There are two rules of thumbs I live by in my pruning work: less is more, and, do what’s needed. Sometimes a lot of pruning is required, but doing less would not achieve the required result. I teach my staff that they should prune with accountability in mind. That is, every cut they make should have a purpose, and if the client should question any cut, they should know exactly why they did what they did. Random, indiscriminate pruning is a disservice to the customer and tree. If you don’t know why you are making a cut, don’t make it.
Trees store energy in their twigs, limbs, trunks, and roots. When we prune branches, we remove stored energy. We must assess the health and vigour of the tree before we decide the pruning “dose.” A stressed, slow-growing tree will have a hard time dealing with a lot of unnecessary pruning, and a healthy, vigorous tree will respond to unnecessary pruning by putting out a lot of new growth in the form of waterspouts and adventitious growth.
The best way to assess the vigour of a tree is to measure the growth rate from the past few years. To do this, you must identify the terminal bud scale scar from the previous year’s growth (figure). Excellent growth over several years is a good indicator that you prune what you need to without too much concern. With a very slow-growing tree, be mindful to prune only what is absolutely necessary. Other ways of assessing health include leaf size, bud size, accumulation of deadwood, tip die-back, and presence of disease (such as fireblight or black knot).
Generally, it is safe to prune up to 10% of the crown of a tree at any time of year. On a very stressed tree, however, I would be concerned about taking off more than 5%. Some species, such as rosybloom crabapple varieties and hawthorn, are prone to extremely congested canopies if left untouched for many years. On these trees, it may be necessary to prune 25% or more of the canopy, and this is fine, provided the growth rate and vigour (bud or leaf size) indicate a healthy tree.
My favourite pruning instructor, Dr. Edward Gilman, formerly of the University of Florida, once told me, when I was humming and hawing about a branch that required removal, “You can cut it now, or you can cut it later, but it has to be cut.”
Less is more. Do what’s needed.
(c) 2018 Shane LePage, Wild Rose Garden & Tree Service Inc., Red Deer