Black Walnut

Juglans nigra

Description & Overview

Black Walnut is a large deciduous tree native to Eastern North America that produces edible nuts in the fall. The nuts are a valuable food source for local wildlife. It is one of North America’s most valuable trees due to its high-quality timber. The main drawback for the Black Walnut is a growth-inhibiting chemical that is produced throughout the tree known as Juglone. In spite of this, this tree can make for an excellent shade or accent tree for larger properties.


Core Characteristics

Mature Height: 50-75′
Mature Spread: 50-75′
Growth Rate: Slow
Growth Form: Oval, rounded
Light Requirements: Full Sun
Site Requirements: Prefers organically rich, moist soils. Tolerates dry soils but grows more slowly.
Flower: 2-4” long catkins. Monoecious.
Bloom Period: April
Foliage: Dark Green, pinnately compound structure.
Fall Color: Yellow
Fruit Notes: Brownish-Green spherical fruit which ripens during summer/fall

Suggested Uses:

The Black Walnut has a broad canopy supported by strong branching which makes it an excellent shade tree. Its large-scale nature makes it a great tree for expansive back yards and parks. Its resilience makes it exceptional in restoration sites and exposed places. The nuts make great eating and are sought by foraging enthusiasts.

It is most well-known for its allelopathic properties. The entire tree produces a growth-inhibiting chemical known as Juglone, but its fruit holds the highest concentration. The Black Walnut is better suited to wide-open, natural areas rather than use in a residential landscape because of its allelopathic effects. This way their beauty can be appreciated, their nuts can provide food for wildlife, and will not compromise the longevity and health of desirable plants.

This is a large, long-lived tree that chemically attacks certain plants. Be sure to site it properly to ensure this tree and nearby companion plants live their best life.

Salable #5 RootMaker Container Black Walnut. Pictures taken mid-July.

Wildlife Value:

The Black Walnut produces a large crop of nuts on average once every 2-3 years. The nuts feed many birds such as turkey and nuthatches. The most notable consumer of the nut is the Eastern Fox Squirrel, 10% of their diet alone can be entirely Black Walnuts. In turn, animals like the Eastern Fox squirrel are a food source for larger predatory animals such as the Red Fox, Red-shouldered Hawk, and Black Bears.

The Walnut caterpillar specializes in the Black Walnut and relatives such as Butternut. This means the leaves of these trees are the only food source these caterpillars can eat. The leaves on both trees also provide food for over 100 other species of moths and butterflies including Luna Moths and Banded Hairstreaks. Since caterpillars are an important food source for birds, such as the Tufted Titmouse, not all caterpillars will see life as an adult butterfly or moth.

Maintenance Tips:

The Black Walnut is adaptable to many different soil types. It can grow in acidic, alkaline, sandy, and clay soils.

Pruning is best done during the dormant season. We invite you to check out the Arborist For Hire lookup at the Wisconsin Arborist Association website to find an ISA Certified Arborist near you.

Black Walnut produces a toxic substance called Juglone. Juglone prevents many plants from growing under or near the tree. Juglone is also produced by trees of the same family

Juglandaceae like Butternut (Juglans cinerea) and Shagbark Hickory (Carya ovata) but in lower concentrations than the Black Walnut. Juglone is present in all parts of the Black Walnut, but it is most prominent in the buds, nut hulls, and roots. The toxic effects of the mature tree can extend 50 to 80 feet from the trunk of the tree. The greatest toxicity occurs within the tree’s dripline. Susceptible plants will wilt and die, these include many vegetables such tomatoes, peppers, and eggplant as well as many ornamental shrubs like lilac, peony, and rhododendron. Notably, many natives are relatively unaffected by this chemical, especially prairie grasses, perennials, and many tougher shrubs.

You can read more about Black Walnut Toxicity as well as view a list of resistant plants.

Pests/Problems:

The Black Walnut has very few major insect enemies. This tree feeds many different insects but few ever become pests that need management. The most troubling insect is the walnut caterpillar. Trees defoliated by heavy infestations may suffer the loss of growth but are rarely killed. The disease is not a major concern when planted in a proper site. A happy, stress-free tree invites few problems.

Thousand cankers disease is a disease that primarily affects Black Walnut trees. The disease is the combined result of the combined activity of a fungus (Geosmithia morbida) and the walnut twig beetle. As the insect bores into the tree, the beetles carry spores of the fungus causing small cankers to form. Over time, the repeated activity disrupts the movement of water and nutrients throughout the tree. The disease is usually lethal within a few years. There is currently no known cure. Thankfully, this disease is currently only present in the Western United States and has not made its way into the Midwest.

Leaf Lore:

Walnut shell blasting is a popular method of cleaning as it can clean surfaces without damaging the underlying original exterior. It is a process where crushed walnut shells are used as an effective medium for blasting away unwanted materials. The automobile industry uses the ground shells of Black Walnut as a nonslip agent in tires, and in products used to deburr precision gears. The airline industry also uses ground shell products to clean jet engines, steam turbines, and mold cleaning.

Black Walnut timber is coveted for its straight, dark, heavy, strong, fine-grained heartwood, which is used to make fine furniture, valuable gunstocks, flooring, oars, and coffins. The easily worked, close-grained wood has been long prized by furniture and cabinetmakers for its attractive color and exceptional durability.

As of 2016, the largest Black Walnut in the United States is on a residential property on Sauvie Island, Oregon. It stands at 112-feet tall, with a crown spread of 144-feet. The diameter of the trunk at breast height is an astounding 8-feet and 7-inches.

Companion Plants:

While there are many plants that are susceptible to Black Walnut Toxicity, there are many that have been observed to thrive under or near one. Companion plants for Black Walnut must be tolerant of Juglone. Trees and shrubs unaffected by Juglone include Maples, Birches, Oaks, Elms, Musclewood, Witchhazel, Elderberry, Sumac, and Ninebark. Many notable perennials include Aster, Coneflowers, Ferns, Daylily, most grasses, Phlox, Sedum, and Lobelia.

You can refer to our Plant Reference Guide for a complete list of Plants observed that are both resistant and susceptible to Black Walnut Toxicity.




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