Pruning Basics

Structural training is a multi-year investment requiring evaluation and corrective pruning on an annual basis.  Young trees require little pruning.  However, the training a tree receives while in the early “growth phase” of its life cycle determines its structural integrity for life.  Many trees become prone to wind and snow damage as they mature due to the lack of structural training while young.  Proper structural training of the young tree makes it especially resilient to storm damage when mature. 

Note: For additional information on a tree’s life cycle, refer to CMG GardenNotes #111, Plant Health Care.  For additional information on branch collar develop refer to CMG GardenNotes #612, Developing Strong Branch Unions.

How much to remove

The amount of foliage/live wood that can be removed per season depends on the growth rate of the tree.  In the “growth phase” of the life cycle, 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. 

In structural pruning, this limitation on the amount of wood that should be removed per year means that it generally takes several years to develop the branching structure desired.  In purchasing trees, select trees that will require minimal pruning to develop the desired structure.

Time of year

Structural pruning is typically done late winter, before trees break dormancy.  Pruning is generally avoided during the spring growth flush as bark is rather tender at this point in time.  Mid-summer pruning is preferred for tree species prone to bleeding if spring pruned (including Birch, Black Locust, Elms, Goldenchain tree, Hackberry, Japanese Pagodatree, Kentucky Coffeetree, Maples, Mulberry, Poplars, Walnuts and Willows).

Size of branches

Ideally, all pruning cuts are two inches in diameter and smaller.  The structural training stage ends when pruning cuts would be greater than two inches.  Any pruning cut four inches and larger must be justified taking into account the potential for decay.

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.  Table 1 shows comparisons in pruning objective of excurrent trees and decurrent trees.

Table 1 – Structural Training Objectives for Young Trees
	Excurrent Trees Single trunk to the top	Decurrent Trees Scaffold branches arise from the trunk becoming the main structural system
Training Objective 1	Remove broken, damaged and competing branches
Training Objective 2	Maintain single trunk to top of tree.	Maintain single dominant trunk to at least 2/3s of the tree’s mature height.
Training Objective 3	Select lowest permanent branch	Select lowest scaffold branch
Training Objective 4	Maintain diameter of all branches less than ½ the diameter of adjacent trunk.	Select other scaffold branches
Training Objective 5	Manage temporary branches removing them over time (before they reach a two inch diameter).

Pruning at planting

Pruning of a newly planted tree should be limited to 1) minimal pruning to maintain a single trunk and 2) removing broken branches.  The hormone Auxin (produced in the twig’s growing tips) stimulates root growth.  Thus heavy pruning slows root regeneration.  As roots establish, Gibberellins (hormones produced in the root growing tips) stimulate canopy growth.  [Figure 1]

Figure 1.  Heavy pruning at planting slows root growth.

Structural training of the young tree begins for the gardener when the canopy shows significant twig growth after the root establishment phase in its life cycle.  This indicates that the root system has adequately reestablished to support branch and leaf growth.

In USDA hardiness zone 5 with good soil and planting techniques, it takes approximately one growing season for the roots of a one inch diameter tree to establish a root system supportive to canopy growth.  Approximately two years on a two inch diameter tree, three years on a three inch diameter tree, and so forth.  (Note: Trunk diameter is measured on small trees at six inches above soil line.)  In Colorado mountain communities with shorter growing seasons and trees planted into poor soil conditions a longer root establishment period may be required.

Training Objective 1 – Remove Broken, Damaged, and Competing Branches

Standard in any pruning job, the first training objective is to remove broken, damaged, or competing branches.  Competing branches are branches growing in the same space with the potential to rub and damage others.  [See Figure 2]

 

Training Objective 2 – Develop Trunk

The primary pruning objective here is to eliminate multiple 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.  It may be helpful to loosely tie the new leader to a stick to bend it to an upward orientation.

Co-dominant trunks

In training trees, arborists have zero tolerance for co-dominant trunks (adjacent trunks of similar diameter).  Co-dominant trunks account for the majority of wind and snow related tree failures in Colorado.

