Crown thinning?

What is crown thinning exactly?

Certain species of trees produce more wood than others, some times so much it creates a very congested canopy. What is the problem with that I hear you ask? In times of high winds, the tree with a congested canopy creates what I call the sail effect. This can have a tremendous affect upon the crown of the tree. This is where crown thinning comes in.

So, does this all really matter, well, yes. Bear in mind in Peter Wohlleben’s book “The hidden life of trees” a mature oak in storm conditions can be taking the stress of 200 tonnes on the main trunk, forces of this magnitude are hard to imagine. Not to sound melodramatic let me say it again 200 TONNES that is a lot of stress. Happily, that can be greatly reduced through crown thinning.

So, what does crown thinning involve?

The normal procedure is to remove the tertiary/smaller branches only. This is normally at the outer crown of the tree but not always depending on the species. Lime for example produces a lot of epicormic growth that crowds out the middle of the tree very excessively in some cases. This technique is predominately used in deciduous trees.

Does this affect the overall shape and size of the tree?

Short answer, no. The overall shape and size of the tree should remain intact. We follow quite a systematic procedure where no more than 30% of the crown is thinned out.

Crown Thinning
Example of a tree before and after crown thinning

What is the direct benefit to the tree?

The big one from my point of view is it vastly reduces the sail affect thereby reducing the stress on the trunk of the tree. This increases the longevity of the tree it decreases the risk of storm damage see more on this in my other blog.

It also reduces the weight of the tree; however, this should not be confused with the leverage of the tree. In some cases, it may be necessary to combine both crown thinning and crown reduction.

A five-year plan is the best view for this type of management as it will need doing again on average every 5 years, depending on species and seasons.

How does it benefit me?

Some species of trees such as sycamore a quite light greedy leaving you and your garden like being in a bit of a dark room. Other plants are stunted and your own pleasure of your garden limited. The result of a crown thin usually leaves dappled shade that is more pleasant and less extreme that total shade. In my experience you gain the best of both worlds some sunshine and some shade, a win-win.

For further information and a good source of background knowledge look up Haynes tree owners’ workshop manual.