by Jeff Wasielewski, former Assistant Curator of Tropical Fruit
Why do we prune? Most of us know the reasons: to create healthy trees by removing dead wood and awkward branches, to promote fruit and flower production, and to create beautiful trees. But all too often we prune for the wrong reasons... for example, because our friendly neighborhood landscaper planted a large tree directly under a power line. Trying to transform a big tree into a small one usually results in an unhealthy tree and an eyesore.
True pruning is an art. It takes intelligence, time and thought. It begins and ends with the tree. Before you make your first cut, study the tree's natural shape. Try to maintain that shape. Study its interior. Look for unhealthy conditions such as included bark, dead wood, or crossed branches.
Included bark occurs when two branches or trunks grow so close that they join as their width increases. A wrinkle occurs where they join and, more importantly, the included area dies, weakening the branch or trunk. A tree with co-dominant leaders tends to have included bark and has a strong chance of splitting. Included bark can be remedied by removing the smaller of the two branches, or the one supporting less of the overall mass. However, if the diameter exceeds six inches, don't remove it. The resulting wound could do more harm to the tree than the inclusion. The tree can be saved by heading back the branch you would like to remove. These cuts will suppress the pruned branch and allow the unpruned branch to grow and become the tree's dominant leader.
Dead wood offers a prime entry point for pathogens like bacteria and fungi. Carefully cut any dead wood away, avoiding the live portion of the tree. Leave a half inch of dead wood. If you make your cut into the living area, the tree will have to re-heal. This is the only time that leaving a stub on the tree is considered correct.
Crossed branches are unhealthy. Remove the smaller branch or the one that has been damaged the most.
Now that you have made the obvious pruning cuts, step back and really look at the tree. Your mind is as important a tool as your saw. You must decide what your goal is. Do you want to head back the tree to keep it at a desired height, or do you want to thin it to open it up?
Heading back a tree is cutting its canopy branches back to the nodes, or growing points.
Thinning a tree is removing a small number of branches back to the trunk.
The earlier you make pruning decisions, the better. Envision the tree in a year, or in five years. How will today's cuts affect the tree's future appearance and health?
Finally, decide if the tree needs to be lifted. Are the lowest branches too low? Will the tree be more attractive if you raise the canopy? Most trees look better when lifted. I like to see a tree with a trunk, not a thirty foot shrub. To lift the canopy, remove the lowest tree limbs all the way to the trunk.
Make your cuts correctly.
The branch "collar" is a swollen area at the base of each branch. Leave the branch collar on the tree, and never remove more than 30 percent of the foliage. Avoid injuring the branch collar; it is critical to healing the pruning wound. You won't need to use pruning paint if you make your pruning cuts correctly.
Many landscapers prune a tree extensively to make sure their customers know they have done their work. Some remove as much as half or even all of a tree's foliage. Others remove the entire top portion of the tree's canopy, a practice called hatracking. You might as well take an ax and hack away at the trunk, because you won't do much more damage. Hatracking opens up the interior of the tree, making previously shaded portions vulnerable to sunscalding. Hatracking also takes away the tree's energy-producing canopy and causes massive regrowth at each cut. When you cut back one branch, it comes back as three or four. This holds true even in the largest of trees. The weight of several new branches can be substantial, creating a dangerous canopy for trees in a hurricane zone.
It is not necessary for a tree to look pruned. Every cut causes some amount of damage. Therefore, a competent pruner removes as little as possible, making only cuts that will benefit the tree in the long run.
The art of pruning requires intelligent decisions from the very beginning, starting with choosing the right tree in the first place. The greatest job of pruning I ever saw was done by my father-in-law. Outside his window he wanted a tree so small that he could always see the entire plant. He planted a Senna polyphylla, a yellow-blooming tree which grows no taller than eight feet. He has never had to prune it once.