How To Prune A Tree

Trees need pruning for a variety of reasons:

  • to remove diseased or damaged branches
  • to thin the crown to allow new growth and better air circulation
  • to reduce the height of a tree
  • to remove obstructing lower branches
  • to shape a tree for design purposes

Once the decision has been made to prune, your next decision is whether or not to tackle the job yourself. Large tree pruning can require climbing and heavy saws or even cherry-pickers and chain saws. In the case of a large tree where you want to remove big branches in the upper area of the crown, it may be best to hire experts.

Getting Started

Illustration identifying the parts of the tree branch.When pruning, it is vital to prune the unwanted branch while protecting the stem or trunk wood of the tree. Tree branches grow from stems at nodes and pruning always takes place on the branch side of a stem-branch node. Branches and stems are separated by a lip of tissue called a stem collar which grows out from the stem at the base of the branch. All pruning cuts should be made on the branch side of this stem collar. This protects the stem and the other branches that might be growing from it. It also allows the tree to heal more effectively after the prune. To prevent tearing of the bark and stem wood, particularly in the case of larger branches, use the following procedure:

Illustration showing the order in which to make pruning cuts.
  1. Make a small wedge shaped cut on the underside of the branch just on the branch side of the stem collar. This will break the bark at that point and prevent a tear from running along the bark and stem tissue.
  2. Somewhat farther along the branch, starting at the top of the branch, cut all the way through the branch leaving a stub end.
  3. Finally, make a third cut parallel to and just on the branch side of the of the stem collar to reduce the length of the stub as much as possible.

A similar procedure is used in pruning one of two branches (or one large branch and a stem) joined together in a 'u' or 'v' crotch. This is known as a drop crotch cut. Make the first notch cut on the underside of the branch you're pruning well up from the crotch. For the second cut, cut completely through the branch from inside the crotch well up from the ridge of bark joining the two branches. Finally, to shorten the remaining stub, make the third cut just to one side of the branch bark ridge and roughly parallel to it.

When to Prune

Dead branches can and should be removed at any time. A tree's dormant season, late fall or winter, is the best time to prune. Pruning during the dormant period minimizes sap loss and subsequent stress to the tree. It also minimizes the risk of fungus infection or insect infestation, since both fungi and insects are likely to be dormant at the same time as the tree. Finally, in the case of deciduous trees, pruning when the leaves are off will give you a better idea of how your pruning will affect the shape of the tree.

How Much To Prune

When deciding how much to prune a tree, as little as possible is often the best rule of thumb. All prunes place stress on a tree and increase its vulnerability to disease and insects. Never prune more than 25% of the crown and ensure that living branches compose at least 2/3 of the height of the tree. Additional pruning could fatally damaging your tree. In some cases, storm damage, height reduction to avoid crowding utility lines or even raising the crown to meet municipal bylaws, your pruning choices are made for you. But even in these instances, prune as little as you can get away with.

Prune the tree for the following:

  • Prune basal suckers (sprouts that grow out at the base of a tree).
  • Do NOT prune terminal leader or branch tips.
  • Prune any codominant leaders or narrow crotch angles.
  • Prune rubbing or crossed branches.
  • Prune any broken branches.

Should I Seal the Wound?

Trees naturally close wounds that result from branch removal, so pruning wounds should be left to close without any help. Also, since most pruning should be done in late fall or winter, insects should not be much of an issue. However, there exist some circumstances when it is preferable to seal the wound with a non-asphalt-based pruning sealer. In particular, you should seal pruning wounds on trees that are susceptible to damaging insect infestation such as birch, oak, and elm trees. Also, if the weather is particularly dry, a pruning sealer will help the tree retain more moisture.