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Philosophy - “Nature as Model”
With a few exceptions, trees have grown and evolved close to each other in forests. Forest-grown trees reach for the sunlight, growing upright with straight trunks. In the relative darkness closer to the ground, lateral branches are shaded, die, and are shed.

Forest trees growing in the open look quite different. They develop large, low branches that get in the way and cause problems. If allowed to develop, codominant stems are weak and can crack.

These problems are expensive to address, with removal often the only option.

Fortunately, pruning can train trees to develop strong architecture, similar to the forest tree. A tree with good structure is an economical tree. It’s easy to care for, holds up well in storms, and is resistant to costly defects.

Young trees
This is a perfect stage to begin structural pruning. Almost the entire tree is “dynamic mass” or living tissue. It responds quickly to training cuts. While the total growth is less after pruning, it is concentrated where it will do the most good—in the central leader. You can think of it like squeezing a water balloon. Squeezing (pruning) the competing stems pushes more growth to the unpruned parts. Along with developing a strong leader, we are planning ahead for the clearance needed under the canopy. 

Contrast this with the way most shade trees are pruned. Low branches are removed as they get in the way. The upper canopy is mostly ignored. Large branches higher up droop down and have to be removed at the trunk, resulting in decay. Interior and low branches are removed to achieve a familiar look, and the structure gets worse. For more extreme examples, see improper pruning.

Established, medium-aged shade trees
Trees that are older than 15 years or so, but are not yet mature.

Assuming the tree has not been carefully pruned since it was planted, it is going to have some issues. You and your trees can still reap the benefits of a more gentle version of structural pruning. Branches with included bark will be thinned or shortened to suppress their growth. While it is still desirable to develop a central leader, it is important not to try to do 25 years of pruning all at once. The pruning dose needs to be smaller than for young trees.

Mature trees
Trees that are at least 75% of their ultimate size, or 40 or more years old.

At this point, most pruning is focused on canopy cleaning. Also, clearance for buildings and traffic needs to be maintained. The older the tree, the less energy it has for closing wounds and walling off decay, so the pruning dose is smaller than for younger trees. If there are structural weaknesses, risk reduction pruning may make them less serious. A support system may extend the life by reducing damage from storms.

At every stage, the well-pruned tree just looks better. The natural beauty of the branching pattern of each species shows. The specimen with good structure has a dignity not found in a sprawling, looming, untrained tree.

Improper pruning
Fortunately, tree topping is much less common than it used to be. Silver maples and crabapples seem to be the favorite targets of this type of abuse. There are plenty of resources online showing the problems with topping. In short, it doesn’t work, it’s ugly, and it makes trees more dangerous.

Unfortunately, over-thinning, sometimes referred to as lion’s tailing, is frequent and widespread. If you look at a tree such as this honeylocust a couple years after this treatment, you will see that it didn’t work. The tree will send out a profusion of sprouts, trying to replace those needed interior branches. Remember the water balloon analogy? When all the interior foliage is removed, all the growth is in the ends of the branches. They don’t develop as much taper, or thickness near the trunk, and are more likely to break.

Specifications
As a tree care consumer, you need to get detailed specifications before having work done. If someone says they are going to “trim it up”, how are you going to know what’s going to happen?

Good pruning follows established industry standards, at a minimum. The  ANSI A300 (American National Standards Institute) standards give performance standards for the care of trees and shrubs. For example, if a tree is to be thinned, ANSI requires the following information be included in the specification: the objective, the part of the tree to be thinned, the size range of parts to be removed, and the pruning dose.

Beyond following the ANSI specs, it takes sensitivity and knowledge of how different species and ages of trees respond to pruning to meet the needs of both the trees and the people. That’s our goal.

Definitions
DBH:  Diameter at 4.5’ from the ground (Diameter Breast Height).

Live crown ratio:  The ratio of the top, foliated portion of the tree to the cleared lower portion.

Central leader:  A dominant stem located more or less in the center of the canopy.

Included bark:  Bark pinched between two stems, caused by a narrow branch angle and resulting in a weak attachment.

Codominant stem:  Two or more stems of approximately the same size, originating in close proximity to each other.

Pruning dose:  The amount of live tissue removed at one pruning, expressed as a percent of the whole tree, or a single stem or branch.

Maximum critical diameter:  The largest diameter pruning cut to be made on a particular species or tree.

Pruning types
Clean:  Removes dead, dying, diseased, damaged, rubbing, or weak branches, 1.5” or greater in diameter.

Thin:  Removes lateral branches from the edge of the canopy, to minimize wind-sail effect and allow air and sunlight to penetrate. Overall size of the canopy is not changed.

Reduce:  Decreases the height and/or spread of the canopy. Reduction cuts shorten a branch or stem back to a lateral branch at least 1/3, preferably 1/2 the diameter, of the cut stem.

Raise/Clearance pruning:  Provides clearance over lawns, walks, drives, and buildings by shortening or removing low-hanging branches. The live crown ratio should not be reduced below 0.6, if possible.

Restore:  Improves the structure of a tree that was previously topped, damaged, or over thinned.

Risk reduction:  Pruning to reduce risk from structural failure, by shortening or removing branches with excessive end weight, included bark, decay or cavities.

Structural pruning:  Pruning that influences the orientation, spacing, growth rate, strength of attachment, and ultimate size of branches and stems, resulting in a strong tree.

See how the aggressive branch on the left side of the tree has been shortened? To develop the strongest structure, this tree will need to be pruned again in about two years.