Some tree trimming tips include only trimming during the tree's dormant season, never removing more than one-quarter of the tree's crown in a season, and trimming the tree branches just above the branch collar. The branch collar is located just above the intersection between the branch and the trunk.
Prune trees during the winter in order to increase the amount of growth during the spring and summer growing seasons. However, it is best to wait until the coldest part of the winter has passed before pruning the tree to avoid harming it. Some trees, such as maples and birches, may begin to excrete sap from the trimmed branches towards the end of the winter as they come out of dormancy. This is normal, and the sap normally ceases flowing after a short time.
Never removing more than one-quarter of the tree's crown during a single season is one of the "one-third and one-quarter" rules of pruning. The other rules are to keep the side branches of the tree to one-third of the diameter of the trunk, avoid pruning so much that there is a space from the base of tree to the first branches that exceeds one-third of the overall height of the tree, and try to encourage side branches to grow at an angle that is one-third off vertical from the trunk of the tree.