I’ve often thought that if I die of a heart attack, I’m sure it will be caused by tree pruning. Many times I have turned a corner, been confronted by a butchered tree and clutched my chest. For some reason many believe it is proper to cut large branches in half, employing heading back cuts (see picture). This improper pruning is bad for the health and safety of the tree, bad for air quality, bad for your neighborhood, bad for your entire community, bad for my health and bad, bad, bad. DO NOT DO IT! No matter clear I make it, I see ruined trees everywhere.
For ornamental shade trees, thinning cuts are most appropriate. Instead of cutting branches in half, you will be removing entire branches. Crossing branches, branches with narrow angles of attachment (they are weakly attached), branches originating on a main leader in close proximity to other branches, and branches in the way of pedestrians or vehicular traffic are prime candidates for removal. The cut should not be made smooth, but a small collar should remain to encourage fast healing and no seal coats of anything should be applied to the cuts. Thinning cuts allow wind to safely blow through trees and for the tree to look structurally appropriate based on the species. Disneyland provides an excellent example of heavily, but properly thinned trees. Trees that are improperly headed back sprout many weakly-attached branches right under the cut and their form is lollypopish rather than a species-appropriate- and frequently majestic- natural form.
If your goal is to prune to control size, then you should prune just after the spring flush of growth. The tree puts on spring growth so that the new growth will feed the tree through photosynthesis. If you cut off much of the new growth, you will be diminishing the tree’s ability to grow more. On the other hand, if your goal is to encourage growth, you should prune during the tree’s dormancy and new growth in the spring will be left to nourish the tree with more food for more future growth.
There are some very specific exceptions to this rule of thinning over heading back (chopping in half). One is a specific pruning style called pollarding which is frequently practiced on London Plane trees such as the famous ones along the Champs de Elysses in Paris, France and those defining Sproul Plaza on the U.C. Berkeley Campus. These trees are pruned to affect a very urban finished product and heading back cuts are used to make these trees almost look like buildings as space definers, rather than a more natural approach using thinning cuts. Another appropriate use of heading back cuts is for many fruit trees in order to encourage lower occurrence of fruit and to produce wood that bears. For instance, Peach and Nectarine bear on one year old wood, so extensive heading back encourages the occurrence of wood that bears fruit.
Other than a few exceptions, we should be thinning as we prune and facilitating a long, healthy life for your trees and for me.