Black Walnut Tree: Benefits, Problems, and Identification

Black walnut (Juglans nigra) is a native hardwood tree that is often planted in large-scale landscapes and valued for its attractive woodworking lumber, edible nuts, and large size. The black walnut tree often causes problems for home gardeners because the tree is allelopathic—it produces juglone, a chemical that inhibits the growth of other plant species in its vicinity. 

Not all plants are sensitive to juglone, including dogwood, juniper, clematis, and daffodils. Many other trees, vines, shrubs, ground covers, annuals, and perennials can grow in close proximity to a black walnut tree.

Black Walnut Tree Quick Facts

• Black walnut trees grow in temperature-varying regions, including USDA hardiness zones 4 to 9. The tree manages well in cold or hot weather.
• Black walnut trees are sensitive to soil conditions and they do best in fertile, well-drained, neutral soil that's kept moist.
• These trees do not tolerate shade well and thrive in full sun conditions.
• Black walnut trees can grow 50 to 120 feet tall.
• The trees produce edible nuts that can be harvested by collecting them fresh from the tree, removing the inner nut from the fruity shell, and drying them for a few weeks before eating. 

Black Walnut Benefits

The black walnut tree is a beloved tree for many reasons. Its large, mature size and canopy make it a desirable shade tree. The tree is also prized for its beautiful, highly desired wood used for woodworking projects, such as furniture and cabinetmaking.

As a native tree, black walnuts provide food and shelter for wildlife. The primary beneficiaries of the nuts are squirrels. The eastern fox squirrel gets 10 percent of its food from black walnuts.

Black walnut trees are the preferred host of the luna moth, an elusive large moth, and the regal moth, which are both native to North America. The eastern screech owl roosts on the limbs of black walnut trees. 

Black Walnut Problems

All parts of the walnut tree naturally produce juglone, with the highest concentration in the buds, nut hulls, and roots. The concentration is lower in the leaves, twigs, and stems, but when the leaves and stems drop and decompose, they also release juglone into the soil. Even after a black walnut tree has been cut down, its remaining roots can still release juglone. 

The highest concentration of juglone is in the soil below the tree’s canopy, but juglone is present in the entire root zone of the tree, which in large black walnut trees can stretch 50 to 60 feet away from the trunk. 

Plants That Won't Grow Near Black Walnut

Vegetables that cannot grow in the vicinity of a black walnut tree include tomatoes, peppers, potatoes, eggplants, cabbage, rhubarb, and asparagus. Flowers sensitive to juglone include peonies, petunias, and chrysanthemums. Affected shrubs include azalea, hydrangea, lilac, and yew. Trees sensitive to juglone are alder, apple and crabapple trees, pine, spruce, silver maple, and birch.

Juglone is not water-soluble so it does not move within the soil. This will allow you to grow sensitive plants such as tomatoes in raised beds. 

Plants sensitive to juglone exhibit wilting, yellow leaves, and stunted or slow growth, which can be confused with a watering issue. The affected plants cannot be treated and usually die within a few months. Because these symptoms can also have other causes such as plant disease or lack of nutrients, a black walnut nearby is not always recognized as the culprit. That’s why it’s important to know how to identify a black walnut tree. 

Other nut trees such as butternut, pecan, shagbark hickory, and English walnut also release juglone but in much smaller concentrations which do not have any effect on nearby plants.

Black Walnut Tree Identification

Black walnut is a deciduous tree that grows 50 to 120 feet tall, with an average height of about 80 feet. The trunk appears especially long because the first branches start at a high level. The bark of young trees is grayish and scaly and over time darkens and develops intersecting diamond-shaped ridges. The dense crown is rounded. 

Black walnut leaves are very large, up to 24 inches long. What appears to be individual leaves are actually single leaflets. Black walnut trees are one of the last trees to leaf out in the spring and one of the first to drop their leaves in the fall. The fall color is yellow to brown. 

The other telltale sign of a black walnut tree is its nuts, which drop off the tree shortly after the leaves in September or October. The yellow-green husk turns black after dropping. 

Plants That Look Similar to Black Walnut

Two trees that can be confused with black walnut because they often grow in the same area are the highly invasive tree of heaven and the native staghorn sumac. 

The three features that distinguish the tree of heaven from a black walnut tree are its off-putting smell, its cantaloupe-like bark, and its non-serrated leaves. Staghorn sumac has fuzzy twigs, leaves that turn bright red in the fall, and eye-catching red flower clusters. Neither tree will grow as tall as a mature black walnut.

How to Get Rid of Black Walnut

Small black walnut seedlings and saplings can be removed with their entire roots by simply pulling them. It might be necessary to loosen the surrounding soil with a shovel to get the entire taproot out.

To get rid of a larger tree, you can cut it down. Another black walnut may sprout from the stump, but it's unlikely. You can coat the freshly cut stump with a stump killer which you can find at your local garden center. Most stump killer products contain potassium nitrate as the active ingredient. It contains a high amount of nitrogen, which speeds up the decomposition of the stump. Another option is to treat the stump with glyphosate, a non-selective herbicide. If you are reluctant to use any of those chemicals, the most low-impact solution is to continue removing the new sprouts.

Alternatively, you can hire a certified arborist to grind the stump out mechanically or remove it.

Girdling the tree is not a great option. That method is best left to be used on trees in dense forests to increase their use as wildlife habitats.