info from agreculture Canada

Pruning trees and shrubs

The rule of thumb to follow when pruning trees and shrubs is: "If you have no good reason to prune, don't". Most trees and shrubs have a natural form to them and may do better and look better if left alone. Each kind of tree has its own characteristic shape or growth habit and when pruning, you should try to maintain that habit. There are times, though, when pruning must be done and proper procedures should be followed.

Reasons for pruning

• Pruning at planting time:
  • just after transplanting, the tops should be pruned back to compensate for the loss of roots and to begin training the tree. This should not exceed one-third of the plants total top growth. 
• Training:
  • to develop a strong framework to withstand winds, a tree should be pruned to a few strong limbs spaced well apart, up, down and around the trunk.
  • to develop a shade tree with limbs coming off the trunk at a height greater than 1.6 to 2.4 metres, lower branches should be pruned off. Ideally pruning should be done over an extended period of time as the tree grows in height. If done all at once this can result in a weak, spindly tree that needs staking. If you are going to prune all at once it is best to prune those lower branches to short stubs. These stubs will eventually be removed. The short stubs act as sap drawers, putting out leafy shoots which manufacture food and draw up water and minerals resulting in a stouter, stronger trunk. These stubbed branches must be kept pruned back and can be removed completely after permanent scaffold branches (main crown) have been established.
• Tree health:
  • prune to eliminate limbs with weak crotches that arise from the trunk at acute angles.
  • prune to eliminate limbs that cross each other or compete for the same space in the trees crown.
  • prune to eliminate dead and diseased branches to improve the appearance of the tree and prevent entrance and spread of diseases and insects.
  • prune to revitalize older trees by pruning out part of the crown of the tree, reducing the leaf area that the root system has to supply. More vigorous growth results in the remaining branches.
  • prune to increase air circulation through the tree both for the trees benefit and to increase air flow into the landscape. More sunlight gets through the tree which is beneficial for lawn growth below.
• Safety:
  • dead, broken, weak or split branches, or low hanging branches which might be a hazard to people, vehicles or buildings should be removed.

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