Gardening with Allen: Topping weakens trees

We planted two maple trees near our house 15 years ago that grew to over 50 feet tall. We are worried that they will break and damage our house during a storm. A friend with a chainsaw says he can cut them down by about a third. Is it a good idea?

Arbitrarily shortening all the branches of a mature tree is called “topping”. Garnishing is never a good idea. Simply reducing the height of a large tree does not make it safer. In fact, improper pruning can make a large tree even more dangerous. The trees will respond by rapidly returning to their normal mature size within a year or two. But there will be a burst of five to 10 times more thin, weaker branches that will be loosely attached and much more easily snapped in a storm. It will take much less wind to knock them down.

Wounds created by crushing large branches are more prone to infection by insects and disease. Some of them will die, creating an even bigger target for the infection. The multitude of small branches growing like a witch’s broomstick from the top of large bare branches destroys the natural shape and beauty of the tree.

If you are concerned about wind damage to trees, the best way to prune them is to open them up by removing some of the inner branches so the wind can blow through the trees more easily. This practice is often called “wind sailing” because it allows the wind to pass through with less resistance. Inward or upward growing branches are removed, leaving outward growing branches.

While there are good reasons to reduce the height of large trees, they can usually be reduced by up to 20% with proper pruning. Larger branches can be shortened into a side branch that is at least one-third the size of the shortened branch. This is sometimes referred to as the “inseam” size. The crotch is the point where two legs connect. If the main branches are simply cut to an arbitrary length without considering the side branches, serious damage can occur. If a weak or damaged branch does not have proper side branches, it is best to remove it completely back to its origin.

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