To the trained eye, the difference between a professional and amateur pruning job is obvious. Whereas the former often leaves the tree attractive, healthy and ready to resist decay, the latter yields a tree that is stressed, covered in wounds and ripe for invading pathogens.
This dichotomy is hardly unexpected -- tree pruning is more complicated than it appears at first glance. Arborists and tree care professionals spend years learning about the biology of trees and the science of maintaining them. Meanwhile, most amateur tree cutters have little experience or education about trees and their care, which manifests in myriad ways.
Improper cut placement is one of the most damaging and common mistakes amateurs make. While it is not realistic or necessary for amateurs to understand tree biology in detail, understanding the basics of branch junctions and proper cut placement will help amateurs to prune trees more like the pros do.
Branch Junction Anatomy
When you prune a branch, the remaining stub, which now lacks bark, is vulnerable pathogens and pests. If disease or decay spread into the primary branches or trunk, the tree could die.
However, trees lose branches naturally, and they have evolved ways to cope with the problem. By taking advantage of their natural defenses, you can help ensure your tree remains healthy.
Look closely at the branch you intend to prune, and trace it back to its origin -- where it emerged from the trunk or parent branch. You will see that the base of the branch is slightly enlarged; arborists call this the "branch collar."
Trunks and branches not only grow longer, they also increase in girth. As the emergent branch and the parent branch increase their girth, the branch collar forms. Often, a ridge -- called the branch bark ridge -- forms in the center of the branch collar.
Inside the branch collar, trees have specialized areas called "branch protection zones." These areas have special chemical and physical characteristics that help protect the tree by quickly initiating the process of compartmentalization, which helps prevent decay from spreading further into the tree.
Correct Cut Location
Most pruning cuts should take place near branch junctions -- places that tree professionals call "nodes". This takes advantage of the tree's branch protection zones, and gives the tree the best chance for surviving.
Find the base of the branch you intend to prune. Identify the branch collar and branch bark ridge if possible (small branches have very subtle branch bark ridges). Find the edge of the branch collar and visualize an imaginary line, encircling the branch at this point.
This imaginary line marks the approximate place at which the branch protection zone ends. You want to remove as much of the wood possible outside this boundary, but leave everything inside this boundary intact.
In other words, this imaginary line marks the final length of the stub you will leave when you have finished pruning the tree. Nevertheless, making a cut in this location is likely to harm the tree, unless you make two other cuts first.
The Three-Cut Method
If you cut a large branch from the top, the branch's weight will cause it to fall before you finish the cut. This often causes the bottom fibers of the branch to remain intact.
When the branch falls, it pulls a long strip of bark and wood with it, leaving horrific scars down the trunk. These scars are not only unsightly; they impart significant stress on the tree and expose it to decay. In a worst-case scenario, these wounds may travel all the way down the length of the trunk.
Avoid this outcome by using the three-cut method.
The first step is to cut halfway through the bottom of the branch, about two feet out from the imaginary line that marks the end of the branch protection zone. This cut will severe the long fibers at the bottom of the branch, and prevent them from stripping the trunk bark.
Place the second cut about 3 inches farther out on the branch than the first cut, and make this cut through the top of the branch. Cut all the way through the branch and the limb will fall.
You are now left with a 2-foot-long stub that is still attached to the trunk or parent branch. Make the third and final cut just outside the imaginary line that marks the end of the branch protection zone. You can make this cut from either direction, as the stump is not heavy enough to strip the tree's bark. Make the final cut smoothly and at a 90-degree angle to the axis of the branch.
Once you have completed the final cut, the task is complete and you can move on to the next branch. While arborists formerly painted these stubs to protect them from decay, research has shown that this practice is unnecessary, unhelpful and, in some cases, detrimental to the tree's defense mechanisms.