There are several reasons for pruning trees:   

• safety
• to keep tree healthy
• to improve the shape of the tree
• to allow more light to reach the ground under the tree

Here are a few rules for pruning trees:

• Remove all dead, damaged, or diseased branches at any time of the year.
• Remove branches that could fall and cause damage to people or property.
• Remove suckers and “water sprouts” at any time of the year.  
• Most deciduous trees are best pruned in the winter when they are dormant. However, some experts prefer to wait until summer to prune trees that bleed, e.g. beech, birch, dogwood, elm, maple and sycamore until the summer.
• Spring-flowering trees, e.g., cherries, redbud and crabapples, should be pruned in late spring or early summer when they have finished flowering.
• Evergreen trees rarely need pruning. If necessary, prune in winter or midsummer.
• Always shape trees when they are young.
• Trees can be thinned but never “topped” (i.e. just removing the tops of all the branches)
• For heavy branches use the 3-cut technique: First, make a partial undercut about 1-3 feet from the trunk. Second, make a top cut through the branch a few inches out from the first cut. Third, make the final cut close to the trunk, leaving the branch collar intact.
• Never prune trees that are near or touching power lines.
• Large trees are best pruned by a certified arborist, as the work can be dangerous (Consult the International Society of Arboriculture.

For more detailed information, see ISA's Pruning Young Trees, Pruning Mature Trees, and the USDA's How to Prune Trees.

For tips on a variety of gardening topics, see our Plant Information Guides. - Courtesy of NYBG Plant Information Service