There has been much research data shared and discussed that supports the idea that it is most beneficial to prune trees of all species in the dormant season (winter). So most practicing arborists will agree that winter is the best time to prune. There are many reasons for this, but since there is such an overwhelming consensus on the matter I think this discussion should be more about how the trees should be pruned.

Pruning Trees

Probably the most common reason to trim or prune a tree is to remove branches that are in the way of something. Maybe satellite reception is poor due to an obstructing limb, or a view is obstructed, a light is covered, a branch is rubbing a building, etc. When pruning to mitigate these issues, we are most often removing live limbs from a tree. When we do this we should take care to make cuts that will heal and leave a proper terminal branch to the leader that is cut. It is not acceptable to simply cut at a convenient place that achieves the clearance that we are seeking. In fact, cutting at improper points on a branch will often encourage the overgrowth of "water sprouts" that will soon make the clearance or obstruction worse than when we began. Guidelines for pruning cuts are outlined in ANSI z133 and the ISA at treesaregood.org and should be followed by anyone who does this work.

The next most common reason for pruning is the removal of dead wood. Dead wood eventually falls from a tree if it is not cut out first, posing danger to anyone and anything below it. Worse yet, dead wood tends to fall from trees on calm weather days rather than in storms, so the risk of it falling on people is greater than we may think. Generally speaking, dead wood should be cut back to where there is live tissue and in a way that promotes the growth of live tissue over the wound. A certified arborist will know where best to make a cut to promote the fastest healing.

Another reason to prune trees is perhaps the most misunderstood of all of the reasons. That is to reduce the risk of a tree failing due to wind exposure. It is true that some trees become more prone to breakage as they grow, but the reasons for this are not well understood by many who prune trees. It would take too much time and space to explain fully, but in general leaders and branches are more or less prone to failure as a result of their angle of growth from the parent stem. Steep angles - sometimes referred to as co-dominant - have a greater potential for failure. Safety pruning should be focused on the reduction or elimination of these kinds of branches and leaders. Simply taking the tops off of leaders does not achieve safety for the same reason mentioned above in the case of pruning live limbs for clearance. The eventual grow-back of the leader in the form of water sprouts just makes the situation worse.

It is true that trimming trees is an essential part of maintaining a property. However, the way in which they are trimmed is critical to achieving positive outcomes. We can do more harm than good if we do not understand the science behind the practice. Consult an arborist and check his or her credentials so you can understand the best practices as they pertain to your trees.