Colorado Master Gardener Program | Yard and Garden Publications | CMG GardenNotes #616

Pruning Maturing Shade Trees

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When to Hire a Professional Certified Arborist

Pruning large trees is a safety issue beyond the training and experience of home gardeners.  Hiring a bonded professional is the best approach for most tree pruning jobs.  Look for arborists with certification from the International Society of Arboriculture, ISA.   Many are listed in the phone book yellow pages and a list of ISA Certified Arborists working in the area can be found on the ISA web site at www.isa-arbor.com.  Also, ask about liability insurance coverage. 

This CMG GardenNotes is written to help the home gardener understand issues around pruning of mature trees and help them communicate with their certified arborist.

Pruning Objectives for Maturing Trees

As trees mature, pruning should be based on pruning objectives.  Do not indiscriminately remove branches.  Pruning objectives (why to prune) determine methods (how to prune) to be used, which in turn determine the type of pruning cuts made.  Table 1 lists common objectives, methods and types of pruning cuts.


Table 1.
Objectives, Methods and Cuts for Pruning Maturing Trees
Pruning Objectives
(why to prune)

Pruning Methods
(how to prune)
Pruning Cuts
Reduce risk of failure (wind and snow)
Improve structure
Maintain health
Improve aesthetics
provide clearance
Improve view
Reduce shade
Influence flowering and fruiting
Structural
Cleaning
Thinning
Raising
Reducing
Restoring
Pollarding

Removal cut
Reduction cut
Heading Cut


Limitation On How Much Can Be Removed

Do not indiscriminately remove branches with live foliage as this can add stress to the tree.  The amount of live wood and foliage that can be removed per season depends on the growth rate of the tree.  As a rule-of-thumb for healthy trees, 10-15% of the live foliage may be removed per season.  For actively growing medium-aged trees without growth limitation factors (such as a dry site or restricted rooting spread), up to 20% of the foliage may be removed per season.  For young actively growing trees (growth phase of life cycle) without growth limiting factors up to 25% of foliage may be removed per season.

More severe pruning slows root growth by shifting the root to shoot growth ratio.  This adds significant stress to the tree.  Heavy pruning also reduces carbohydrate reserves, making the tree less tolerant of insects, diseases, and drought stress.  Do not remove live wood and foliage from trees showing stress.

General Pruning Guidelines

  • To minimize the potential for decay, make removal cuts on branches with a branch collar when ever possible.  For details, refer to CMG Garden Notes #613, Pruning Cuts.

  • Ideally, pruning cuts are made on branches two inches and less in diameter.  These small wounds minimize the potential for internal decay.  Unless there is a strong justification, (taking into account the potential for a decay column) avoid removing branches larger than four inches in diameter.  Large wounds predispose some trees to internal decay.

  • To maintain overall tree vigor, at least one-half of the foliage should be in the lower two-thirds of the tree.  The lowest limb should originate in the bottom one-third of the tree’s height.

  • Pruning should maintain the tree’s natural shape. 

  • Avoid “lion-tailing” where the small twiggy inner foliage is removed on the lower scaffold branches and secondary trunks.  This shifts weight to the ends of branches increasing the potential for wind damage.  It also reduces the carbohydrate reserves in the lower branching structure decreasing resilience to stress factors.

  • Avoid topping a tree.  Topping opens the tree to internal decay.  Regrowth of watersprouts (adventitious shoots) is structurally unsound. 

  • Written specification for any pruning job should include the following:

  • Clearly state which tree(s) will be pruned.

  • Clearly indicate the objectives (whys)for pruning (such as reduce risk of failure due to wind damage or snow loading, reduce shade, manage health, improve aesthetics, provide clearance, improve view, influence flowering and fruiting).

  • Specify pruning methods (hows) to meet the objectives (that is structural pruning, cleaning, thinning, raising, reducing, restoration pruning).

  • State the size specification for the minimum and/or maximum branch size to be removed.  For example, “Cuts shall be made on branches two inches and less in diameter” and “In a reduction cut, the side branch pruned back to shall be at least one-third the diameter of the branch removed.”

  • Specify the maximum amount (percentage) of live tissue that can be removed.  For example, “Pruning shall not remove more than 15% of the live crown.”

