Of the six types of walnut trees that grow in the United States, four are in the Far West. One type, the butternut, has a wide distribution in the eastern U.S., but not as wide as that of Juglans nigra, the black walnut. This tree is one that the early settlers recognized as an extremely useful commodity, using the fine wood as well as the nuts, and exporting it as well. The features that the black walnut tree possesses make it an easy one to identify in its natural habitat.
The first thing someone might notice about a black walnut, especially when there are no nuts yet on the tree, is the bark. Besides being black, the trait that helps to give the tree its name, it is quite thick and furrowed deeply. Scraping the upper surface of the bark will expose another layer resembling chocolate in color. The ridges that develop on the bark crisscross to form a diamond-like pattern on the trunk.
The actual walnut grows on the tree by itself or in clusters of twos and threes, developing from the flowers during the summer into the fall. These nuts are round and have a greenish husk around them. This thick protective layer will eventually turn a yellow-black shade as the nut within it ripens. Once ripe, the entire fruit will have a brownish-black dye in and on it that gets all over anyone who decides to pick it from the tree or ground. The nut on the inside is brown and enclosed in a hard shell.
One of the quickest ways to narrow a tree down as being a walnut is to study the leaves. Walnuts have compound leaves, with an assortment of smaller leaflets arranged on one stem that botanists consider as one leaf. Walnuts have between 11 and 23 of these 3.5-inch-long leaflets arranged on the stem, and the whole compound leaf might be over one foot long. The upper side of the leaflet is a greenish-yellow, with the bottom side much paler; the leaves have a hairy feeling on the undersides.
At one time it was not an uncommon sight to see walnut trees as tall as 150 feet in the eastern forests, but nearly all the trees this large have fallen victim to the axe or the chain saw. However, a black walnut will still grow easily as tall as 70 to 100 feet, with a wide-spreading top. The trunk typically is as wide as two or three feet.
One attribute of the black walnut is that very few plants have the ability to grow underneath it. This is due to the presence of a toxic compound called juglone that exists in the root system and infiltrates the surrounding soil, making it difficult for other plant species to take hold and thrive.
A black walnut will respond to drought conditions in the summer months by prematurely dropping some of its leaves. The presence of the recognizable leaflets on the ground beneath a tree often gives away the presence of a black walnut.