It has been said that the best time to prune trees is whenever the blade is sharp. This is true to some extent. Routine pruning can be done at any time of the year but growth is maximized and wound closure is fastest if pruning takes place before the spring growth.
Heavy pruning after the spring growth causes stress to the tree. This is because the tree has just used all of its energy to produce leaves and shoots and may not be able to recover.
All pruning cuts should be made outside of the branch collar. Parent branch or trunk tissue must not be damaged.
When you are pruning a tree, be careful not to remove too much of the tree at one time. Leaves produce food for the tree and removing them will temporarily starve the tree. In response, the tree will force the rapid growth of multiple shoots below each cut.
Be sure to regularly inspect your trees throughout the growing season for disease, insect and environmental problems. Often by the time a tree shows obvious signs of infection, infestation, or stress it is too late to do anything about it.
Dutch Elm Disease
Please remember that in order to prevent the spread of Dutch Elm Disease it is illegal to prune elm trees in Alberta between April 1 and September 30.