The walnut tree is one of the largest in the Juglandaceae family of trees and shrubs. It is also one of the longest-living. One walnut tree in Melbury Park, Dorsetshire, United Kingdom, is documented at about 200 years old.
Walnut trees are fast growers. They can reach a height of 20 feet in about 10 years, which is also the age when they start to produce fruit. A full-grown tree will reach a height of 60 to 70 feet. A walnut tree's trunk measures about five feet in diameter or wider, and the branches spread out as far as 40 feet. Walnut trees have a thick, uneven bark on the trunk, but on the young branches, bark is smooth and gray. The leaves are composed of five to11 small leaflets. They grow opposite each other on the stem, with the last one on the tip.
Walnut trees can grow in just about any type of soil, as long as it is well-drained. The trees should be planted away from other, shorter plants. The wide spread of the walnut's branches makes it hard for anything to grow under them. Black walnut trees secrete a substance called juglone, which is toxic to many plants. Walnut trees should be planted 60 to 70 feet apart.
Walnut trees should be watered right after they are planted. They need approximately a half gallon of water a week for the first year, and one to two gallons a week during dry spells. Mulching keeps the soil moist and stops weeds from growing. Small branches can be pruned off at any time. Heavy pruning needs to be done in the late winter or early spring.
Walnuts are harvested by hitting the ends of the branches with a long pole. This not only knocks off the walnuts, but the tips of the branches as well. When the tips are knocked off, it encourages the growth of new spurs which will, in turn, produce new flowers. The wood of the tree is also harvested to make furniture.
Young walnut trees can be damaged by spring frosts. The wood of young trees is soft and can also be damaged by worms. Walnut trees are susceptible to diseases and infestation by insects. Diseases include walnut blight, walnut leaf blotch, honey fungus, blackline, crown gall, downy spot, nectria and botrytis. Insects include walnut leaf gall mite, codling moth, red spider mite, walnut weevil and husk flies.