Thinning and Reducing

Thinning the crown

Thinning is the selective removal of live branches to reduce crown density. Thinning removes branches from the end portion of limbs. Pruning cuts to thin the crown of a large tree typically range in size from one to four inches in diameter. For purposes of reducing damage from wind, cuts smaller than about one inch in diameter appears to be less effective. However, small cuts of one-quarter to one-half-inch in diameter can be used to thin small ornamental trees such as Japanese maple (Acer palmatum), crabapples (Malus spp.), and citrus (Citrus spp.) for aesthetics and other reasons. Proper thinning does not change the overall size of the tree.

Thinning, of various types, is the most commonly-practiced method of pruning on mature trees in some regions. Thinning allows wind to pass through or into the pruned section of the crown, which can temporarily improve durability in a storm or sunlight penetration to the ground. Perhaps the best use of crown thinning in the landscape is to reduce mechanical stress on selected limbs from gravity, wind, ice, or snow on branches with structural defects such as cracks, cavities, and hollows, and on codominant, overextended, or long branches.

A word of caution:
Unfortunately, many people prune when it is not necessary, or they misapply or misunderstand thinning, and subscribe to the practice of only removing branches from the interior of the crown. This is often referred to as lions-tailing, overlifting, overthinning, stripping out the interior, or cleaning out. Little or nothing is removed from the ends of the limbs, and this is a mistake. Regions of the world where thinning is rarely practiced do not appear to experience more storm-related tree damage than regions where thinning is common. Thinning is conducted on large or small trees for a number of other reasons. Although entire-tree thinning can be useful, structural pruning should be considered before the entire crown is thinned. In fact, structural pruning is probably the best way to thin. A practical approach would be to structurally prune to reduce defects, then thin to shape and balance to the desired crown aesthetics if maximum pruning dose has not been exceeded. Other than to avoid storm damage, entire-tree thinning of landscape trees is mostly an aesthetic practice that lasts only a short time.

Questions to consider when deciding to thin the crown

Does this tree have a healthy tree structure (i.e. straight singular central leader)?

Can this tree tolerate increased pruning?

Progression of tree thinning:

Is there a short-term need for pruning?

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Reducing the Crown

Unlike a removal cut, reduction cuts back to a weak natural boundary resulting in internal wood discoloration and decay behind pruning cuts, especially in weak compartmentalizers, in drought, or in trees in poor health. Although reduction cuts are preferred when reducing crown size, heading cuts are sometimes necessary. Crown reduction can lead to dead bark on top of retained branches from sudden sun exposure; this is very damaging to trees. For these reasons, it is best not to reduce the entire crown if at all possible, especially on mature trees. However, reduction is a useful pruning method which has many applications, especially when applied to selected portions of the crown for specific objectives. For example, it is a useful means of retaining very old trees. Trees have endured crown reduction since arriving on the planet having been reduced in size as they break in storms.

The objective is to make reduction cuts so lateral branch tips remain intact on the outer edge of the new, smaller crown (right photograph). Ideally, pruning cuts should be hidden and the crown periphery should look a bit jagged.

A word of caution:

Resist the temptation to create a smooth crown outline by pruning retained lateral branches because these are destined to become the main branches on the new, smaller crown. Heading (topping), shearing, tipping, lopping, or rounding-over are considered substandard techniques for reducing the size of a shade tree because they compromise the tree’s structure. If more than twenty-five or thirty percent of the foliage will be removed on anything but a young tree, consider dividing the job into two sessions, one growing season apart, to minimize starch (energy) removal and stress-induced sprouting.

Reducing the crown should not be used to bring down the size of trees of large diameter. This is called topping and is not recommended because it damages trees permanently. Heading large-diameter branches causes many problems including massive decay in many species, weakly attached sprouts, and many dead branch stubs. Topping occasionally kills trees. If a tree needs regular pruning to keep it small, perhaps the wrong tree was planted in that location. Consider replanting with an appropriately sized plant, or else move the obstacle you were pruning the tree away from and structurally prune to develop strong structure.

Maintaining old trees

Before reduction

Examples of pollarding which maintains trees at the same height forever. New shoots are removed back to the same point, the pollard head, annually. Trees in the photograph are 140 years old.

Reduce breakage risk

Huge limb with severe crack

Closeup shows two large cracks

After reducing the left side of crown only

Case 1:

The enormous limb with two large cracks (second limb from the ground on left side of tree) is very weak. The closeup shows both cracks clearly. This limb is a good candidate for reduction which should reduce the likelihood of the branch breaking.

Case 2:

The tree forks into two codominant stems about five feet from the ground. The entire tree needs to be reduced to help prevent the tree from splitting apart. Only the left side of the crown was reduced in the right photograph. The right side of the crown also needs to be reduced.

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