  • In writing pruning specifications, the word “should” refers to a practice that is routine and recommended.  The word “shall” refers to a practice that is mandatory.

  • Include these generic safety statements to reduce the homeowner’s and pruning crew’s liability.  “All work shall be preformed in accordance with American National Standards Institute (ANSI) A300 Pruning Standards and Z133.3 Safety Standards.”  and “All work shall be preformed under the supervision of a licensed, ISA Certified arborist.”

Structural Pruning

In Colorado (and other snowy climates), most storm damage in trees is due to co-dominant trunks (trunks of similar size).   Structural problems of this type should have been corrected while the tree was in the early growth stage.  Arborists have a limited potential to correct structural defects on middle-age and mature trees without predisposing the tree to internal decay and creating an unsightly shaped tree.  [Figure 1]

Storm damage on codominant trunks

Figure 1.  Co-dominant trunks (adjacent trunks of similar size) account for the majority of storm damage in Colorado landscapes.



Structural training centers around developing a dominant trunk with subordinate side branches and secondary limbs.  It requires annual pruning over a period of years, rather than an occasional one-time pruning.  For additional details on structural training, refer to CMG GardenNotes #614, Structural Training of Young Shade Trees and #617, Structural Pruning on Maturing Trees.

Cleaning

Cleaning is the removal of dead, diseased, detached, cracked, and broken branches.  This type of pruning is done to reduce the risk of branch failure, improve tree appearance, and to reduce the spread of insects and diseases.  Most pruning of middle-aged and mature trees falls into this type.  Trees under stress or declining trees may need cleaning every few months to years.  All dead wood may be removed at one time.  In cleaning, do not remove healthy branches and live foliage.  Do not clean out healthy growth in the tree’s interior.  [Figure 2]

Dead branches on old cottonwood

Figure 2.  This old cottonwood needing cleaning to remove dead branches and reduce the risks associated with branch failure.



Removing dead branches – To minimize risk if the branch were to fail, it is advisable to remove any dead branch larger than a two-inch diameter and higher than 30 feet.  Dead branches may also become a source of insect and disease pressure in the tree. 

Remove the dead branches using the three-step pruning technique.  For details refer to CMG GardenNotes #613, Pruning Cuts.  Do not cut into the branch collar which would open a high potential for decay to spread into the trunk.  If live wood has began to grow out along the dead limb, cut just beyond the live wood being cautious not to nick the live tissue.  Never “flush cut” the dead branch.  [Figure 3]

Removing dead branch

Figure 3. When removing dead branches, do not cut into the living tissues.



Written specifications for cleaning should specify the minimum size of dead branches to be removed.  For example, “Clean branches one inch diameter and larger” or “Clean branches two inches in diameter and larger that are 30 feet and higher above the ground.”  The location of the branch to be removed should be specific if the entire crown is not going to be cleaned.

Thinning

Thinning is the selective removal of smaller branches (½ inch to 2.5 inches in diameter) to reduce crown density.  Because the majority of small branches are in the outer canopy, thinning focuses in this area.  Thinning should retain crown shape and size, and provide an even distribution of foliage throughout the crown.  Removal cuts are primarily used.   [Figure 4] 

Branch thinning

Figure 4.  Thinning is the selective removal of small branches, growing parallel to each other, in the leafy upper/outer tree canopy.



Because thinning is in the upper/outer canopy, it requires a trained arborist with a high level of skill.  Thinning is expensive, often running $500 to over $1,000 per large tree when done correctly.   

Thinning can include removal of suckers from the base of the tree and some waterspouts on the interior.  Excessive removal of watersprouts at one time often promotes growth of additional watersprouts, and should be avoided.

Benefits of Thinning

  • Thinning is the best way to minimize potential damage caused by snow loading, a primary situation leading to tree failures in Colorado.  Thinning can reduce limb weight in order to compensate for structural defects.

  • Thinning increases light penetration into the tree interior.  This can invigorate the tree and help retain the tree’s natural shape.  Thinning may adequately reduce shade for shade tolerant under story plants below the tree.  However, thinning middle-aged and mature trees will not adequately promote growth of sun loving plants like Kentucky bluegrass.

