Black walnut trees are found throughout the eastern and central United States and into southern Ontario. Naturally occurring trees prefer deep, rich bottomland soils, mostly along forest edges. They are frequently found in pastures and along roadsides, where there is plenty of light. It is a tree capable of reaching heights of more than 100 feet, with some specimens reported to have reached 150 feet tall and 8 feet in diameter. Trees of that size are extremely rare now, however.
The bark is black and deeply grooved, and the top branches spread out to form a wide crown. The leaf of the black walnut is pinnately compound, which means it is made up of numerous leaflets on a central leaf stem (petiole). The number of leaflets per leaf varies, usually numbering around 15 or 20. The leaflets are yellowish-green during the growing season, changing to a dull yellow in autumn. Separate male and female flowers occur on the same tree. They are small, with no petals, hanging in long strings called catkins (like the flowers of oak trees). One distinctive identification trait found on black walnuts and their close relative, the butternut (Juglans cinerea), is the pith. When a twig is cut and the interior material examined, it will appear chambered, like a honeycomb.
The fruit is a large nut, around 2 inches in diameter and surrounded by a thick green husk that turns dark brown to black after the nut falls from the tree. The outer part of the nut (shell) is very hard, black in color with fine ridges. Nuts grow singly or in clusters of two or three. The meat of the nut is high in oil and very nutritious; it’s a favorite food of squirrels and deer, not to mention people. Walnut trees begin producing nuts at around 10 years of age and continue to produce their entire lives (up to 200 years).
Walnut uses
The black walnut tree has been prized for more than 300 years for its many excellent qualities. The heartwood (the wood closest to the center of the tree) of a mature black walnut is dark brown with a straight grain. It’s an easy wood to work and extremely durable. During colonial times, when large trees were still abundant, solid black walnut wood was used to make all manner of furniture.
By the mid-1800s, most of the really large trees were gone, but walnut continued to be used to make everything from fence posts and railroad ties (because it didn’t rot easily) to gunstocks. Today, black walnut is still prized for use in gunstocks, because it doesn’t warp or splinter and absorbs recoil better than any other wood.
Another major use of black walnut is as a veneer. Veneer is made by slicing the wood to create very thin (about 1/28th of an inch thick) sheets and gluing the sheets to paneling and cabinets. Trees used for veneer are typically at least 16 inches in diameter and preferably larger. The best trees for lumber grow in natural, woodland settings, where the trunk tends to grow straighter with a smaller crown of spreading branches at the top. These trees produce better and larger logs, as well as the darker heartwood prized for furniture.
Black walnut tree value
There are numerous “urban myths” associated with the value of black walnut trees, especially those growing in residential environments. Claims have been heard of a single tree bringing thousands of dollars. As with any tree species, the value is based on many factors, such as current demand, supply and the quality of the wood. If you have black walnut trees you are thinking about selling, first check with a consulting forester or your state forestry agency and determine the value of the trees in question.
In addition to the tree’s valuable lumber, the walnuts themselves are extremely valuable. While most commercially sold nuts (Persian walnuts, pecans, macadamia) are grown in orchards, a substantial portion of the black walnut harvest comes from naturally growing stands of trees. The best nut trees grow in open areas where they develop a widespread crown of branches. While the annual harvest of black walnuts is small compared with Persian (sometimes called English) walnuts, a demand for them always exists.
The nut meat is used for baking and in ice cream and walnut stain is derived from the husks of ripened walnuts. During World War I, not only were gunstocks made from walnut wood, the woody hulls of walnuts were ground up to make activated charcoal for use in gas masks. Walnut hulls have also been used as abrasives to clean jet engines and to assist in drilling oil wells. Native Americans ground up the green husks of black walnuts to use as a poison (that didn’t affect people) to catch large numbers of fish. Extracts from black walnut hulls are used to treat a variety of internal and external parasites in both humans and animals.
The problem with walnuts
For all the wonderful products we derive from the black walnut tree, it is not without its drawbacks. Walnut and butternut trees get part of their scientific name, Juglans, from a chemical they produce called “juglone.” Juglone is an allelopathic chemical, meaning it inhibits the growth of other plants in the vicinity of the tree, reducing competition for water and nutrients. While not all plants are sensitive to juglone, tomatoes, potatoes, hydrangeas, blueberries and apple trees are very sensitive.
Horses seem to be sensitive to black walnuts as well, often developing respiratory problems when exposed to pollen and fallen leaves of this tree. Bedding material that contains black walnut shavings has also been shown to cause inflammation of the hooves in horses. Dogs have also gotten sick from eating the seeds and chewing on the hulls of black walnuts.
Of course, many folks with black walnut trees close to their homes complain about the large nuts falling in the fall. To them it represents one more hassle in maintaining their yard. Black walnuts are also difficult to crack open, owing to their thick hulls, and most people don’t want to bother with them.
Walnuts for the future
While black walnut trees are not the major source of furniture and other products that they were two centuries ago, demand for this remarkable tree and its many products remains high. Having black walnut trees on your land or planting a large stand of them could potentially produce a steady source of income for years to come. As with most hardwood species, black walnut trees take many years to mature and may not be ready for harvest in the landowner’s lifetime. They may, however, represent a significant legacy for children or grandchildren, while providing a variety of other marketable products for landowners.
John Marshall teaches environmental and safety technology and biology at Pulaski Technical College in North Little Rock, Arkansas. He grew up on a small farm in northern Alabama and has never lost his love of the land. John, his wife, three children, a dog, a cat and a parakeet live in the small town of Benton.