|
Pruning
Deciduous Shade Trees
Cooperative
Extension, College of Agriculture & