Structural Training Summary
for Young Shade Trees

How much to remove

The amount of foliage/live wood that can be removed per season depends on the growth rate of the tree.  Young, vigorously growing trees without stress or growth limiting factors may have up to 25% of the foliage removed by pruning per season.  On trees with reduced growth rates (due to growth limiting factors such as dry sites and restricted root spread) generally 10% to 15% of the foliage is maximum per season (based on actual growth of the tree).  Do not remove live wood and foliage on trees showing stress.  Excessive removal of foliage puts the root system under stress. 

Ideally, all pruning cuts are two inches in diameter and smaller. 

Growth habit

The desired branching structure depends on the natural growth habit of the tree.  Trees with an excurrent growth habit develop with a central leader (single trunk) to the top.  Examples of excurrent trees include Aspen, Linden and pines.  Trees with a decurrent growth habit develop a more rounded form with multiple scaffold branches (secondary trunk-like branches) or secondary trunks originating from the trunk.  Examples of decurrent trees include Maple, Ash, Elm and Honeylocust. 

Developing Trunk

The primary pruning objective is to eliminate multiple secondary and co-dominant trunks.  If multiple trunks start to develop, remove all but one.  If the leader is killed, select a side branch to become the new leader, removing competition.  [Figure 1]

Figure 1. Co-dominant trunks – A branch union with two trunks of similar size is structurally weak and prone to storm damage.  “Included bark” (hidden bark) between the trunks prevents the wood from growing together.  In structural pruning, there is zero tolerance for co-dominant trunks.

On excurrent (central leader) trees, maintain a single trunk to the top of the tree.  If a side branch begins growing upright in a trunk-like fashion, prune it back, redirecting growth to an outward direction.  Do not prune or “head back” the central leader (trunk). 

On decurrent trees, maintain a single dominant trunk to at least 2/3s of the tree’s mature height.  For example, if the mature tree height is 30 feet, a single trunk should dominate to at least 20 feet.  If the mature tree reaches 60 foot, a single trunk dominates to at least 40 feet.  Scaffold branches become the secondary framework of the tree.  Through training, secondary trunks are avoided.  Do not “head-back” (prune) the central leader.

The overall objective with decurrent trees is to develop a structural system of scaffold branches rather than secondary trunks.  Scaffold branches are the major structural, trunk-like branches that originate off of the trunk.  By definition, a scaffold branch must be less than one-half the size of the adjacent trunk.  Less than one-third is preferred.  This allows for a branch collar to develop creating a structurally strong branch union.  In contrast, “secondary trunks” lack the size relationship for branch collar development creating structurally weak branch unions.  In an open landscape setting, most decurrent trees naturally develop multiple secondary trunks arising at the same location predisposing the tree to storm damage. 

Managing Side Branches

Selecting lowest branch

The objective is to identify what will become the lowest permanent branch early in the tree’s life, allowing the gardener to manage and remove lower branches over time.  Branches below the lowest permanent branch are called temporary branches.

Figure 2. To develop a strong trunk taper, at least one-half of the foliage must be in the lower two-thirds of the tree.  Temporary branches below the lowest permanent branches will be removed over time, as the tree grows in height.

On excurrent trees, select the lowest permanent branch.  Branches below this point become temporary branches. 

On decurrent trees, select the lowest (first) scaffold branch.  Other scaffold branches will be selected based on the location of this branch.  Branches below the lowest (first) scaffold branch become temporary branches.

Developing branching structure

For a branch collar to develop (creating a structurally strong branch union) the side branch must be less than one-half the diameter of the adjacent trunk.  Less than one-third is preferred.  If a side branch is growing too fast, compared to the trunk, prune back the side-branch to slow the growth.

In structural training of decurrent trees, the overall intent is to guide development of the branching structure, creating scaffold branches and eliminating secondary trunks.  Minimum spacing on scaffold branches is based on the mature height of the tree, based on the formula of six inches per ten feet of mature height.  For example, a tree with a mature height of 30 feet should have scaffold branches spaced at least 18 inches apart.