The Four Inch Rule:
The 4-inch rule to maximize root branching in containers should be a consideration at every transplant. Anyone that has ever pruned shrubs or trees to make a hedge has experienced the "4-inch rule". It has long been known that when a twig or branch is cut in the pruning process branching occurs, but did you notice where? Typically from the point of the pruning cut and extending back about 4 inches. Allow the numerous branches produced as a result of the first pruning to grow out 4 to 6 inches then prune again and more branching occurs. Branching occurs as a result of the removal of the terminal bud, and the hormone that suppresses branching (also known as apical dominance). What has been known for only a few years is the fact that the four inch rule applies to roots as well. This is especially dramatic with young roots, which are the below ground counterparts to the twigs pruned to create a hedge. As roots grow older they become less responsive to pruning just as occurs with larger limbs and branches. Root tips exert an apical dominance just like twigs. With roots, the white tip is most responsive, and when air root pruning , or copper pruning, occurs at the proper location, secondary roots typically begin to forum quickly and within 3 to 5 days the 4-inch rule is obvious. By utilizing the 4-inch rule, root branching is maximized through the growth medium, which in turn stimulates top growth and improves the tree's quality. Combine the 4-inch rule with alert and timely transplanting, plant growth and quality of both tops and roots take a giant step forware. - Millet