Pruning Deciduous Shade Trees - January 24, 2001
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
I hesitate to write on pruning of deciduous shade trees because people will immediately head for the yard and start cutting. There are good reasons to prune shade trees just like there are bad reasons. A threat to human safety and/or health of the tree: good reasons. Bought a new pruning saw or have too much time and/or energy on your hands: bad reasons. I think you get the idea. Prune deciduous shade trees only if you have specific goals or objectives in mind.
One of my colleagues, Terry Mikel, defines a tree as "a self-pruning shrub". In natural settings without irrigation, the crown shades the lower branches. Over time, these branches naturally die and usually fall off. In cultivated landscapes, we irrigate and fertilize trees, which causes rapid growth creating a need for artificial pruning.
Pruning a tree always causes a wound. The tree responds to pruning in several ways: 1) the loss of leaves lowers the amount of energy the tree can harvest from the sun (photosynthesis); 2) the wound is a potential entry site for pests and diseases; 3) new distribution of plant growth hormones within the tree causes sprouting (suckers, water sprouts, or lateral buds); and 4) overall vigor decreases.
Trees from nurseries are often sold staked with their side branches pruned to create a trunk. This practice is not recommended because these side branches provide energy to the trunk at their point of attachment. The energy from these side branches will increase diameter growth and result in a sturdier tree. As the tree increases in height, these side branches may be pruned away.
Given the advantages of having side branches on young trees, now I can sound contradictory and recommend some pruning for branch spacing when a tree is young. If a young tree has too many lateral branches that originate at the same level, they can become crowded as it matures. The lateral branches coming directly off the trunk are called scaffold branches. The strength and spacing of the scaffold branches will determine the structure and shape of the mature tree. In general, scaffold branches should be spaced 12 inches or more vertically from the next scaffold branch above or below that point. They should also be evenly spaced in the radial dimension and not all be growing out of one side of the trunk.
Narrow branch angles are prone to splitting and are weak points on trees. Think of a clock face with the trunk rising upward through the "12". Ideally, branches will at least have angles at the "10" or "2" o-clock angles. This is equivalent to a 60-degree angle. Branches angles narrower than this are weak and prone to breakage. Where there is a pruning choice between a narrow and a wide branch angle, keep the branch with an angle closest to 60-degrees. Some trees have naturally narrow branch angles. Arizona ash trees are a good example of this.
Crossing branches are undesirable. They rub against each other causing injury to the bark and entry points for diseases and insects. In this situation, a choice must be made. It may not be an easy choice, but if not pruned, the problem usually gets worse. Once the least of two evils is selected, the pruning cut should be made at a location that does not leave a dead stub or promote extensive sucker growth. It is best to cut the offending branch of where it is attached to a larger branch or the trunk.
To make a proper pruning cut on a large branch, do it in three steps. First, go several inches above the point where the final cut will be made and cut the underside of the branch about a third of the way through. Second, go above the undercut and saw through the branch. Third, make the final cut leaving the branch collar (the swollen area where the branch is attached). This method takes the weight off the branch first and prevents stripped bark where the final cut is made. Do not use pruning paint or sealant on the wound.
The best time of year to prune deciduous shade trees is right now. They are dormant and the branching structure can easily be observed. Summer pruning should be done only when branches are broken or there is a safety hazard. Dormant pruning also decreases the risk of injury from torn bark. For a complete illustrated publication, download "Pruning Deciduous Shade Trees" from the University of Arizona Cooperative Extension web site at http://ag.arizona.edu/pubs/garden/az1139.pdf or request it from Yavapai County Cooperative Extension (see below).
Deciduous fruit trees are pruned similarly but for different reasons. Come to one of my upcoming fruit tree pruning demonstrations. They will be on February 3, 2001 at the Freitas Orchard in Cottonwood, February 17, 2001 at Bent River Ranch in Clarkdale, and March 3, 2001 at Young's Farm in Dewey. All workshops start at 10:00 AM. Watch for detailed press releases in local newspapers.
The University of Arizona Cooperative Extension has publications and information on tree pruning and care. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number. The Yavapai County Cooperative Extension web site is http://ag.arizona.edu/yavapai/.
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Last Updated: March 15, 2001
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