Tree crown thinning is the selective removal of small, live branches from throughout the tree crown to increase light penetration and air movement.
The crown of the tree consists of leaves, twigs and branches but not the trunk or the roots. It is a job primarily carried out on hardwood tree species. The intent is to maintain a tree’s structure and form while making the overall tree crown less dense and congested.
Tree crown thinning in moderation
So as not to stress the tree unnecessarily and prevent excessive production of epicormic shoots, no more than 30% of the living tree crown should be removed at one time. Epicormic buds, which produce epicormic shoots, lie dormant beneath the bark and their growth is suppressed by hormones from active shoots higher up the plant. Their growth is triggered by a change in conditions such as the removal of a branch or when light levels are increased following removal of nearby plants.
Repeat tree crown thinning
Crown thinning is rarely a once-only operation. It is very likely that repeat pruning will be necessary in order to thin out the tree crown when dense re-growth begins to appear. This is even more true of species that produce abundant epicormic shoots as a reaction to pruning, such as Acer negundo (Box Elder) or Acer sacharinum (Sugar Maple).
Uneven tree crown thinning or over-thinning can look unsightly and prove more costly in the long run as the tree will over compensate by producing masses of epicormic shoots. A common mistake by inexperienced tree surgeons is to remove all the branches from the centre of the crown leaving only the growth at the ends of branches. This is known as “lion tailing”. Lion’s tails can give rise to sun scalding, abundant epicormic growth and can make future tree surgery work much more difficult due to the removal of vital hand and foot holds for the climber.to top