Avoid pruning during the fall or early winter because, like fertilization, pruning in fall encourages tender new growth that may not be sufficiently hardened to resist the winter cold. Some shade and flowering trees tend to bleed or excrete large amounts of sap from pruning wounds. Among these trees are maple, birch, dogwood, beech, elm, willow, flowering plum and flowering cherry. Sap excreted from the tree is not harmful, but it is unsightly. To minimize bleeding, prune these trees after the leaves have matured.
Leaves use plant sap when they expand, and the tree excretes less sap from the wound.
To understand why one pruning technique is preferred over another for a particular plant and why cuts are made the way they are, it helps to understand a basic physiological principle of pruning. As long as the terminal bud is intact, auxin suppresses the growth of lateral buds and shoots behind the terminal. However, when you remove the terminal bud by pruning, lateral buds and shoots below the pruning cut grow vigorously. The most vigorous new growth always occurs within 6 to 8 inches of the pruning cut. If it is necessary, for instance, to prune beneath power lines or to clear a tree from interfering with a structure, always prune back to a fork where there is a live branch that is at least half the diameter of the limb being removed.