  • Thinning is a technique to partially open a view without removing or structurally impacting a tree.  This is often referred to as vista pruning.

  • On A tall tree, thinning may not be an effective technique to reduce wind sail and potential for breakage in strong winds.  Reducing is the most effective way to deal with wind loading issues.

As a point of clarification, thinning is done on relatively small branches in the leafy upper/outer canopy.  Thinning is not removing large lower branches which could create gaps in the crown and encourage watersprouts.  Thinning is not removal of the small twiggy branches in the inner canopy.  Thinning will not significantly lower a tree’s height.  [Figure 5]

Tree thinning

Figure 5. 
Left – Thinning focuses on small branches in the upper/outer tree canopy. 
Right – Thinning does NOT remove large branches, creating a gap in the tree canopy.



Avoid “lion-tailing” which is the removal of the live small leafy twigs down in the tree’s interior.  These small interior branches are critical to the trunk’s structural integrity and vigor.  They also serve to dampen tree sway in wind.  Lion-tailing shifts the wind loading to the outer canopy increasing the tree’s potential for wind damage.  [Figure 6]

lion tailing

Figure 6.  Do not “Lion Tail” trees as in the photo. Removal of the smaller twiggy wood in the inner tree canopy decreases vigor on the major branches and trunk and shifts the weight to the top increasing the potential for wind damage.



Written specifications for a thinning job should specify the following:

  • Clarify what percent of the tree’s canopy may be removed.  For example, “Pruning should not exceed 15% of the total live canopy.”
  • Clarify where in the tree the pruning will occur.  For example, “Pruning shall occur in the outer one-third of the crown.”
  • Clarify size of branches to be removed.  For example, “Pruning shall remove branches from one-half inch up to two inches in diameter.”

Raising

Raising is the removal of lower branches to provide clearance for people, traffic, buildings, or a view.  When removing lower branches, maintain at least one-half of the foliage in the lower two-thirds of the tree.  The lowest branch should originate in the bottom one-third of the tree’s height (live crown ratio).  [Figure 7]

Raising tree limbs

Figure 7. When removing lower branches, maintain at least one-half of the foliage in the bottom two-thirds of the tree.  The lowest branch should originate in the lower one-third of the tree.



Raising should be part of the tree’s structural training while young.  Ideally raising would be done before branches to be removed exceed a two-inch diameter.  The potential for decay is high when the branch removed is larger than four inches or when a two inch and larger branch is greater than half the diameter of the adjacent trunk (no branch collar to suppress decay).

On many trees, lower branches make-up a significant portion of the tree’s entire canopy and cannot be removed without significantly impacting tree health and appearance.  When the branch to be removed is larger than two inches, consider other alternatives.  Can the clearance required be achieved with removal and reduction cuts out along the branch rather than removing the entire branch?  Leaving some small diameter branches on the lower trunk for a year helps close pruning wounds and lessens the potential for trunk cracking.   [Figure 8]

raising tree limbs

Figure 8.  In raising branches on maturing trees, consider if required clearance can be achieved with removal and reduction cuts out along the branch rather than removing large branches entirely.



Excessive removal of lower branches increases the potential for tree failure by decreasing trunk taper, causing trunk cracks and decay, and transferring weight to the top.    

Written specification for raising should include the following:

  • Clarify the clearance required.  For example, “The tree’s crown will be raised to seven feet.”
  • Clarify what branch(es) will be pruned and the type of pruning cuts (removal or reduction cut) to be used.  For example, “The lowest branch on the south side shall be removed back to the trunk with a removal cut.  The lowest branch on the north side will be reduced with a reduction cut at the branch five feet out from the trunk and A removal cut to the lowest side-branch.”
  • Clarify what size of branches will be pruned.  For examples, “All cuts shall be two inches in diameter and smaller.”

Reduction

Reduction is the selective removal of branches to decrease the height and/or spread of a tree.  It requires the use of reduction cuts which remove larger branches back to smaller side branches.   [Figure 9]

Lower tree height

Figure 9.  Reducing is the selective removal of branches to decrease a tree’s height and/or spread.  Just being tall does not indicate that a tree is structurally weak and prone to storm damage.



Reducing is the most effective method to reduce potential wind damage on large trees with structural problems.  Reducing and thinning both decrease potential failure from snow loading; however, the removal cut used in thinning may better address issues without predisposing the tree to internal decay.

