Pruning Trees

David S. BienemannMunicipal Arborist

March 28, 2006

Contact Information

David S. BienemannMunicipal Arborist

Bowling Green, Ohio419-353-4101

arborist@bgohio.orgwww.bgohio.org/arborist

Discussion Topics• Tree Anatomy• Why Prune• Pruning Methods• Practice Pruning• Questions

What is a Tree?

What is a Tree?

Crown

TrunkRoots

HowTreesWork

TranspirationO2

H2O & Nutrients

PhotosynthesisC6H12O6

PhotosynthesisC6H12O6

RespirationCO2

Sugar Storage

Cross Section of a Woody StemCambium

The meristem of the trunk of the tree, the only cells in the trunk that divide.

A layer of vascular cells two cells thick

Cross Section of a Woody Stem

Cambium Phloem

Vascular tissue that transmits “tree food”down the tree

Cross Section of a Woody Stem

Cambium Phloem

Old non-functioning phloem

Protects the tree from physical injury, attack from pathogens and prevents tree from drying

Bark

Cross Section of a Woody Stem

Cambium Phloem

Vascular tissue that transmits water and nutrients from the soil to the leaves

Xylem functions for 3 to 7 years depending on species

Bark

Xylem

Cross Section of a Woody Stem

Cambium Phloem

Non-functioning xylem cells

Gives structural strength and stores by-products

Bark

Xylem

Heartwood

Cross Section of a Woody Stem

Cambium Phloem

Vascular cell turned perpendicular to xylem cells

transports by-products from phloem to heartwood

Bark

XylemHeartwood

Vascular Ray

Cross Section of a Woody Stem

Cambium Phloem

Rings of different sized vascular cells

The large cells are formed in the spring the small in the summer

Bark

XylemHeartwoodVascular Ray

Annual Rings

Compartmentalization of Decay in Trees

• CODIT• How Trees Respond to Wounds• Develops barriers around wound• Weakest barrier is up and down

the trunk/branch/roots• Response dependent on tree’s

health, species, size of wound, etc.

The Urban Environment is an Unnatural Environment

Forest Trees• Organic Matter• Soil Compaction• Water Filtration• Protection from Elements• Native Soil• Tree Species Adapted to Site• Trees Compete for Top Spot

“Survival of the Fittest”

Urban Trees• Organic Matter• Soil Compaction• Water Filtration• Protection from Elements• Non-native soils• Variety of Urban Stressors• Dead Tree = Hazard

Why Prune?

Reasons For PruningThe main reasons for pruning ornamental and shade trees include safety, health, and aesthetics. In addition, pruning can be used to stimulate fruit production and increase the value of timber. Pruning for safety (Fig. 1A) involves removing branches that could fall and cause injury or property damage, trimming branches that interfere with lines of sight on streets or driveways, and removing branches that grow into utility lines. Safety pruning can be largely avoided by carefully choosing species that will not grow beyond the space available to them, and have strength and form characteristics that are suited to the site.

Pruning for health (Fig. 1B) involves removing diseased or insect-infested wood, thinning the crown to increase airflow and reduce some pest problems, and removing crossing and rubbing branches. Pruning can best be used to encourage trees to develop a strong structure and reduce the likelihood of damage during severe weather. Removing broken or damaged limbs encourages wound closure.

Pruning for aesthetics (Fig. 1C) involves enhancing the natural form and character of trees or stimulating flower production. Pruning for form can be especially important on open-grown trees that do very little self-pruning.

Industry consensus document presents performance standards for the care and maintenance of trees, shrubs, and other woody plants

Intended as a guide for drafting specifications

Compliance voluntary, although the standards will be the benchmark used in tree care malpractice litigation.

ANSI A300National Tree Care Standards

Proper Planning• Right Tree, Right Place• Correct Tree Planting• Correct Tree Pruning• Long-term Care

Pruning living branches

To find the proper place to cut a branch, look for the branch collar that grows from the stem tissue at the underside of the base of the branch (Fig. 6A). On the upper surface, there is usually a branch bark ridge that runs (more or less) parallel to the branch angle, along the stem of the tree. A proper pruning cut does notdamage either the branch bark ridge or the branch collar.

