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Pruning Shade Trees
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. Many are listed in the phone book yellow pages. Ask about liability insurance.
This is written to help home gardeners understand issues around pruning of mature trees and to help them to communicate with their certified arborist.
Pruning Objectives for Mature Trees
As trees mature, pruning should be based on pruning objectives (why to prune).
Do not indiscriminately remove branches. Pruning objectives 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 for Pruning Maturing Trees Objectives (Whys) Methods (How) Pruning Cuts
Reduce risk of failure (wind & snow) Structural Removal cut
Improve structure Cleaning Reduction cut
Maintain health Thinning Heading cut
Improve aesthetics Raising
Provide clearance Reducing
Improve view Restoring
Reduce shade Pollarding
Influence flowering and fruiting
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
age 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. The amount of live wood and foliage removed should be reduced for trees with growth limiting factors, such as nonirrigated sites and limited root spread.
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 whenever possible.
· 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 and cracking.
· To maintain trunk taper resilient to winds, 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 cleaned-out 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 and cracking. Regrowth of watersprouts (adventitious shoots) is structurally unsound.
· Written specification for any pruning job should include the following:
o Clearly state which tree(s) will be pruned.
o Clearly indicate the objectives for pruning (why prune), such as reduce risk of failure due to wind damage or snow loading, manage health, improve aesthetics, provide clearance, and improve view.
o Specify pruning methods (how to prune) to meet the objectives, such as structural pruning, cleaning, thinning, raising, reducing, and restoration pruning.
o State the size specification for the minimum and/or maximum branch size to be removed. For example, "Cuts should be made on branches two inches and less in diameter" and "In a reduction cut, the side branch pruned back to, should be at least one-third the diameter of the branch removed."
o Specify the maximum amount (percentage) of live tissue that can be removed. For example, "Pruning should not remove more than 15% of the live crown."
o In writing pruning specification, the word "should" refers to a practice that is routine and recommended. The word "shall" refers to a practice that is mandatory.
o Include these generic safety statements to reduce the homeowner's and pruning crew's liability. "All work shall be performed in accordance with ANSI A300 Pruning Standards and ANSI Z133.3 Safety Standards."
Structural Pruning
 In snowy climates, most storm
damage is due to codominant 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, cracking, and creating an unsightly
shaped tree. [Figure 1]
Figure 1. Codominant 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.
Cleaning
Cleaning is the removal of dead, diseased, 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-age and mature trees falls into this type. Trees under stress or declining
trees, may need cleaning every few months to ever few years. All dead wood may
be removed at one time. It does not count in the total of live wood/foliage
removed. In cleaning, do not remove healthy branches and live foliage. Do not
clean out healthy growth in the tree's interior. [Figure 2]
Figure 2. 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
opening 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]
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. Since 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]
Figure 4. Thinning is the selective
removal of small branches,
growing parallel to each other, in
the leafy upper/outer tree canopy.
Since 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-age 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
the removal of small twiggy branches in the inner canopy. Thinning will not
significantly lower a tree's height. [Figure 5]
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.
Lion-tailing
 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]
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 1/3 of the crown."
· Clarify size of branches to be removed. For example, "Pruning should
remove branches from 1/2 inch up to 2½ 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]
Figure 7. When removing lower branches, maintain at least 1/2 of the foliage in the bottom 2/3s of the tree.The lowest branch should originate in the lower onethird of the tree.
Raising should be part of the tree's structural training while young. Ideally raising would be done before the 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]
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 removes larger branches back to smaller side branches. [Figure 9]
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]
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 will be at least 1/3 the
diameter of the trunk/parent branch removed. Under ANSI pruning standards, if
the side branch is less than 1/3, it is considered a heading cut, which is generally
unacceptable in pruning standards.
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]
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.
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 should be at least 1/3 the diameter of the branch
removed."
· Percentage of foliage to be removed. For example, "Pruning should 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 of 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 products
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
reduction methods. [Figure 13]
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, in regards to 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 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 fall? 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.
Codominant 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.
How Should Trees with Root Damage 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.
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