Types of Walnut Trees image by JoJann:commons.wikimedia.org. The walnut tree genus, known botanically as juglans, contains at least 17 species and hybrid cultivars of walnut trees. Walnut trees are a fine hardwood trees related to the hickory genus and are grown for their nuts and timber wood. All species fall into one of three types of walnut tree: those grown for their fruit or nuts, those grown for timber, and those grown for both their fruit and timber.