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 (including 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.

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 in an outward direction.  Do not prune or “head back” the central leader (trunk). 

On decurrent trees, maintain 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 a 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 at this early time in life, allowing the gardener to manage and remove lower branches over time.  Branches below the lowest permanent branch are called  temporary branches. 

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 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.

	Excurrent Trees Single trunk to the top	Decurrent Trees Scaffold branches arise from the trunk becoming the main structural system
Objective 1	Remove broken, damaged and competing branches.
Objective 2	Maintain single trunk to top of tree.	Maintain single dominant trunk to at least 2/3s of the tree’s mature height.
Objective 3	Select lowest permanent branch	Select lowest scaffold branch
Objective 4	Maintain diameter of all branches less than ½ the diameter of adjacent trunk.	Select other scaffold branches (Scaffold branch must be less than ½ the diameter of adjacent trunk.)
Objective 5	Manage growth on temporary branches by routinely pruning them back, and eventually removing them over time as the tree grows in height.  Ideally, temporary branches are pruned back to a few buds.  On temporary branches that have grown significantly before training begins, start by cutting them back by about 50%, removing more over time.  Temporary branches are removed before they reach a two inch diameter

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