Gardening with Allen

Is Your Tree Too Big?

I am often asked to evaluate a situation where home owners want to drastically reduce the size of a large tree. When I ask them why they want to reduce a mature tree’s size, the response is often “It is just too big. We didn’t realize it was going to get this big when we planted it.” They are often worried about what might happen if the wind blew the tree over.

I start by explaining that the tree is just growing to its natural height. The maximum size reduction that should be made in a tree is 20 to 25%. When properly pruned by that much it normally takes 2 years until it regrows to its former natural height with strong branches. Home owners or unscrupulous arborists will sometimes prune a tree more than 25%, but the damage done to the tree destroys its natural growth and often kills the tree within a few years.

The worst type of pruning that can be done is to stub back all major branches to an arbitrary height. This is referred to as “topping”. Topped trees typically regrow many slender, weak branches. A mild wind storm will often snap a number of branches and they end up littering the ground. The tree will still regrow to its former size in 2 years or less. The major wounds made by shortening large branches are often infected with damaging insects or diseases.

So What Can Be Done?

The first and best response is to accept the size of the tree and appreciate it for its beauty and shade producing qualities. Unless it is growing in some kind of precarious situation, it is not likely to suffer from normal storm damage. Tree species which are more subject to storm damage can be identified by an arborist for removal.

If the tree has developed branches which are too close to a building and could cause damage, some of the offending branches may be removed entirely. When branches are removed back to their origin, only limited regrowth occurs.

If the tree is causing so much shade that other plants are having a difficult time growing underneath it can be pruned in 2 ways. First, some of the lower branches can be removed so that more light shines underneath the tree. Second, some upper branches can be thinned out so there is more sunlight coming through the tree. Branches should be removed back to their origin or to a side branch at least 1/3 the size of the branch being removed.

And if none of these solutions will solve the problems caused by the tree, it should be removed. If the tree is pruned incorrectly in order to reduce it in size it will need to be removed any way in a few years after it develops the problems described above.

A licensed arborist who is certified by the International Society of Arborists will give correct advice about pruning large trees.

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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