As with most questions, I can offer you multiple answers and suggestions.
First, let me show you a little comparison that should peak your financial interest!
Example 1: 14" DBH black walnut tree with one 16-ft log contains approx. 50 board feet (Doyle Tree Scale). 50 board feet multiplied by $1.00/ft stumpage value = $50.
Example 2: 18" DBH black walnut tree with one 16-ft log contains approx. 100 board feet. 100 board feet multiplied by $1.00/ft stumpage value = $100.
Example 3: 24" DHB black walnut tree with one 16-ft log contains approx. 220 board feet. 220 board feet multiplied by $1.00/ft stumpage value = $220.
*** Black walnut can add 4-inches of diameter growth every 10-12 years on deep, fertile, well-drained soil***
Maturity, in regards to timber, comes in two forms: financial maturity and biological maturity. Financial maturity comes first in the life span of tree, followed by biological maturity second. As best explained by Mills and Calahan (1993), financial maturity is that point in the life of the tree beyond which the expected value increase no longer equals or exceeds the net return that would be obtained if the tree were sold and the cash value were invested elsewhere. In contrast, the biological maturity of a tree or a stand of trees occurs when the tree or stand achieves maximum merchantable volume. Financial maturity differs from biological maturity by imposing economic and business management constraints on the production process (Mills and Callahan, 1993).
Additional Tips: Grow your black walnut trees to a "minimum" of 24-26” DBH for veneer markets. The great thing about veneer quality trees is typically the price per board foot increases as the diameter and log length of the tree increases. Thus, a 22-inch DBH black walnut tree with one 8-foot veneer log may bring $510 (170 board feet x $3.00/ft), whereas a 26-inch DBH black walnut tree with one 8-foot veneer log may command $1300 (260 board feet x $5.00/ft). The moral of the story with black walnut – grow’em big, grow’em tall, grow’em straight, grow’em with minimal defect!
Most of the sawmill owners I have talked to, including veneer buyers, do not want logs smaller than 12-14" in diameter at the small end.
As always, I hope this forum helped you out!