Walnut trees prefer rich moist soil, but squirrels will take walnuts from the tree and then hide them, meaning you can encounter one anywhere in the woodlands within the tree's geographic range. The walnut species with the widest range is the black walnut of the eastern United States, but very few large ones exist in the wild today. The wood of the black walnut is among the finest grained of all trees, meaning the big walnuts of the colonial forests are long gone. You can learn to identify a walnut tree, looking its leaves, fruit and other features.
Look at the shape and size of a tree to determine whether it is a black walnut. Do this from a distance if possible to get a better comparison to surrounding trees. The younger trees have what the Ohio Department of Natural Resources website calls an "irregular" appearance when young, with several dividing branches partway up the trunk. When the walnut matures, its crown has a rounded shape. The mature black walnut may reach heights of 70 feet and be just as wide in its canopy of branches.
Examine the leaves of the tree closely. All the walnut trees growing in the United States have compound leaves, with small leaflets growing on a long stem-like "rachis." The black walnut has between 11 and 23 of these leaflets arranged on one stalk, which itself may measure 2 feet in length. The leaflets are green but they may turn yellow in autumn.
Inspect the unique fruit that hangs from, or is on the ground below, the black walnut tree. Walnuts develop from the tree's flowers and have a kernel encompassed by two very hard brownish shells that seem fused together. These possess a covering of a greenish husk for protection. The ripening walnuts will have their outer husk change to a yellow-black color. If you want to verify a tree is a walnut, put on a pair of gloves and pick one of the ripened nuts. The gloves will protect your hands from a brown-black dye that seeps from the husk, one that can stain your bare hands for a long time if you are not careful.
Scrutinize the area right underneath a black walnut tree. Walnut trees produce a compound called juglone that acts as a natural herbicide. Many species of plants will not grow beneath a walnut because the juglone gets into the soil that surrounds the tree, preventing plants from growing. Often you will notice bare ground beneath a black walnut tree for this reason.
Observe the nuts of the butternut tree to help you recognize this kind of walnut tree. This type of walnut, also known as white walnut, does not grow as tall as a black walnut. The nuts have a rust-colored shell covered with sticky hairs. They have the shape of an egg, with one end tapered to a point. The husks contain a hard shell in which there exists an oily nut that gives the trees its name.