The black walnut (Juglans nigra) is one of about 15 species in the walnut genus. A member of the Juglandaceae family, it is related to the pecan and wingnut trees. Black walnut is native to North America and can be found in the wild from southern Ontario to Florida and as far west as Texas. It is valued for its edible nuts, but even more prized for its wood, which is in high demand for furniture veneers. Breeders and growers have a continued interest in increasing production by stimulating and optimizing growth.
The first consideration in optimizing growth is good site selection. Black walnuts have extremely deep taproots, so soil should be deep, high in organic matter and pH neutral. The trees also cannot tolerate standing water, so swampy sites and flood plains are undesirable planting locations. A north- or east-facing slope favors good growth, as does avoidance of ridgetops. Though trees can be planted in woodland openings, fields or areas already in cultivation make the best locations for black walnut plantations.
Other species are sometimes interplanted among the young walnut trees to provide certain benefits that enhance growth. Several varieties of trees fix beneficial nitrogen in the soil by way of their root nodules. The best of these species is European black alder (Alnus glutinosa), which grows alongside and protects the young walnuts. Juglone, a toxin produced by the walnuts, eventually kills the alders at about the 15-year point, a time when the increased size of the walnuts would have made it necessary to thin the alders anyway.
Weed and Predator Control
Weed control eliminates competing plant species, thereby stimulating growth. This control can be done by manual means, by application of herbicides like glycophosphate (Round-Up) or by the use of mulches under the trees. Browsing by predatory animals, especially deer, can destroy trees. To keep animals at bay, some growers use tree shelters, plastic tubes that are placed around the young trees and supported with stakes. These can be raised by 6 inches during the fall and winter months to provide necessary ventilation while continuing to discourage animals.
Pruning helps ensure healthy growth and also can optimize the value of black walnut timber. The trees have a natural tendency towards straight, unbranched trunks for as much as half their mature height. When they are also free of knots, those trunks are the most valuable for timber purposes. Pruning any side branches that sprout from young trees helps promote tall, straight growth. Experts, like Professor Walter F. Beineke of Purdue University's Cooperative Extension Service, warn against overpruning trees, which can diminish leaf surface area available for photosynthesis.
Black walnut trees planted in good soil need little fertilization and none in the first year of growth. Nitrogen, in the form of urea, should be added on a regular schedule after the trees are three years old. Acidic soil can be neutralized by the addition of agricultural lime. The best way to determine soil pH is to use a soil test kit, available through nurseries and garden centers.