Works Tree Service is ready to help Nashville and Middle Tennessee homeowners.