Black walnut (Juglans nigra) is a native hardwood tree that is often planted in large-scale landscapes and valued as a lumber tree. It 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; many trees, vines, shrubs, ground covers, annuals, and perennials can grow in close proximity to a black walnut tree.
Which Parts of the Black Walnut Tree are Toxic?
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 stem, 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.
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, which allows you to grow sensitive plants such as tomatoes in raised beds.
Plants sensitive to juglone exhibit wilting, yellow leaves, and stunted or slow growth. 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.
Nurseries often sell English walnut scions that have been grafted onto black walnut rootstock because black walnuts have a better resistance to pests and diseases than English (Persian) walnut trees. If you plant such a tree in your yard, be aware that it will have the same effect on juglone-sensitive plants as a non-grafted black walnut tree.
How to Identify Black Walnut
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 appear 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 are 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
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 Black Walnut is Beneficial for Wildlife
As a native tree, black walnuts provide food and shelter for wildlife. The primary beneficiary 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.
Where Does Black Walnut Grow?
Black walnut is a hardy, tough tree that can grow in many locations. In fact, it is often used in the rehabilitation and reforesting of disturbed sites such as former mined areas. When a walnut seedling pops up in the middle of your flower beds, it was most likely brought there by squirrels ithe previous fall. Squirrels like to bury the nuts in multiple locations to come back to in winter. Under the right conditions the nuts easily sprout after going through a 3- to 4-month cold stratification period.
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. It's possible that another black walnut will 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 snag abundance for a wildlife habitat.
“Black Walnut Toxicity.” Mortonarb.org, 25 Apr. 2021