By far the most common walnut species in North America is the Black Walnut (Juglans nigra), which is native to most of the Eastern half of the United States and Canada, from Quebec to Florida and the Atlantic coast to the eastern edge of the Great Plains.

The Butternut (Juglans cinerea) is another of the most common native walnuts. It is even hardier than the Black Walnut and some people prefer its flavor.

Other native walnuts include:

• Southern California walnut (Juglans californica)
• Northern California walnut (Juglans hindsii)
• Arizona walnut (Juglans major)
• Little walnut (Juglans microcarpa)

Black walnuts make attractive but somewhat troublesome additions to the woodlot or home landscape. The leaves, roots, husks, and woody parts of the tree contain a toxin called juglone that is harmful to many other plant species, including many common garden plants. The toxicity may spread well beyond the tree’s drip-line and last in the soil for years after the tree dies or is removed. In nature, the toxicity helps protect the tree’s “territory” and ensure that it gets enough sunlight. In the garden or woodlot, it can be a real bother. The Michigan State University Department of Horticulture offers a substantial list of plants that can and cannot be grown near walnuts.

Horse and dog owners should also be wary of planting black walnuts. Dogs become ill after eating the nut husks (clean up debris regularly to prevent this) and horses can develop respiratory problems from black walnut pollen and leaves, and laminitis if bedded down in wood shavings that contain 20% or more black walnut.

However, large pigs will eat walnuts eagerly with no ill effects, as will smaller pigs and poultry if the nuts are broken open for them.

Juglans regia, variously known as the English, Persian, or Common Walnut, is a non-native walnut that can be successfully grown in some regions of North America. It is hardy to zone 5 and produces juglone in much smaller quantities than Black Walnuts.