Ask the Denver Arborist

~Helpful Tips for your Trees~

Pruning Mature Shade Trees

When to hire a professional Licensed arborist

Pruning large trees is a safety issue beyond the training and experience of the common home owner. Hiring a licensed professional tree care company is the best approach for most tree pruning jobs. Look for licensed arborists within the city where the work will be performed. Many are listed in the phone book yellow pages. Also ask about liability insurance coverage.

Pruning objectives for mature trees

As trees mature, pruning should be based on pruning purposes. Do not just indiscriminately remove branches. The pruning purpose determines how to prune, which in turn determines the type of pruning cuts used. Our tree care professionals and licensed tree care arborist are experts in these determinations. Request a free estimate from A Family Tree Service today.


Techniques For Pruning Mature Trees
Purposes How Pruning Cut
Reduce risk of failure Structural Thinning cut
(Wind loading) Cleaning Reduction cut
(Snow loading) Thinning Heading cut
(Structural pruning) Raising
Develop structure Restoration
Improve aesthetics
Provide clearance
Improve view
Reduce shade
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, 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.

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General pruning guidelines

  • To minimize the potential for decay, make thinning 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 inch 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 be in the bottom 40% 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 breakage in winds. 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. Re-growth of water sprouts (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 purpose 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 how to prune to meet the purpose (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 1/3 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”.

Dealing with structural defects

Most storm damage in Colorado 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. Little can be done to correct structural defects on mature trees without predisposing the tree to internal decay and creating an unsightly shaped tree.


Cleaning is the removal of dead, diseases, detached and broken branches. Most pruning of mature trees falls into this category. 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.

Removing dead branches – To minimize risk if the branch were to fail, it is advisable to remove any dead branch larger than 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.

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.

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 inch diameter and larger that are 30 feet and higher above the ground.”


Thinning is the selective removal of smaller branches (two inch diameter and smaller) in the leafy upper/outer canopy. Thinning cuts are primarily used. Since thinning is in the upper/outer canopy, it requires a trained arborist with a high level of skill in professional tree care techniques.

Thinning is expensive, often running$500 to over $1,000 per large tree when done correctly.

At A Family Tree Service, our professional tree care personnel and licensed arborist, are highly skilled in the evaluation and execution of the proper pruning techniques required to maintain the health and beauty of your trees. Request a free tree care estimate and a licensed arborist will come to you to give a complete evaluation of your tree care needs. Or call (303) 424-1222 and talk to our arborist. We are happy to answer any questions you may have.

Tree thinning

Benefits of thinning

  • Thinning is the best way to minimize potential damage caused by snow loading, the primary factor 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 reduce shade to under story plants below the tree. Shading by maturing trees often limits the vigor of lawn and flowers under the tree. However, increased light penetration into a lawn may invigorate the lawn adding stress to an old or declining tree due to root competition for water and nutrients.
  • Thinning is a technique to partially open a view without removing or structurally impacting a tree. This is often referred to as “vista pruning”.
  • Thinning is not 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 (less than two inch diameter) in the leafy upper/outer canopy. Thinning is not removing large lower branches which could create gaps in the crown and encourage water sprouts. Thinning is not removal of the small twiggy branches in the inner canopy. Thinning will not significantly lower a tree’s height.

Avoid “lion-tailing” which is 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 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 shall remove branches from 1/4 inch up to two inches in diameter”. In the majority of professional tree care operations, by licensed arborist, there will be several different types of pruning cuts made. Where and how to make the tree pruning cuts take a tree specialist like the ones on our staff.


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 be in the bottom 40% of the tree’s height (live crown ratio).

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 thinning 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.

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(s) will be pruned and the type of pruning cuts (thinning or reduction cut) to be used. For example, “the lowest branch on the south side shall be removed back to the trunk with a thinning cut. The lowest branch on the north side will be reduced with a reduction cut near the end and thinning cut to the lowest sid branch.”
  • Clarify what size of branches will be pruned. For examples, “all cuts shall be two inches in diameter and smaller.”

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Reducing is selective removal of branches to decrease the height and/or spread of a tree. It requires the use ofreduction cuts to remove larger branches back to smaller branches.