With co-dominant trunks, no branch collar develops to knit the two trunks together.  The branch collar is the area where trunk wood wraps around the branch wood creating a structurally strong branch union.  The branch union (crotch) is structurally weak and prone to breakage as the trunks reach a size greater than 3-4 inches in diameter.  [Figure 3]

Note: In selecting a tree, it is advisable to avoid purchasing trees with co-dominant trunks.

Excurrent Trees – Maintain single trunk to top of tree

On excurrent (central leader) trees, maintain a single trunk to the top of the tree.  If a side branch beings growing upright in a trunk-like fashion, prune it back to redirecting the growth to in an outward direction.  Do not prune or “head back” the central leader (trunk).  [Figure 4]

                                                                    Figure 4.  On excurrent trees, maintain a single trunk to the top.

Decurrent Trees – Maintain single dominant trunk to at least 2/3s of the tree’s mature height

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. 

On decurrent trees maintain a single dominant trunk to at least two-thirds 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.  By training, secondary trunks are avoided.  [Figure 5]

                 Figure 5.  On decurrent trees maintain a single dominant trunk to at least two-thirds
                 of the tree’s mature height.

If vigorous growing side branches being to choke out the central leader, heavy pruning will be required over time to reestablish a central leader.  Some tree species naturally put out many upward growing secondary trunks.  Again, heavy pruning over a period of years will be desirable to establish a dominant central leader with subordinate smaller side branches.

Do not “head-back” (prune) the central leader.

Training Objective 3 – Select Lowest Branch

It is often desirable to raise the canopy (remove lower branches) so they are out of the way of people activities like mowing the lawn and lawn games.  For shade trees in lawns, patios, and along sidewalks, the lowest permanent branch generally starts 7 to 10 feet above ground level.  On smaller specimen trees in a garden bed, lower branching may be preferred.  Along streets, lowest branches start at 14 feet.  In wooded settings, raise the canopy to 10 feet as a fire prevention technique.

Many gardeners mistakenly plan to remove lower branches as the tree reaches a more mature size.  Removing these larger branches as the tree matures opens the tree to internal decay.  On decurrent trees, these lower branches typically make-up a significant portion of the tree. 

The objective is to identify what will be the lowest permanent branch at this early time in life, allowing the gardener to management and remove lower branches over time.   Branches below the lowest permanent branch are call temporary branches.  Management and removal of the temporary branches are discussed in Objective 5.

The lowest branch on any tree should never be higher than 40% of the tree height.  In establishing the lowest branch, don’t “limb-up” a young tree too early in its growth.  To develop a trunk taper resilient to wind, one-half of the leafing area should be found in the lower 2/3s of the tree.  Lower temporary branches should be removed only as the tree expands in height, but before they reach two-inch diameter.  (Refer to Objective 5 for details).  [Figure 6]

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. 

Training Objective 4 – Developing Branching Structure

In Objective 4, branches are management differently for excurrent and decurrent trees.

Excurrent Trees — Maintain diameter of all branches less than 1/2 the trunk diameter

For structural integrity, side branches must be less than one-half the diameter of the adjacent trunk.  Less than one-third is preferred.  Without this important size ratio, the branch collar fails to develop, creating a weak branch union.  [Figure 7]

                        Figure 7.  For a branch collar to develop the side branch must be less than one-half the
                        diameter of the adjacent trunk.

If the diameter of a branch is growing too fast compared to the trunk, prune the branch back to slow its growth rate.

Spacing of branches along the trunk is not a critical structural issue on excurrent trees, as long as the trunk to side branch ratio is within limits.  Many species of excurrent trees develop branches in a whorl.  This is structurally acceptable as long as the branch to trunk size ratios are is within limits.  On some species of tree, thinning of competing branches (branching growing in the same space with the potential to rub and damage each other) may be desirable.   

Decurrent Trees – Select other scaffold branches

In structural pruning of decurrent trees, an overall intent is to guide development of the branching structure, creating scaffold branches and eliminating secondary trunks.  The intent is create strong branch unions with a branch collar.  For the branch collar to develop the branch must be less than one-half (less than one-third preferable) the size of the adjacent trunk.  Without the branch collar, secondary trunks are structurally weak and prone to breakage as the tree matures.

