Juglans regia has been cultivated for many centuries and grows to become a large tree approx. 15m tall. The almost round crown consists of heavy branches. The old trunk is light grey with deep, irregular vertical grooves. The imparipinnate leaves grow to a length of 20 - 30 cm and have 2 - 4 (6) opposing pairs of leaflets. The leaf segments are ovoid to oval, approx. 8 - 12 cm. After the appearance of the leaves the large male catkins (approx. 7 cm) and the smaller female flowers appear, the latter in groups of 2 or 3. The green, almost round fruit has a smooth husk that has short gland-like hairs at the beginning. The light brown nut is grooved. Very tasty. The nuts do not appear until the tree is approx. 15 years old though. Has deep roots and much prefers open soil that the roots can penetrate. The tree is an important producer of wood.
Walnut trees grow best in rich, deep soil with full sun and long summers. Other plants often will not grow under walnut trees because the fallen leaves and husks contain juglone, a chemical which acts as a natural herbicide. Horses that eat walnut leaves may develop laminitis, a hoof ailment. Mature trees may reach 50 feet (15 m) in height and width, and live more than 200 years, developing massive trunks more than 8 feet (2.4 m) thick.
In Skopelos, a Greek island in the Aegean Sea, local legend suggests whoever plants a walnut tree will die as soon as the tree can "see" the sea. Most planting is done by field rats. In Flanders, a folk saying states: "By the time the tree is big, the planter surely will be dead." (Dutch: Boompje groot, plantertje dood). These sayings refer to the relatively slow growth rate and late fruiting of the tree.
Benevento in southern Italy is the home of an ancient tradition of stregoneria. The witches of Benevento were reputed to come from all over Italy to gather for their sabbats under the sacred walnut tree of Benevento. This legend inspired many cultural works, including the 1812 ballet Il Noce di Benevento (the walnut tree of Benevento) by Salvatore Viganò and Franz Xaver Süssmayr, a theme from which was adapted into a violin piece called Le Streghe by Niccolò Paganini. The Beneventan liqueur Strega depicts on its label the famous walnut tree with the witches dancing under it.