Southern catalpa trees (Catalpa bignonioides), also called "catawba" trees, are impressive but short-lived trees that grow to a height and spread of 40 to 50 feet. The objective when pruning this grand hardwood tree is to keep it small and well-shaped to prevent splits. The seed pods of the tree and long branches place a heavy load on its broad crown, often necessitating bracing with metal rods in mature trees or wooden poles in young specimens.
Start a pruning program when Southern catalpas are young. Your objective should be to limit spread and height. Prune after catalpas have flowered in May to slow tree growth. Remove branches that grow close to each other on the trunk, or "leader"; they will completely grow together, forming one weak branch. Raise the crown by taking branches off the leader as the upper part or "crown" of the tree grows.
Remove branches over 2 inches in diameter by sawing up from underneath about 5 inches out on the branch. Then make a downward cut parallel to the leader just outside the "branch collar," the thickened bark where the branch joins the tree. Never make a "flush" cut that cuts into this collar; it is the tissue that will grow over the edges of the cut, protecting the branch from infections. Remove any branches that meet at less than a 35-degree angle in a sharp "Y" shape; they will not develop a healthy collar and are more likely to split in storms or high winds.
Choose a leader, and shorten any competing branches; this is called "subordination." Late winter is a good time to scout potential candidates for subordination pruning and "crown reduction." Reduce crown density by removing branches that cross each other and remove "water sprouts"--vertical shoots rising from lower branches.
Remove branches where they have become congested so that light and air can filter through the crown of the tree. Remove any branch that has grown to over half the diameter of the leader at the same height; it is too heavy and may threaten the structure of the tree.
Remove dead or diseased branches anytime, and cut suckers when they appear in the spring. Although catalpa's growth habit is to spread out, it will need some shaping to avoid becoming irregular. Never trim more than a quarter of the length of a branch. Take branches that are growing downward rather than upward-growing branches. Prune "up" lower branches on a mature tree from 5 to 8 feet to raise the crown and provide clearance for movement underneath the tree.