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 thinning may better address issues without predisposing the tree to internal decay. Reducing is also the selective removal of branches to decrease a tree’s height or girth.

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.

Not every tree should be reduced and just because it is, “very big,” does not mean that it is structurally unsound. Different types of trees are designed, by God, to grow to a certain height and width. Our licensed arborist and professional tree care staff can help you to determine the proper course of action. If the tree is so large that it makes you uncomfortable where it is, then perhaps the complete tree removal by a professional tree care company is the answer.

In a proper reduction cut, the side branch pruned back to will be at least 1/3 the diameter of the trunk/parent branch removed. Under ANZI 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 thinning 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.

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. Hazard potential should be evaluated by an experienced arborist based on branching structure, branch union integrity, signs of internal decay and previous damage.

Written specification for reducing should include the following:

  • Clarify what is 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 1/3 the diameter of the branch removed.”
  • Percentage of foliage to be removed. For example, “Pruning shall not exceed 10% of the total foliage”.


Frequently asked questions about pruning mature shade trees

What about topping a tree?

Shade trees should never be topped. The re-growth 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 reducing.  

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 crown reducing, 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 co-dominant trunks, weak branch unions or internal decay? This should be evaluated by an experienced Licensed 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 hazards without compromising the shade. In some situations the risk of failure can not be reduced without removal. Remember that healthy structurally sound trees are generally wind fast even when mature.

Storm damage is usually, but not always, related to structural problems that could have been corrected 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 re-growth of trees that have been “topped”. Consult an Licensed 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 was 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 aesthetically 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 1-3 years.

Training Of Young Trees

Developing a Strong Branch Union – Most storm damage in Colorado landscape trees results from failures at the branch union (crotch) primarily with co-dominant trunks (adjacent trunks of similar size). Primary objectives in training young trees are to develop strong branch unions and eliminate structurally weak co-dominant trunks.

Structural strength of a branch union is based on the development of a branch collar. The branch collar is where the annual growth rings of the trunk overlap the annual growth rings of the side branch like shuffling a deck of cards. In lumber the branch collar is called the knot. As the branch collar develops, side branch tissues connect into the trunk in a wedge-shape making a structurally strong unit. For the branch collar to develop the side branch must be less than half the diameter of the adjacent trunk. Less than one-third is preferred. If the side branch is too large in diameter, our licensed arborist can properly prune back the side branch significantly to slow growth or remove the branch entirely.

A branch collar will not develop on co-dominant trunks (adjoining trunks of similar size) making this branch union structurally weak. Multiple branches arising at the same location also compromises the branch collar’s structural strength. Some tree species (like elm, maple, and crabapple) naturally develop multiple branches at one location. This predisposes the tree to storm damage if the situation is not corrected by structural training when the tree is young. You should consult a licensed arborist to determine the proper pruning to be performed.

Minimizing the spread of decay

The development of a branch collar significantly reduces the potential spread of decay. Also branch unions with a right angle of attachment are more effective in preventing the spread of decay. To reduce the potential for decay, 1) prune to develop branch collars (side branch must be less than half the diameter of the adjacent trunk) and 2) select branch unions with a wide angle of attachment. In pruning, remove co-dominant trunks and narrow branch unions while young (smaller than two inches).

Implications for pruning

In training trees for structural integrity, our licensed arborist has zero tolerance for co-dominant trunks. Co-dominant trunks should be corrected immediately when they start to develop, at least before the trunks grow to a two-inch diameter.

In training trees for structural integrity, our licensed arborist give special attention to developing strong branch unions with branch collars. For the collar to develop, the side branch must be less than half the diameter of the adjacent trunk. Less than 1/3 is preferred. If the side branch is too large, significantly prune back the side branch to slow its growth rate. In pruning, our licensed arborist will select branch unions with a wide angle of attachment. Ideally, all pruning cuts are two inches and smaller in diameter. Any pruning cut four inches and larger must be justified taking into account the high potential for decay.

Licensed arborists can quickly identify any problems that your tree may have to provide the proper maintenance care. To contact our licensed arborist, go to Request Estimate and describe your situation, or call us at: (303) 424-1222