Thinking about proper tree pruning reveals some interesting presuppositions on our part about what’s natural and what is not. It’s important to know some basics about pruning — in particular what not to do.
Pruning, it turns out, is quite complicated. Far from random cutting of branches that are in the way, it requires forethought and an understanding of why trees are shaped the way they are and how a particular species grows. I used to think that, yes, trees need pruning if they are ill, have damaged limbs, or are impeding our busy-ness, but there’s a lot more to it.
I thought too that a tree knows how to be a tree — we don’t need to shape it to suit our preconceived beliefs, barring the above reasons for pruning. And this is true, in a natural setting. But a tree standing in relative isolation along a street is not a natural situation.
In woodland, trees rarely stand alone. Surrounded by others, they buffer each other against the wind, leaning and swaying in tandem. Better to take small damage and help your neighbor than be uprooted and toppled completely. After a storm, most damage will be to trees at the perimeter of a forest where they are exposed.
Trees in a dense population are always competing for sun, and as the lower limbs get crowded out and shaded, they tend to shed leaves and eventually fall off. Walking through the woods, you often need to look up a ways to find the first horizontal branches on larger trees. It makes sense too: Why produce leaves to photosynthesize when there’s insufficient light? Why maintain the limbs to support the branches and leaves? In this way trees may shade out their own branches.
Now imagine the tree in your front yard. It probably has complete access to the sun even to the very base of the trunk. The lower limbs won’t be shaded, so are far less likely to drop, but rather can just keep on growing. This is where the tree needs to be shaped by pruning, since this unnatural shape can cause instability and increased danger of falling in a storm. So in a case like this, proper pruning is keeping the tree shaped like a tree — as it would be in a natural setting.
For a street tree, limbs are usually kept to about 15 feet above the ground for vehicle clearance. When young, a street tree needs to be gradually pruned to become the tree that gives the shady canopy we desire, while allowing traffic beneath. And over-pruning too soon in a young tree’s life can rob it of food.
To properly redirect growth on a young tree, heading cuts are made. These are “internodal” cuts made between areas of branching.
Solitary trees also need their crowns thinned out. In nature, any thinning needed will likely happen on its own, but in landscaping, trees can become top heavy enough to topple in high wind, especially when winds are combined with oversaturated soil. Thinning out removes enough to retain the tree’s shape while encouraging healing and new, branching growth.
Thinning is unlike topping, a terrible and overaggressive removal of foliage. The unhealthy growth resulting from topping (or hatracking) is a cluster of shoots that are weak, unattractive and more likely to break in storms.
There are many other reasons to prune, like having two dominant trunks where one may weaken and fail for example, or to safely remove decaying or sick limbs. Fairchild’s arborist, Bob Brennan, who recently passed away, was a highly skilled arborist who shared this knowledge freely. He once let me peruse the books used for preparing to become a certified arborist. They were as thick as phone books!
Are you just pruning some in-the-way lower branches? Avoid cutting flush to the trunk, as this can introduce disease and decay. In most cases, there is a swelling called the branch collar where a branch attaches to the trunk. Cut at an angle to leave the collar intact, while not leaving much of a stub beyond the collar.
Every situation and species is unique, a good reason to call on a certified arborist if you have tree trouble, or think your trees need pruning for the upcoming storm season. You can easily search The International Society of Arboriculture at www.isa-arbor.com to find a certified arborist in your area.
Kenneth Setzer is writer and editor at Fairchild Tropical Botanic Garden.