Crown thinning is the removal of smaller / tertiary branches, usually towards the outer portion of the crown.
The aim is to produce a uniform density of foliage and an evenly spaced branch structure. Crown thinning is only really effective on non-coniferous trees, like birch, oak, cherry and sycamore. Crown thinning does not alter the overall size or shape of the tree.
Material should be removed systematically throughout the tree; should not exceed the stated percentage; and not more than 30% overall.
Crown thinning allows more light to pass through the tree; reduces wind resistance; reduces weight – but does not necessarily reduce leverage on the tree structure; and is rarely a once-only operation, particularly on species that are known to produce large amounts of epicormic growth, such as lime.