This answer is from the USU Extention Ctr. http://extension.usu.edu/htm/faq/faq_q=100
There are many reasons a tree or shrub may need to be pruned. For ornamental and shade trees:
* Remember that the tree limbs and branches will stay at the same height for the entire life of the tree. The growing point for the tree is located in the top terminal bud, and the rest of the tree will only grow in circumference. If the branch is four feet off the ground today, it will be four feet off the ground in 20 years.
* You can safely prune most trees through the end of May. Most pruning is done before the tree leaves out because it is easier to see where to prune and easier to get into the tree. I recommend pruning in March and early April.
* Do very little pruning on ornamental trees. Prune wood that is dead, diseased or injured and branches that cross (rub) or grow back into the center of the tree or are out of place. Be sure to keep the natural shape of the tree intact.
Fruit trees are normally trained and pruned to increase their productivity and keep their size under control. It is best to prune them on an annual basis, starting the first year they are planted. Too many people wait until the tree is five or ten years old before they consider pruning. Begin training a fruit tree the first year it is planted.
* Don’t let firsttime pruning intimidate you. Decide for yourself how you want the tree to look in five, 10 or even 20 years, then start to shape it as you prune. The main objective of pruning fruit trees is to keep the tree open, allowing light to penetrate into the center of the tree.
* New fruit trees normally need four to six branches to form the lower scaffolding. In orchards, many trees are trained with a central leader, or main trunk, with many scaffolding layers. For backyard orchardists with only a few trees it’s best to prune fruit trees as an open vase. An open vase tree has only one scaffolding layer and the center of the tree remains open.
* To create this shape keep five or so branches that are kept should be three to five feet off the ground, and spaced evenly around the tree. This is the framework for the open vase. As these branches grow they become the major wood which produces the fruit. Picture the tree as a giant solar collector, and space the branches around the tree to optimize the amount of sun it can collect.
* Apple, pear and cherry trees naturally try to grow a central leader, or a main trunk system. Removing the leader to create an open vase makes the tree take on an unnatural shape. The branches may each try to become the main trunk and grow upright. Discourage this by training branches to grow in a horizontal direction by either tying them down with string, placing weights out on the ends of the branches or placing a spacer in the fork to force a wider angle.
* Most spacers are made with a piece of 1 x 1 wood with finishing nails placed in both ends to keep it steady. Be careful whenever bending and forcing a branch down. Do this after the sap starts to flow in the tree so the wood is pliable. Begin forming only young wood branches. Do not try to bend any wood more than a year or two old. This is another reason it’s important to begin training a tree when it is young and the tree is still pliable enough to bend.
* Leave spacers in the tree for a year or two, then remove them or place them in another part of the tree. Trained wood will remain that way for the rest of the tree’s life.