Apple trees are hardy and grow well throughout most of America. However, for the best production, they need to be pruned from time to time. As young trees, they need annual pruning to help them grow into strong, sturdy trees. Older trees need to be pruned to clear out old wood, encourage fruit production, and maintain a healthy shape.
Apple trees should only be pruned in late winter or early spring, and only before new growth appears. In the summer, you can prune away suckers, or the new growth that comes up from the rootstock. All other pruning must be done during the tree's dormant season. Winter pruning will invigorate the next season's growth. Summer pruning will inhibit growth.
Apples are produced on shoots that grow from the main branch. This year's shoots will grow next year's apples. So the point of pruning is to balance the number of new shoots and those produced last year.
The first pruning of a new apple tree should occur right after planting in November or December, if the tree is a one-year-old or "maiden" tree. Make sure the tree has at least four buds or new branches below the cut, and lop off the top half of the tree. This will force the tree to start developing a nice shape.
If the tree is at least two-years-old or older, prune it in December through February when it is dormant. Last year's growth should not be pruned, but all other growth should be cut by 1/3. Make sure you prune to just above an outward facing bud to avoid having branches grow into the center of the tree.
By the time a tree is five years old it is considered mature, and pruning should only be done in the dormant months of December to February. The goal here is to keep the interior of the tree open and get rid of dead or diseased wood. As always, leave the bulk of last year's growth alone.