The Hidden Secret of the Black Walnut

February 17, 2023

The black walnut tree is one of North America’s most valuable and beautiful native trees. When planted in the open, the tree will branch out closer to the ground, making it easier to harvest its 2–3-inch nuts.

On larger properties, black walnuts make great shade trees, they can grow to 50 feet or taller. The bark will darken and develop intersecting diamond shaped ridges. The black walnut tree leaves are pinnate feather-like, there are usually two to nine pairs of leaflets on each blade and one more leaf at the tip, the leaves are very large, up to 24 inches long. Black walnut trees are the last to leaf out in the spring and the first to drop them in the fall. The walnuts will drop shortly after the leaves in September or October. The green walnuts typically grow to the size of a baseball and can weigh between 11-12 grams. They can make a mess in your yard while they rot and mold. If you want to crack the nuts, the meat on the inside is edible.

Uses for Black Walnut

The European settlers found the tree’s natural resistance to decay made the wood perfect for use as poles, shingles, fence posts, and sills. The settlers used the nut inside as snacks, added nuts to soups, and ground the nuts into meal for baking. The hard shells were perfect for storing the walnuts over the winter.

The sweet nutmeat inside is so worth the effort. When I was growing up, my grandmother would collect the black walnut meat and take it to a lady who would make a black walnut cake with cream cheese icing. It was delicious! There are so many different ways to cook with the walnut meat. Some people harvest the walnuts to be eaten raw or added to cookies and bars. They can also be added to ice cream as a topping.

And of course, black walnut tree offers food and shelter for wildlife.

Squirrels munch on the walnuts.
The Luna moth uses the tree as a host, they will feed on the foliage and oviposits on the leaves.
Eastern screech owls will roost on it.

The Hidden Secret of the Black Walnut Tree

Although black walnuts have many uses and benefits, you’d be surprised to find out that it can be deadly to other plants and cause skin rashes, eye irritation, and respiratory problems in humans. Nearly every part of the black walnut exudes a natural herbicide known as juglone. The highest concentration of juglone is in the buds, nut hulls, and roots of the tree. The concentration is lower in the leaves, twigs, and stem, but these parts of the tree can still have an effect on nearby plants and people. When the leaves and stems fall to the ground and decompose, they also release juglone into the ground. The roots, which can extend 50 feet or more from the tree, will still release juglone into the surrounding soil even after the tree has been removed.

Many woodworkers and people in the tree service industry talk about getting a rash or having respiratory problems after working with black walnut. The particles of sawdust that are released during pruning or removals can cause an allergic reaction in people who are sensitive (similar to poison ivy).

Certain plants, if grown too close to the tree’s roots or under the canopy, may die or experience stunted growth. Affected plants include potatoes, tomatoes, peppers, and berries. Bushes such as rhododendrons, azaleas, and lilacs. Trees affected are alder, apple, crabapple, pear, pine, spruce, silver maple, and birch. One year we moved our garden, and our tomatoes and peppers did awful, the plants did not grow, wilted, and we had no tomatoes or peppers to harvest. Later we found out from researching that the cause of the issues was the black walnut tree just 15 feet away.

Did you know? – other nut trees such as butternut, pecan, shagbark hickory, and English walnut also release juglone but in much smaller amounts, it doesn’t have any effect on nearby plants.

References:

Black Walnut Trees: Facts About the Infamous Black Walnut | The Old Farmer's Almanac

Black Walnut Tree: How to Identify and Remove It (thespruce.com)

black walnut pictures - Canva images

History of Black Walnut (herballegacy.com)

Habits and Traits of Luna Moth, Actias luna (thoughtco.com)

Two green pine trees are silhouetted against a white background.

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