Crown thinning
Also called: thinning a tree, crown cleaning, canopy thinning, tree thinning, selective pruning, lion tailing
Selectively removing whole branches at their origin throughout a tree's crown — usually 10–20% of the live foliage — to let light and wind through without changing the tree's size or shape. Whole branches cut at the collar, never the ends shortened; 25% a year is the ceiling.
Arborists divide pruning into four named operations, and thinning is the one most residential trims are sold as. Crown cleaning is the base: dead, dying, diseased, broken, crossing and rubbing branches removed. Crown thinning goes a step further and removes a share of the healthy branches too — the small interior ones, the weakly attached ones, the ones competing for the same space — spread evenly through the crown so the tree ends up the same size and silhouette but more open. The reasons are light to the lawn or the house below, less wind resistance in a storm, and less weight on long limbs. The ANSI A300 standard and the ISA put the ceiling at 25% of the live crown in a year, and 10–15% is normal on a mature tree.
Two ways it goes wrong. Over-thinning removes too much and the tree responds with a flush of weak water sprouts along the limbs, sunscald on bark that was shaded, and a loss of the reserves it needed. Lion-tailing removes all the interior branches and leaves tufts of foliage at the ends of the limbs — it’s fast, it looks tidy from the ground for a year, and it puts all the weight and wind load at the tips, where limbs then break. A properly thinned tree has foliage distributed along each limb, from the trunk outward.
On the quote, the word to look for is a percentage and a description: “thin 10–15%, interior and crossing branches, to A300.” On the cost page thinning is in the medium tier’s scope; crown raising is its companion, and topping is what a tree service that doesn’t know the difference does instead.