Thinning reduces the density of the crown through the removal of lower order branches, whilst retaining the main structural branches of the tree.
Thinning pruning is carried out to increase light penetration and air movement through the crown. It is also used when formative pruning, or when restoring views.
There are limits to the extent of thinning that can be conducted in trees without adverse impacts on tree health and appearance. These limits are tree age and species specific.
The percentage of thinning conducted and the maximum diameter of branches being removed must be specified by a qualified arborist with suitable experience and training prior to the works being conducted.
A common failure of misapplied thinning is the effect known as ‘Lions tailing’ where only lower order branches are removed. This can lead to branch instability and a higher likelihood of branch failure.