Proper Pruning

Tree Response To Head Cuts “Topping”

Water Sprouts from Topping

Two Years of Sprout Growth on Silver MapleTwo Years of Sprout Growth on Silver Maple

Objectives of Pruning

• Reduce risk of failure – Structural pruning begins at planting and carries through the first 25 years.

• Provide Clearance – Growth can be directed away from an object such as building, security light, or power line by reducing limbs on that side of the tree.

Objectives of Pruning

• Reduce shade and wind resistance – A lawn, ground covers or shrubs can receive more sunlight when live foliage is removed from the canopy.

• Maintain health – Cleaning the canopy, especially in medium and mature trees. Remove dead, diseased, and rubbing branches.

Objectives of Pruning

• Influence flower or fruit production – The number and/or size of flowers or fruit can be influenced by pruning.

• Improve a view – A view can be enhanced or opened by removing live branches.

• Improve aesthetics – A tree can be pruned to make it looks more appealing.

Pruning Time Line• At planting

- all branches will eventually be removed on trees less than 4" caliper- do not remove more than 25% of live foliage- shorten or remove all leaders and branches competing with the one stem that would make the best leader- remove broken, cracked or severely damaged branches

Pruning Time Line• Two years

- all branches will eventually be removed on trees less than 4" caliper- do not remove more than 40% of live foliage- shorten or remove all competing leaders (may have to do in two stages if there are more than 3 leaders) - shorten or remove large, low vigorous branches to improve clearance- shorten or remove branches within 12" of largest diameter branches in top half of trees greater than about 4 inches caliper

Pruning Time Line• Four years

- most branches are still temporary and will eventually be removed from the tree- do not remove more than 35% of live foliage- shorten or remove competing leaders- shorten or remove large, low vigorous branches to improve clearance- shorten or remove branches within 12" of largest diameter branches in top half of tree- there should be only one large branch per node (no clustered branches) so shorten branches that are nearby so only one is present

Pruning Time Line• Eight years

- shorten or remove competing leaders- do not remove more than 25 to 35% of foliage- determine where you want the lowest permanent scaffold limb and shorten any aggressive branches lower than this limb- shorten branches within 12-18" of largest diameter branches (there should be only one large branch per node (no clustered branches)- shorten low branches that will have to be removed latter so they do not become large

Design a Plan for Training Shade Trees

• Establish and maintain a dominant leader by subordinating all but one codominant stem.

• Space main scaffold limbs apart by removing or shortening nearby branches.

• Anticipate future form and function by training and pruning early to avoid cutting large branches latter; don't remove large branches because this initiates decay in the trunk (i.e. instead of allowing a low branch from growing large then removing it when it is too low, anticipate this by shortening it earlier).

• Position the lowest main scaffold limb high enough so it will not droop and have to be removed latter.

• Prevent all branches from growing larger than half the trunk diameter.

• Maintain a live crown ratio of greater than 60%.

Structural Pruning

Structural Pruning Before• Problem - Before pruning, the

young to medium-aged tree has three developing leaders or codominant stems (a, b and c). These are likely to develop into multiple leaders, which are considered weaker than trees with one trunk. Large maturing trees usually perform best and last longest if they grow with one main leader. Structural pruning helps the tree develop one main trunk.

Structural Pruning Light• Solution - Reduce the length of

(subordinate) leaders 'a' and 'c' using a reduction cut to encourage leader 'b' to grow faster. This will help leader 'b' become the dominant trunk by slowing growth on leaders 'a' and 'c' and allowing more sunlight to reach 'b'. In most cases on large-maturing trees, branches in the lower 15-20 feet of the tree should be kept smaller than half the trunk diameter using this technique.

Structural Pruning Moderate• Solution - This moderate

pruning treatment is similar to the light pruning treatment. The main difference is here we are more aggressive. Larger holes are left in the canopy following moderate pruning. This might be appropriate if trees cannot be pruned for several years, or if they might never be pruned again. Up to about 40% of foliage could be removed on young trees, if necessary.

Thinning the Canopy

• Before pruning, the canopy is very dense toward the outer edge. The canopy might be shading desirable plants below.

Thinning the Canopy

• Remove small branches from the outside portion of the canopy, not from the interior.

Thinning the Canopy

• After thinning, the outer portion of the canopy is thinner. Little has changed on the interior of the canopy because few branches were removed from there.

