My sister Ola, an avid gardener, would always say when spring approached that “her sap was rising.” Spring is not here yet, but as our collective sap starts rising, it might be a good time to talk about the science of pruning. If this sounds as exciting as watching paint dry, move along, please. But for those of you who would like to protect your “green” investment, hang in there for a little bit as I attempt to shed some light on the effects of proper and improper pruning.
In order to understand where and how to prune, I first need to provide a bit of tree biology. You may have noticed that trees can’t run away from danger, but that doesn’t mean they are totally defenseless. When a tree’s leaves are being chewed by a caterpillar, it talks to nearby trees by releasing chemicals that alert them to manufacture nasty tasting substances in their own leaves to ward off the caterpillars. You could call it social media for trees. Trees have also developed ways to prevent the spread of disease within the trunk and branches by laying down chemical and physical barriers. A proper pruning cut is done to maintain the integrity of these barriers, especially the branch collar (the swollen area where the trunk and branch join). When pruning larger branches, be sure to use the 3-cut method to prevent branch bark being stripped past the branch collar.
The preferred place to make a pruning cut is back to the parent branch or trunk, just to the outside of the branch collar, if present. This is called a thinning cut, and it most closely mimics how branches shed naturally. A reduction cut is made when it is necessary to reduce the length of a limb by cutting it back to a side branch. Cut back to a side branch that is at least 1/3 the diameter of the branch being removed. Reduction cuts are not as good for the tree because you are destroying some of the tree’s natural barriers. The final type of cut is called a heading cut, where a branch is usually cut to a stub.
You can see heading cuts everywhere, because the other name for this is called topping. People who hire companies to top trees are spending good money on well-intentioned but erroneous observations. Maybe the tree is topped because they are concerned that it has become too big and hazardous and, besides, a topped tree will put out a flush of new growth. That’s a good thing, right? Wrong for so many reasons! First, this will serve to starve the tree and set it up for susceptibility to insects and/or diseases because the tree’s natural barriers have been breached. Secondly, the wood that sprouts from that cut is more weakly attached than a limb that would develop normally. If rot develops at the cut, the weight of the sprout has created a dangerous situation for people walking underneath the tree. Thirdly, if the tree was topped to control the height and spread, the new growth (called water sprouts) will grow so rapidly that the tree will soon be as big or bigger than before. Finally, tree topping may sound like a bargain since it is easier and quicker than using the skill and judgement of a professional arborist. (A professional arborist will NEVER suggest topping your trees.) However, you have just locked yourself into an endless cycle of maintenance and potential liability. Do yourself a favor and “Just Say No” to tree topping.
Proper pruning is a large subject, and impossible to fully address in one article. Watch for upcoming articles that will continue to explore this and other tree health topics.