Not all trees can be reduced without predisposing the tree to decline and death.  Crown reducing requires the extensive use of reduction cuts which can predispose the branch/trunk to internal decay.  On older trees showing stress or decline, reduction cuts can accelerate decline and death.  [Figure 10] 

Dieback from pruning cut

Figure 10.  Not every tree should be reduced.  Notice the dieback associated with the previous reduction on this old cottonwood.  On old trees and trees showing stress or decline, reduction cuts and heading cuts may accelerate the decline cycle.



In a proper reduction cut, the side branch pruned back to will be at least one-third the diameter of the trunk/parent branch removed.  Under ANSI pruning standards, if the side branch is less than one-third, it is considered a heading cut, which is unacceptable.  For additional details on proper reduction cuts, refer to CMG GardenNotes #613, Pruning Cuts.

It is very difficult to use crown reducing to permanently maintain a tree at a small size without causing tree decline.  Ideally, trees were selected with adequate space for their mature size.  Where size control is necessary, it is best to begin reduction pruning as the tree reaches acceptable size, rather than when the tree becomes overgrown.

In crown reducing, first visualize the new outer edge of the smaller canopy.  Then prune the tree back to appropriate branch unions for a proper reduction cut or removal cut.  Some branches will be left taller than the visualized outer edge while others will be cut back below the visualized canopy edge.  Don’t make heading cuts and avoid rounding off the tree canopy.  [Figure 11]


Reducing tree height

Figure 11.
Left – In reduction, visualize the new outer edge of the smaller canopy.  Prune back to branch unions that make proper reduction and thinning cuts.  Some branches will be taller than the new outer edge, some shorter.
Right – This tree is incorrectly rounded off with heading cuts. 


In shortening primary upward growing trunks/primary branches to a lateral branch, a side branch that is somewhat upward growing with a narrow branch union angle may be stronger than a branch union with a wide angle.  [Figure 12] 

Reducing tree height

Figure 12.  In shortening a main upward growing branch, pruning back to a narrow branch union may be stronger than a wide branch union.



Just because a tree is tall does not indicate that it is structurally unsound.  Potential risk of failure should be evaluated by an experienced arborist based on branching structure, branch union integrity, signs of internal decay, and previous damage.

Written specifications for reduction pruning should include the following:

  • Clarify the desired reduction in height/spread.
  • Specify criteria for reduction cuts.  For example, “All cuts should be made on branches less than two inches in diameter. Diameter of the side branches pruned back to shall be at least one-third the diameter of the branch removed.”
  • Percentage of foliage to be removed.  For example, “Pruning shall not exceed 10% of the total foliage.”

Restoration

Restoration is the selective removal of branches, sprouts, and stubs from trees that have been damaged by improper pruning, vandalism, and storms.  The objective is to restore the trees structure, form and appearance to the extent possible.  Restoration generally requires annual pruning over a period of years.

Actual pruning procedures vary with the situation.  When dealing with situations of excessive watersprouts, a rule to thumb is to remove one-third and reduce one-third with each annual pruning.  Removing all of the watersprouts at one time often stimulates the growth of more watersprouts.

Pollarding

Pollarding is a training system that involves heading the first year followed by annual spout removal to maintain the tree at a predetermined size or shape.  Pollarding started as a method to produce shoots for fuel, shelter, and produced made from the young shoots.  Today, it is used as an art form.  Pollarding is not topping and should not be considered a routine method to keep large trees small.  Due to annual labor involved, it is uncommon in the United States.

Frequently Asked Questions About Pruning Mature Shade Trees

What About Topping a Tree? 

Shade trees should never be topped.  The regrowth of a topped tree is structurally unsound.  Topping required by utility right-of-way pruning is starkly obvious and sets an unfortunate community standard followed by others.  Instead of topping, use cleaning, thinning, and/or proper crown reduction cuts. [Figure 13]

Don't top trees
Figure 13. Never top a tree, the regrowth is structurally unsound, making it very prone to wind and storm damage.



 What About Utility Right-of-Way Pruning?  