Thinning the Canopy

• Removing only lower and interior branches results in a weak tree. This so-called lions-tailing causes problems. DO NOTprune trees in this manner.

Lion-Tailing Pruning

Crown thinning - branches to be removed are shaded in blue; pruning cuts should be made at the red lines. No more than one-fourth of the living branches should be removed at one time.

Pruning Techniques

Pruning TechniquesCrown raising: removes the lower branches from a tree in order to provide clearance for buildings, vehicles, pedestrians, and vistas.

Pruning TechniquesCrown reduction: removes the upper branches from a tree in order to reduce the height of the crown yet maintains the structural integrity and natural form of the tree.

An alternative to “topping”

Pruning TechniquesLateral/Directional Pruning: removal of a branch to the trunk or a significant lateral branch to encourage growth away from a utility conductor or other obstruction.

An alternative to “topping”

Practice Pruning Trees

Subordinate Competing Leaders

• This Norway maple has three or four leaders or trunks up into the canopy making it a potentially weak tree as it grows older. Select the one that will make the best leader and reduce the length of the others using reduction cuts.

Too Late to Save Tree

• The structure of this zelkova tree is too committed to the multi-leader form to attempt to prune it to a dominant leader. The tree could be kept small using reduction techniques to help prevent it from falling apart.

Reducing Competing Leaders• Two stems are competing

with the stem in the center of the canopy. Left unpruned, a weak multi-trunked form often results. Reduce the length of these two stems using reduction cuts to slow their growth. This will allow the stem in the center of the tree to dominate the structure and result in a stronger, more durable form.

Sprouting from Over Pruning

• Sprouts often emerge from the branches when the tree is over-pruned. Let these sprouts grow for a number of years to allow the tree to recover the lost energy-producing capacity lost when the tree was over-pruned.

Topped Tree

• This is not the way to prune shade trees. In addition to looking terrible, topping causes many serious problems for trees and their owners. Do not allow tree companies to prune trees like this.

Topping Damage• A look inside a tree topped

several years ago reveals some of the problems with topping. Note 1) the brown decay that begins just behind the topping cut, 2) the discontinuity of structure as shown by the abrupt ending of the wood at the cut, and 3) the barrier zone indicated by the arrows on the left. The barrier zone can be the source of cracks that develop inside the tree making it less strong.

Reduction Pruning• Most of the foliage on the large

limb on the lower right side of this tree is at the end of the limb. This can make a limb susceptible to breakage. Consider shortening the limb to bring the center of gravity closer to the trunk. The top of the tree divides into two more-or-less equal sized limbs about 3/5s up the tree. Consider shortening one or both of these to help prevent failure if you have reason to believe the tree could break in a storm.

Structural Pruning• The round head on this young

tree is attractive but the tree structure is poorly formed inside the canopy for sustainable landscapes. Most or all branches in the lower 2/3s of the canopy should be shortened now using reduction cuts so they do not grow up and become part of the permanent canopy. If they are allowed to develop up into the canopy then a weak multi-trunked form develops poorly suited for urban landscapes.

Structural Pruning• This young tree is developing

three main stems, one in the center of the canopy, one on the lower right, and one a bit farther up on the left. While these are secured to the tree at the moment, continued growth is likely to result in a weak structure. It is better to preventively prune now than to attempt to correct this problem later. Remove the two upright stems (large arrows) but leave the lateral branches each is growing from (smaller arrow).

Structural Prune• Three stems are growing up

into the canopy. Pick the one that is positioned in a desirable location (typically the one in the center of the canopy) and reduce the length of the other two. Cut back to a lateral branch located in a desirable position using a reduction cut. The amount of wood you remove (in other words how far back you cut) should be in proportion to the amount of growth-slowing effect you desire. Cut far back into the tree for maximum growth slowing effect.

Poor Pruning• This tree was just pruned by

university crews. The canopy was lifted to enhance clearance under the canopy. What should have been done is a combination of lifting (also called raising) and subordination of codominant stems to encourage development of one dominant trunk. The pruning you see above is very common and it encourages development of weak codominant stems. Raising in this fashion forces both main stems to elongate. The stem on the left should have been reduced in length so the stem on the right side could dominate the structure.

