When you start reading about pruning or attend a class, one of the first things you will hear is "never remove too much or you might kill your tree."
How to Prune: The Rules of Removal Percentages
But, what is too much? And, can we really kill a plant by over-pruning it?
The answers, in order: It depends. And, yes.
Generally speaking most woody plants can easily tolerate having one quarter of their living mass removed each year. So, if you begin pruning by taking out the dead material, do not count that in your "safe amount" rule. However, the moment you begin removing suckers and other living branches, begin your tally. And total amount you remove will depend on a number of factors.
How old is the tree? In many cases, the older the tree, the less material it will tolerate losing. I think about it this way: the older I get, the harder it is for me to recover from illness or injury. It's kind of the same for older trees and shrubs.
What kind of plant is it? Some shrubs, like Hydrangeas and Nandinas, function really well if about one third of their living branches are removed each year. But, if you cut the entire shrub to the ground, it may not come back at all. Other plants like Twig Dogwoods and Butterfly bushes look their very best if most of the plant is cut to just a stump each late winter. If you're in doubt, ask at your local nursery or stick with removing only a third of the plant each year.
Does it really need to be cut?Think about a forest. Trees and shrubs in the woods do just fine without people coming in to prune them every year. So, quite often the "less is more" rule applies. Keep your cuts to a minimum, and if you do not have time to trim your rhodies each spring, they will probably survive just fine without a trim.
There are several reasons why cutting less material is ideal. Plants put a lot of energy into sealing up each cut to protect themselves from potential infections, pests and disease. Too, plants need branches on which leaves form in order effectively feed themselves. If they can't photosynthesizesufficiently, they will starve and die.
So, cut just a little and only when necessary. When it comes to pruning, a military buzz cut will do more than humiliate your tree. Quite likely, it will kill it.