Pruning for utility line clearance does not always follow desirable pruning techniques regarding appearance and health of the tree.  In this situation, the needs of the utility right-of-way take priority over the tree.

When a tree under a power line requires frequent crown reduction, consider having the tree removed.  Utility companies are generally eager to accommodate.  In planting trees, selection criteria (i.e., size and placement) should be followed so that a tree’s health and appearance will never be compromised by the need for utility pruning. 

I’m Concerned About My Tree Breaking in Storms, But I Really Don’t Want to Lose the Shade.  Do I Really Need to Have the Tree Pruned or Removed? 

This is a two-part question.  First, does the tree show signs of being highly susceptible to storm damage, (i.e., previous storm damage, dieback or dead branches, structural problems such as codominant trunks, weak branch unions or internal decay)?  This should be evaluated by an experienced ISA Certified Arborist.

Second, if yes, what would the tree or branch hit should it fail?  If it would cause significant property damage or threaten life, the tree should be pruned or removed as a preventive measure. 

Cleaning and thinning may reduce the potential storm hazard without compromising the shade.  In some situations the risk of failure cannot be reduced without removal.  Remember that healthy structurally sound trees are generally windfast even when mature. 

Storm damage is usually, but not always, related to structural problems that could have been addressed with proper structural training when the tree was young.  Co-dominant trunks account for the majority of tree failures in Colorado.  The hazard of wind damage is higher on the regrowth of trees that have been “topped”.  Consult an ISA Certified Arborist for additional details.

How Should Storm-Damaged Trees be Pruned? 

First, focus on cleaning (removing broken and damaged limbs) keeping in mind the structural integrity of the tree.  Realize that you may have to accept less than ideal pruning techniques by “Mother Nature”. 

Second, focus on thinning and/or reducing to restore the tree’s structural integrity and shape to the extent possible.  This may take place over a period of years. 

The maximum amount of tree canopy that can be removed without putting the tree and its root system under stress includes the live wood/foliage removed by the storm.  When Mother Nature removes too much live wood/foliage, limit pruning to cleaning.   

On storm damaged trees where excessive live wood and foliage were removed by storm damage, wait until the roots and crown stabilize (as measured in canopy growth) before doing thinning, reducing, or other structural pruning.  This may be a multi-year period.

Keep the tree if it can be pruned back to structurally sound wood and will be esthetically pleasing.  Often when one side of the tree is gone, the best option is to remove the entire tree. [Figure 14]

Storm damaged tree
Figure 14. Keep storm damaged trees when it can be pruned back to structurally sound wood and has an acceptable appearance. This yard would look better if the tree was removed.



How Should Root Damaged Trees be pruned?

Focus on cleaning.  Avoid removing live wood and foliage as this could speed the decline.  Removing live wood lowers the auxin content which is the hormone that promotes root growth.  Removing foliage reduces photosynthesis and levels of stored carbohydrates that the tree is living on during the recovery period.  Trees in a construction site with damaged roots may require cleaning every 3-12 months for five plus years.

How Should Declining Trees be Pruned?

Focus on cleaning.  Avoid removing live wood and foliage as this could speed the decline.  Removing live wood lowers the auxin content which is the hormone that promotes root growth.  Removing foliage reduces photosynthesis and levels of stored carbohydrates that the tree is living on.  Old declining cottonwoods and poplars may warrant cleaning every 1-5 years.


Additional Information

CMG GardensNotes on pruning

Books (available from the International Society of Arboriculture)

  • An Illustrated Guide to Pruning, Third Edition. Edward F Gilman. Cengage Learning. 2011
  • ANSI A300 Pruning Standards, Part 1. American National Standards Institute. 2008
  • Best Management Practices (Revised 2008). Edward Gilman and Sharon Lilly. International Society of Arboriculture. 2008.

Web


Authors: David Whiting with Carol O'Meara; Colorado State University Extension

  • CMG GardenNotes are available online at www.cmg.colostate.edu
  • Colorado Master Gardener/Colorado Gardener Certificate Training is made possible by a grant from the Colorado Garden Show, Inc.
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  • Copyright. 2010-12. Colorado Master Gardener Program, Colorado State University Extension. All Rights Reserved. CMG GardenNotes may be reproduced without change or additions, for nonprofit educational use.

Revised July 2012