Pruning is done for a variety of reasons. Most pruning is aimed at improving structure and safety, or for controlling size. Other objectives can be to open vistas, repair storm damage, and provide clearance for structures and traffic. Good pruning includes removal of diseased, dying, or dead branches, crossing or rubbing branches, branches with thorns below eye level, sucker growth from rootstock, water sprouts from limbs, and other objectionable growth.
Though mature trees may need only occasional pruning to remove dead wood, young trees should be pruned regularly to provide strong branch structure for future growth. Select scaffold (central structural) branches that are at least 18” apart and evenly distributed around the trunk. Branches should not grow directly above one another. Maintain a single leader as long as possible.
Pruning cuts should always be made near the base of a branch (Figure 1). Do not leave a stub. When heading back (drop-crotching) a tree to reduce its size, always cut the leader back to a lateral branch large enough to assume the terminal role. The lateral branch will eventually become the new leader. Indiscriminately sawing-off large branches (topping) is not acceptable. The new branches produced on the stubs are very weak.
Large branches are removed with a 3 step process (Figure 2). First, cut partially (no more than 1/3) through the branch from the underside. The second cut is made through the branch from the upperside a short distance beyond the first. The third and final cut is the most critical. This final cut is made where the branch arises from the trunk at the swelling called the branch collar. The final cut should not be made flush with the trunk. This results in a very large wound and a greater possibility for poor healing and the introduction of decay.
The best time to prune is between mid-February and early May. Trees pruned at this time in early spring develop a callous around the cut much more rapidly than those pruned at other times. However, there are a few exceptions to this rule.
- Maples, walnuts, birches, beeches, hornbeams, and yellowwood are known as “bleeders”. The “bleeding” may be unsightly, but it does not harm the tree. Bleeding results from copious sap flow, and can be avoided by delaying pruning until after the foliage has fully emerged.
- Spring flowering trees should be pruned after flowers have dropped.
- To avoid the introduction of disease pathogens to oaks and elms, avoid pruning between April 15 and October 15.
- Prompt pruning of storm-damaged limbs and dead branches should be done to encourage wound closure and avoid potential hazards.
Visit the Gateway to Tree Science exhibit on the East Side of the Arboretum to see ongoing pruning demonstrations and experiments. Learn more about the long-term benefits of proper pruning, other best practices for tree care, and more about the research taking place at the Arboretum for a greener, healthier, and more beautiful world.