Crown thinning involves the removal of a portion of smaller or tertiary branches, usually situated at the outer section of the crown. It is important to produce a uniformity within the density of the foliage around an evenly spaced branch structure.
Crown thinning is usually confined to broad-leaved species because of the density of the foliage. It is important to note that crown thinning, unlike crown reduction, does not alter the overall size or shape of the tree when the work is completed.
When carrying out a thinning exercise, the material should be removed systematically throughout the trees foliage, and so as not to cause harm to the tree. This should not exceed the recommended percentage of not more than 30% of the foliage overall.
The common reasons for crown thinning are to allow more light passing through the tree, therefore helping other plants and shrubs to gather the light they need to survive and flourish, but is also a recommended treatment to reduce wind resistance where high winds are expected or common in unsheltered areas. The treatment can also greatly reduce weight (but this does not necessarily reduce leverage on the structure) and is rarely a once-only operation particularly on species that are known to produce large amounts of epicormic growth.