Most deciduous trees are pruned and shaped in winter, after leaf fall, which varies according to the tree species. Some spring-blooming trees including dogwoods, redbuds and deciduous magnolias are pruned in spring after they've finished blooming. Evergreen trees such as Southern magnolias are also best pruned in winter to avoid sunburned on newly exposed bark. Redwoods and deodoras can be shaped lightly in winter to remove congested interior branches and hazardous limbs, but "thinning" or the removal and stripping of major branches is not recommended. Different tree species require different types of pruning to maintain a strong structure and good health.
These are a few examples of basic rules of pruning that every tree trimmer, including handy homeowners, should know. Homeowners can and should educate themselves on pruning techniques (at least on what not to do) as well as who should be hired to do pruning.
Ortho's "All About Pruning," available at most garden centers, has excellent illustrations of both proper and bad pruning. If you're considering having major work done on trees on your property, it's worth the price of the book ($11.95 the last time I checked) to get the job done right. Poorly pruned trees actually will cost more to maintain over the shortened life of the tree. Restructuring a badly pruned tree takes about seven years of good, expert annual pruning. Trees that are so badly pruned that limbs die and the tree structure is unbalanced are hazardous and are liabilities that cost money to remove. Take the book with you as you walk around your neighborhood. You might be surprised at how many trees have been improperly pruned.
Certified arborists have received extensive training in tree care, have taken and passed licensing exams and are expected to follow a code of ethics. They're also the only professional tree pruners, by law, who can work on trees taller than 15 feet or trees near power lines. Their bids often are comparable to other tree pruners and you'll have the security of knowing that their work is guaranteed. Look for tree care companies that have a certified arborist on staff and get three bids before hiring.
One of the major reasons to avoid hiring unlicensed and uninsured tree pruners is the potential for major liability costs to the homeowner in case of injury to the pruners while working or if the tree falls and causes injury or property damage. Do not hire any tree pruners who cannot produce a business card listing their contractor's license number or who do not display the contractor's license number on their work trucks. Then check their record with the Better Business Bureau and the Contractor's Licensing Board.