Gardening with Allen

How to Prune (and not Prune) Large Trees

Large trees growing near homes and other structures can be dangerous during windy weather, especially if they have wounds, rot or weak branch connections. However, far too many healthy large trees are damaged by incorrect pruning. Simply reducing the height of a large tree does not necessarily make it safer. In fact, improper pruning can make a large tree even more dangerous.

Large trees such as fir, pine, maple, oak, and ash grow naturally to heights of 70 feet or more. With the exception of older poplar or cottonwood trees, they are not dangerous just because they have reached their normal mature size.

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 that wind can blow through the trees more readily. This practice is called “wind sailing” because it allows the wind to sail through with less resistance. Wind sailing is not recommended for large spruce and cypress trees, but it is for firs, pines and most deciduous trees. If there is good reason for reducing the height of large trees, they can usually be reduced up to 25% with proper pruning. A large branch can be shortened back to a side branch which is at least one third the size of the branch being shortened. This is sometimes called “drop crotch” pruning. The crotch is the point where two branches connect. If major branches are simply stubbed off at an arbitrary length without regard to side branches, severe damage can occur. If a weak or damaged branch does not have appropriate side branches, it is better to remove it completely back to its origin.

Needle evergreen trees should never have the center trunk topped or shortened. Stubbing or “topping” large branches of broad leaf trees causes them to produce many weak “water sprout” branches which grow straight up. These water sprouts grow very rapidly, quickly reaching the tree’s previous height in a year or two. Because of their weak connections to the larger branches, they are very subject to breakage. The wounds created by stubbing large branches are more subject to insect and disease infection. Some branches will die back, creating an even larger target for infection. The multitude of small branches growing like a witch’s broom on the end of bare larger branches destroys the natural shape and beauty of the tree.

Large poplars and/or cottonwood trees (including the new hybrid poplars) should not be planted close to structures. Their wood is more subject to wind damage than slower growing trees. Large branches are more likely to break regardless of how they are pruned.

If you are concerned about the health or danger of large trees, get an arborist to inspect them. We provide free tree assessment consultations.

Allen Wilson

Allen has been writing about gardening for over 30 years. He is a retired professor of Horticulture. Additional gardening information is available on his web site: naturalpruningnw.com under "how to guide". A monthly email garden newsletter can also be signed up for on this site or by sending a request to allenw98663@yahoo.com.

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