Crown thinning is the removal of a portion of smaller/ tertiary branches, usually at the outer crown, to produce a uniform density of foliage around an evenly spaced branch structure. Crown thinning  does not alter the overall shape of the tree.  Material should be removed systematically throughout the tree, should not exceed the stated percentage and not more than 30 % overall.

Common reasons for crown thinning are to allow more light to pass through the tree, reduce wind resistance and to reduce the overall weight.