There are three basic reasons why we prune trees. The first is to improve the look of a tree; the second is to improve the health of a tree; and the third is to make a tree more storm resistant.
If you like a tree to be really thick, impenetrable and with no visible branches, then you've come to the wrong place. In fact I think I have just given a definition for a bush. In my opinion a tree's full beauty is only available when the structure of the branches is visible. Trees simply look better to me when they are evenly thinned out. Another factor is the general shape or silhouette of the tree. If some branches stick up like Alfalfa's rooster tail, this probably diminishes the aesthetic appeal. Most trees have a mind of their own. When a tree has departed from a symmetrical look it is often preferable to work with what you have instead of trying vainly to make it more evenly shaped. In a bonsai garden you will find both symmetrical and asymmetrical trees. So thinning can improve the appeal of a tree and some general shaping can also help at times.
As far as its health goes, a tree will benefit from the removal of dead branches as these attract a myriad of wood-eating insects and fungi. The other health-related issue is that of crossing branches. When branches cross and rub together it not only interrupts the aesthetic flow of the tree's scaffolding but also causes wounds to form in the bark. These open wounds are very attractive to pests. It is best and least expensive to remove crossing branches when they are small-- as each of the crossing branches enlarge over time, so does the problem.
Where we live in the Seattle Wa there are occasional summer storms called micro-bursts. Wow, they can wreak some havoc! As far as I'm concerned, the principle reason for keeping a tree thinned out is to help it weather a storm. If you can see quite a bit of daylight through a tree's foliage then it's a good bet that the wind can slip through without knocking it over. A tree that is too thick is like a boat with too much sail up in high winds--it will have a greater chance of suffering a "knockdown."
We prune a tree to improve the way it looks, improve its health, or make it more storm resistant. If your tree looks good, has no crossing or dead branches, and is not overly thick, then you might be able to wait a year or two before hiring an arborist. I hope that this article will help you assess the needs of your trees and leave you ready to make informed decisions for their care.
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