Much of our horticultural literature in Canada is based on European knowledge and in the case of pruning, Europeans do not remove more than 30% of a canopy in one year. Their climate is significantly gentler, the winters are shorter, and the summers rarely have long droughts or baking temperatures for multiple weeks at a time. Therefore, the plants have the length of growing season to rebuild the resources to be able to tolerate losing as much as 30% of their structure. Here in central North America, if we remove that much, particularly during the growing months of May to August, the plant is left without the resources to survive these long periods of climatic extremes and frequently goes into decline. This decline can sometimes be rapid but will inevitably result in a shortened life. Basically, the plant is ‘shocked’ - very similar to a more familiar term, ‘transplant shock’, which means that for some reason a transplanted tree dies even when it receives correct care and support.
As a result of this climatic reality, the removal of 10% of a canopy in a year will minimize the resource loss to a plant and enable it to adjust to living with a mildly reduced canopy. An analogy I favour is that if a plant was in a natural setting and was browsed by an animal, this 10% would be a normal foliage loss to the plant. In other words, a 10% loss of total canopy, particularly if it is new spring growth, is built into their genetics. They can replace that each year during the short growing season before dormancy.
On occasion, every 3 to 5 years, removing an additional 5%, particularly if it is going to allow light to pass more deeply into the center of the plant, is a choice we can make if the plant is still showing vigor. If the plant has not responded with normal extension growth after the previous year’s pruning then do not prune at all until extension growth is seen. If there is little extension shoot growth there is no need to prune anyway, so let the plant build up its resources again, otherwise it will steadily fail. This is particularly true for plants treated as hedges or those that are sculptured. Only a few plants like a Privet, Yew or Boxwood can tolerate having their new growth continually removed, and even then, a plant such as a Yew, which can live for many centuries, will live only a quarter of its normal life span if its natural desire to grow vigorously is continually denied.