The selection of other scaffold branches takes place over a period of years as the tree grows in height.  Branches along the trunk not destined to become a scaffold branch are managed as temporary branches or removed entirely. 

In selecting other scaffold branches, consider branch spacing and branch union (crotch) angles.  In an open landscape setting, decurrent trees naturally develop more branches than is desirable predisposing the tree to wind and snow damage as the tree matures.  The objective of training is to correct this situation while the tree is young. 

Branch spacing — Spacing for scaffold branches depends on the mature height of the tree.  The rule of thumb is to allow at least 6 inches per 10 feet of mature tree height.  The table below show spacing for various mature heights.

Table 2 – Minimum Spacing for Scaffold Branches
Mature Tree Height	Minimum Scaffold Branch Spacing
20 feet	1 foot
30 feet	1.5 feet
40 feet	2 feet
50 feet	2.5 feet
60 feet	3.0 feet
70 feet	3.5 feet
80 feet	4.0 feet

Select scaffold branches with even distribution around the tree trunk.  Where a scaffold branch is growing directly above another, vertical spacing should be at least 60 inches on trees with a mature height of 30 feet and taller (18 to 36 inches on smaller trees).  [Figure 8]

Multiple branching at one location — When multiple scaffold branches arise from the same area, the branch collars cannot knit together into a strong branch union.  These branches become vulnerable to wind and snow damage.  In training a young decurrent tree, eliminate multiple branches arising at the same location.  Many common shade trees, including maples, cottonwoods, poplars, and elms naturally develop multiple branching at the same location.  [Figure 9]

Branch union angles – The problem with a narrow branch union (crotch) angle is the development of included bark (bark against bark inside the branch union) as the tree grows.  With included bark, the branch collar can’t wrap the trunk wood around the side branch wood, creating a weak branch union.   A branch union with a wide angle of attachment is also more resistant to the spread of decay.

In selecting scaffold branches, select outward growing branches with a wide angle of attachment rather than upward growing branches.

Training Objective 5 – Manage Temporary Branches, Removing Them Over Time

Temporary branches on the lower trunk are important to the tree’s early growth.  Photosynthates (carbohydrates and proteins produced by photosynthesis) produced in lower canopy help develop the natural trunk taper giving wind resilience.  Shading by the lower foliage helps reduce sunscald of the tender bark.

Manage growth on temporary branches by keeping them short and 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.   Pruning back a temporary branch slows the growth giving more time before the branch must be removed due to size.

Keeping temporary branches short suppresses their rapid growth while encouraging the desired growth up in the scaffold branch structure.  During the early training process, a young tree will have a cylinder of short temporary branches along the lower trunk (below the lowest permanent branch), with the tree’s significant growth developing up in the permanent branch structure.  [Figure 10]

Preferred vertical spacing of temporary branches is 4 to 6 inches.  Thus some branches would be removed outright.  On decurrent trees, no temporary branch should be within 6 inches of a scaffold branch.   Branches between scaffold branches are also considered temporary branches.  Maintain these temporary branches for 1 to 5 years, removing them before they reach a two-inch diameter. 

On decurrent trees it generally takes several years to manage and eventually remove temporary branches.  Remember that the total amount of foliage that can be removed per season depends on the growth rate of the tree.  In purchasing, select trees that require minimum corrective pruning to make them structurally sound.

Additional Information – CMG GardenNotes on pruning

• #611   Tree Growth and Decay
• #612   Developing Strong Branch Unions
• #613   Pruning Cuts
• #614   Structural Training of Young Shade Trees
• #615   Structural Training– Pruning Flow Chart
• #616   Pruning Mature Shade Trees
• #618   Pruning Evergreens
• #619   Pruning Flowering Shrubs
• #620   Structural Pruning Summary

Books – Edward F Gilman. An Illustrated Guide to Pruning, Second Edition. Delmar. 2002

Web – http://hort.ifas.ufl.edu/woody/pruning/

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