There are a number of reasons to prune woody plants. One of the most important is to maintain a healthy, safe plant. This is particularly true of trees since dead limbs, topped branches, and poor form can lead to unsafe conditions. We can reduce the amount of pruning needed by selecting the right tree for the site. Trees can range in size from 20 to over 100 feet, and many can get large very quickly. Trees grown for flowers are typically smaller in stature than are those planted for shade. Ask yourself why you are planting this tree and then research the mature size of your selections to make sure they will fit in your landscape. How do you know if it will fit? Investigate your surroundings. If you are planting a tree that matures over 60’ tall, make sure there are no overhead utilities within 35 feet of where you intend to plant. Look for any horizontal obstructions. Allow sufficient room for air circulation between your home and large maturing trees. No matter what tree species you choose, be sure to give it good maintenance from the time it is young. This means training it for good form during the first 10-20 years of its life to build strong branch structure (see AG-71, Pruning Trees and Shrubs). If you are inexperienced at pruning or must get off the ground to remove branches, call an International Society of Arboriculture (ISA) certified arborist. They understand tree biology, know when and where to make cuts, and how to inspect trees for hazards. If you feel comfortable pruning, here are a few tips to ensure that you are making the right cuts, during the right time of the year.
Remove branches growing in toward the trunk, crossing branches, and smaller laterals to get spacing of 12-18 inches between main structural branches. Thinning cuts are made to “open-up” the canopy to allow better wind movement and light penetration (Figure 1).
You should also remove limbs with narrow crotches (V-crotches) as these often lead to branch failure, as limbs get larger (Figure 2). Cut back to the branch collar of another lateral, at the main trunk, or even to a bud on very small branches. Always prune larger branches back to a lateral that is at least 1⁄3 the diameter of the branch you are removing, as this ensures little loss of photosynthetic activity. Cutting into the collar, termed “flush” cutting, removes the chemical zone that promotes good wound closure (Figure 3). Without wound closure sometimes referred to as “healing,” the tree may be subjected to decay-causing organisms, and a potential decline in tree health. For years, much of the tree care recommendations indicated that flush cutting, along with the application of wound dressings were appropriate practices. Research now indicates that both of these practices can lead to unhealthy trees. Wound dressings seal in moisture, creating an ideal environment for decay organisms.
Another pruning practice that leads to decay is topping. Topping was and, unfortunately, still is a common practice used to reduce the height of a tree. Good plant selection can help eliminate this improper practice. Additionally, knowledgeable homeowners can insist on the use of good pruning techniques by hiring certified professionals.
If a tree has grown too tall, a technique known as “crown reduction” can be used (Figure 4). Crown reduction uses the same principles as thinning, by removing branches back to a lateral. Use reduction pruning only as a last resort, as wound closure may be slow or inconsequential if done later in the life of the tree.
Before picking up a saw or shears, understand why you are pruning. Know how to make the proper cuts and when it is appropriate to prune each particular species. Avoid unneeded pruning by selecting a tree that grows within the bounds of the site.
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Publication date: Oct. 1, 1997