The black walnut (Juglans nigra) is a hardwood tree native to North America. The tree is valued for nut production, timber production, ornamental use and wildlife habitat. It is has slow growth and long life. The tree's point of maturity depends on the planned use. Trees for nut production may mature sooner than trees planned for harvest for timber.
Early Nut Production
The earliest sign of maturity with a black walnut is the beginning of nut production. This is reproduction and an obvious sign the tree is beginning to mature. This commonly occurs at about 10 years of age. Nut production is slow at first, and the tree would still be too small to have commercial value for timber. At this stage, the tree is about 4 inches in diameter.
Optimum Nut Production
The best years for production start when the tree is about 30 years old. A 30-year-old tree may have a trunk about 14 inches in diameter. The tree produces an optimum crop about 40 percent of the years for the next several decades.
Growers prune black walnut trees planned for timber production to produce a single trunk that is as long and thick as possible. The harvest size varies depending on the intended use. A 15-inch diameter trunk size is considered minimal and takes about 35 years to mature under ideal conditions. The highest quality logs, about 20 inches in diameter, take about 50 years to achieve.
Because of the pruning requirements of the black walnuts planned for timber use, most growers plan their trees for one use or the other. Pruning all branches below 25 feet in height limits the amount of nuts the tree will set. Growers still harvest some nuts, but the production level is below a commercial level.