There are many reasons to prune a tree branch but completing the work properly is vital to the tree’s health.
Prune only what you can reach with hand tools while standing on the ground. It is best not to do this from a ladder or to climb higher to reach a branch; use a certified arborist for such a task. You can find an arborist in your area, by consulting the International Society of Arboriculture www.isa-arbor.com.
For small branch pruning, use sharp bypass shears, not anvil shears. Larger branches use bypass loppers, and for bigger branches, use a sharp pruning saw.
Improper pruning can threaten your tree's health. Understanding how your tree's tissues will react to the cuts you are making can help you prepare properly for pruning.
When you prune the branch from a tree, woody plant parts are sacrificed to protect the healthy. In the vicinity of a wound area, trees activate living cells that store oils, starches and other materials. The cells convert their stores into compounds that coalesce around the wound, forming a boundary that restrict the spread of pathogens and insect pests. The isolated wood dies, but the rest of the tree lives.
When conductive tissues are injured, as they are by improper pruning, rot tends to spread farther above and below the wound than it does to the sides or into the tree. Trees have another defensive zone at the base of branches, where they form a boundary that inhibits the spread of pathogens. Proper pruning preserves the tissues where this protective barrier forms and does not injure conductive tissues.
When tree cells have ample reserves, the tree will have a strong defense system and the tree can form strong barriers against diseases and pests. If reserves are low, the barriers will be weak. Tree energy reserves increase and decrease annually in the following way: the tree's fine roots start absorbing water and elements in late winter, often when snow is still on the ground. Then the tips of the twigs awaken and the buds begin to open. Once the leaves have opened up, new wood begins to form in the branches, and then in the trunk. During these phases of time, the tree draws mainly on stored energy to fuel its growth. Energy reserves are usually at their lowest during the time the leaves have formed. Then, within days, new leaves begin trapping the much-needed sun’s energy and the reserves increase very rapidly.
Pruning when your tree has the strongest natural defense system will give it the best chance to defend itself against pests and disease. The best time of year to prune is when the tree has the highest reserves and is ready for new growth. This is during late dormancy, before buds begin to form (swell).
The worst times to prune are when leaves are forming (because energy reserves are low) and when leaves are falling (because new absorption roots develop then and pruning drains energy that would have gone into their formation).
Other times to refrain from pruning living branches due to possible low energy reserves include: during or after a season of severe drought, if the tree has been pruned harshly in the last few years, and if a massed amount of leaves have been lost due to pests or disease problems. Under such circumstances, it is best to prune in a year or two. But, do remove damaged or dead branches, which can be done at any time as needed.
The key to proper pruning is a locating a distinctive bulge at the branch base, consisting of a series of collars. The branch collars are built up with a layer of tissues yearly during spring after leaves form. These collar tissues have extraordinary resiliency, and strength. You can actually feel the collars popping past each other when tugging downward on a three or four year old branch. Leaving the branch collar intact when pruning protects the tree from multiple attacks of pathogens or insect injury.
Branch collars vary widely. Some tree species have nearly flat collars, others are thick and prominent. So the pruning cut will differ a lot in distance and vertical direction from the trunk. The position of the branch collar determines a proper cut. The proper pruning cut will come close to the tree trunk but avoid the protective collar.
When a tree has a highly prominent branch collar, people may hesitate to prune properly. They feel preserving it leaves the tree with an unsightly volcano-like wound. The temptation to cut through the branch collar is greater still when a branch has been dead for some years and the branch collar has grown like sleeve around it. On large dying and dead branches, the branch collar may grow outward several feet. But no matter how prominent the branch collar, leave it intact. Also, do not cut above the collar, which can invite pathogens and insect problems.
It is equally important that you never cut behind (into) the branch bark ridge. The branch bark ridge is raised bark between a branch and the trunk. As the tree grows, the raised bark persists on the trunk in a line to each side for the branch. Never cut behind the branch bark ridge, ever! If you have trouble distinguishing the branch collar, you can make a proper pruning cut by taking an angle that mirrors the angle between the branch bark ridge and the trunk picture below.
A very important step before cutting a large branch off at the branch collar, is to shorten the branch to a 6 inch stub. This precaution is necessary to prevent the branch tearing away from the tree as you near the end of the pruning cut, ripping out the tissues at the bottom of the branch collar and below; Make the initial stub cut on the underside of the branch, 6 inches from the branch collar and about a third of the way through the branch. Then start the second stub cut on the top of the branch, out from the first cut, about 1/4 inch. Now you are ready to make the ideally placed pruning cut.