Too Much Crown Lifting• Canopy lifting or raising on

young trees should almost always be combined with structural pruning to create one dominant leader. The three leaders or trunks that remained after this tree was pruned will all grow at about the same rate creating a weak multi-trunkedhabit. You sure would not want this tree to grow to a large size with this inferior and weak form because of its proximity to people and property. If it breaks people may get hurt.

Structural Pruning• The three main branches in the

lower canopy originating about eight feet from the ground could be shortened now to keep them small so when they are eventually removed the pruning wounds will be small. Allowing all three lower branches to grow rapidly is likely to result in four trunks all growing from about the same spot. This form is weaker than when branches are much smaller than the trunk. This tree could also use some subordination of codominantstems in the upper canopy.

Trouble Tree• The casual observer would not

know that there is a huge potential problem with this tree. Its nice rounded even canopy hides the fact that the huge limb on the lower right is very poorly attached to the trunk and could fall anytime. There is almost always a vehicle parked under the large defective limb. Click to the next photograph for a close-up of the weak connection.

Included Bark• The four feet of included bark in

the branch union on right combined with the huge weight associated with this large limb on this 22 year old tree spells trouble. This limb is likely to split from the trunk in the very near future. Preventive structural pruning 15 years ago could have avoided this. Now, one option is to cable and brace the limb to the rest of the tree combined with reduction pruning on the right hand limb. Another option is to remove the parking spaces from under the limb so when the limb falls chances of property damage are diminished.

What is the best treatment?• Two trunks are growing close

together with bark included between them. There is also included bark in the branch unions of each tree. Some trees in this situation fall over. Some arborists would install cables in these two trees in an attempt to reduce the chances of them falling over and to prevent branch breakage. Reduction and thinning could be used in conjunction with cabling.

Structural Pruning• 1) Three branches on the lower left

side of the tree should be reduced in length now. Since each will be removed from the tree at some point in the future keep them small in diameter by shortening them. This will slow their growth rate. Small branches will leave a smaller wound when the branches are eventually removed. 2) Pick one stem to be the leader in the top of the tree and shorten the competing leaders to slow their growth rate. Regular pruning in this manner will produce a tree with one dominant trunk which is considered a better structure for many shade trees.

Reducing Competing Leaders

• The Siberian elm is known to break in storms. Bark inclusions, long limbs, and weak wood are largely responsible for this. Reduce the length of over-extended limbs, especially those with bark inclusions, to minimize failure potential.

Structural Pruning• This Scholar tree has a

spreading open habit with aggressive low branches. In the coming years, many branches on this tree will droop and get in the way of mowers and have to be removed. If the tree was situated close to a street or parking lot, the lower branches would have to be removed for vehicle clearance. Shorten many of the branches on this tree now to force more growth in the stem that will make the best upright trunk. Use reduction cuts back to lateral branches.

Weak Attachments• Three stems are attached at the

same spot on the trunk and there is bark included in each union. This makes the connections weak and susceptible to splitting out from the tree. Because the stems are so large and tree is committed to this weak structure, cabling and bracing stems together perhaps combined with some reduction pruning on the left and right limbs, can help keep this tree together.

Bark Inclusion = Weak Link• This weak attachment is likely

to split in the future. Preventive pruning now can prevent this from happening. Reduce the length of the left hand stem so the one on the right will grow faster. Eventually the left hand stem will be removed from the tree because it is way too low on the trunk. In many cases, the lowest permanent limb on shade trees should be at least 15 feet off the ground.

Crown Reduction

• Two large limbs originate from a point 20 feet up the trunk and there is bark included in the union. The top of the tree has grown over the building and is now exposed to the wind. To reduce the chances for failure, reduce the right hand stem and cable the two large limbs together.

Restoring Topped Trees• Note the topping cut in the center

left of the photo. There is a short stub sticking out from the tissue. The large diameter sprout on top and the smaller one on the bottom grew from just below the topping cut made several years earlier. A collar is developing around the stub and the smaller sprout because one of the sprouts grew to much larger in diameter than the other one. This indicates that the connection between the two is stronger than if there was no collar. Attempt to improve structure on topped trees by reducing the growth rate on competing stems so that one stem dominates.

Questions?