Juglans nigra Black walnut hulls come from the immature fruit of the black walnut tree (Juglans nigra). References to the use of black walnut hulls date back to the ancient Greeks and Romans, and they have been used by many cultures throughout history as an herbal tonic. The black walnut tree is indigenous to the Middle East, but is currently grown in the U.S., Canada and Europe as well. Natural color variation may occur in this product.