Typically you'll tackle tree trimming for three reasons: safety, tree health and tree aesthetics. Pruning can accomplish many goals, including:
Removing dead wood
Trimming branches that obscure driving views
Removing diseased or insect-infested branches
Increasing airflow inside a tree crown (especially important in windy areas)
Allowing light to penetrate to the ground below
Removing branches that rub against one another
Limiting branches growing into utility lines (never prune limbs near utility lines – call your local utility company)
Enhancing a tree's appearance
A tree isn't like a shrub that might require routine thinning. Because trimming permanently changes a tree's structure and appearance, you want to trim intentionally. The most beautiful mature trees undergo intentional trimming when they're young. Although the general principles of trimming are the same regardless of the size of the tree, here are some other tips for trimming young trees.
When trimming mature trees, always have a purpose for making cuts. Trees recover more easily from smaller wounds than larger ones. That means it's always better to remove smaller branches. Follow these guidelines, provided by the U.S. Forest Service.
For branches less than 2 inches in diameter, make the cut.
For branches 2-4 inches in diameter, think twice before cutting.
For branches more than 4 inches in diameter, cut only if you have a very good reason.
Never remove more than one-fourth of a tree's total leaf-bearing capacity.
Types of Trimming Cuts
Whether you're dealing with young or mature trees, trimming cuts fall into two categories: thinning or heading.
Thinning cuts remove an entire branch or prune one branch back to another branch. These cuts stimulate growth throughout the whole tree and often remove weak, diseased or problem growth. You also use thinning cuts when you want to improve air circulation in a tree canopy or enhance light penetration to interior leaves or the ground below. Thinning enhances a tree's natural shape.
Heading cuts reduce tree height by cutting ends of lateral branches back to a set of buds that in turn start growing as a result of the pruning. Heading cuts destroy a tree's natural shape and are very hard – if not impossible – to correct. Don't use heading cuts on branches over a year old.
The Problems With Topping
When done in the worst possible way, heading cuts are known as topping. This is the most common trimming error and the most damaging to tree health. Topping reduces overall tree size by cutting branches and even the main trunk back to stubs. Topping can also occur naturally when heavy winds break the tallest growing point in a canopy.
Topping removes significant portions of the leaf-bearing crown, which causes the tree to enter starvation mode. Many buds quickly produce shoots to replace the lost leaves. These shoots cluster together and are spindly, weak and prone to breaking in windstorms. They also require frequent trimming.
The practice of topping creates large, gaping wounds that trees struggle to seal. These wounds form openings for decay or disease organisms, which can quickly move throughout a tree. Decaying stubs in turn create weak points in the branch structure.
Topping (see photo above) is a terrible trimming practice. There are other trimming methods that reduce the height of a mature tree while protecting it and preserving its beauty, as shown in the illustration below. Contact a certified arborist for help.
Make Proper Cuts
To perform heading cuts on young branches, make the cut one-quarter inch above a lateral bud. Slope the cut down and away from the bud.
To make thinning cuts on larger branches, cut outside the branch collar at a 45-to 60-degree angle to the branch bark ridge.
If you're cutting limbs larger than 1 inch in diameter, follow a three-cut procedure to first reduce limb weight and avoid tearing bark.
Make an undercut about halfway through the branch 12-18 inches away from where the branch joins its supporting structure.
Then cut from the top of the branch a few inches beyond the first cut (farther out on the limb). The weight of the limb will cause it to break between the cuts.
Remove the stub by placing a cut outside the branch collar at a 45-to 60-degree angle to the branch bark ridge.
Do not apply paint or wound dressing to cuts. Trees heal trimming wounds best when left to do so naturally.
Call A Professional
In general, it's best to call a certified arborist if you're trimming trees taller than 10-15 feet, if you need to use a chain saw or if you plan to use a ladder. If you have some experience trimming trees, you may be able to use a rope saw on limbs higher than 15 feet, but it's always best to err on the side of caution, especially if falling limbs could damage property. Understand that limbs and branches represent significant weight. Even a 1-inch-diameter limb, if it's long enough and high enough, can thud to the ground with enough force to knock a person down.
The International Society of Arboriculture (ISA) recommends hiring a tree service company that has ISA-certified professionals on staff. For help finding ISA-certified professionals, visit the ISA website.