Juglans nirga, the black walnut tree, is native to North America over much of the United States' eastern half. It is rare north of New York City but grows southward into Georgia and as far west as the Great Plains. One of six types of walnut tree in the country, the black walnut has many features that make it among the most useful of trees. It is a picturesque tree which many landowners are proud to have on their property.
The spreading crown of a full-grown black walnut gives it much of its appeal as an ornamental species. The tree can grow to be 70 feet tall and just as wide at the top. The compound leaves, composed of a series of leaflets on a long-stemmed rachis that attaches to the branch, are as long as 2 feet. The individual leaflets can number between 15 and 23 on each rachis; these are smooth on their top sides but hairy on their bottom sides. The bark of the black walnut is very dark brown and sometimes nearly black, with furrows and ridges in mature specimens.
Walnut wood has properties that have made it a most sought-after commodity, as it neither swells nor shrinks as much as other woods. This makes it valuable for such purposes as gunstocks and cabinets in addition to other uses such as veneer. Black walnut has excellent grain and is an easy wood for artisans to work with, but has great durability. The wood has a brownish color and over the years has shown up as logs and flooring in countless homes.
Handling a black walnut fruit will have the effect of staining your hands if you fail to prepare and wear gloves. The husks of the walnut are round and light green before they ripen, at which time they turn a yellow-green and leak a brown dye that is difficult at best to remove from the hands. Inside this husk is a hard two-halved shell that is so strong that pulverized pieces of it can scour clean jet engine parts. The husk contains the prized walnut that cooks use in various recipes.
Juglone is a toxic substance produced by the root system of a black walnut tree. It exudes from the roots, where it has the highest concentration, into the nearby soil around the tree. Juglone has the ability to kill many types of plants, and its effects range outward from the tree as it grows. Plants such as tomatoes, blueberries, blackberries, azaleas, apple trees, rhododendrons and potatoes cannot survive when juglone is present in the soil. Horse owners should never use walnut trees for sawdust or wood chip bedding as the juglone can cause reactions in the animal.
Black walnuts are monoecious when it comes to their flowers, meaning that both female and male flowers will grow on the same tree. The male flowers are catkins and they hang down from where they develop in the same spot year after year. The female flowers are smaller and emerge in different places on the tree as it grows over time. These flowers will emerge in the middle to latter part of spring.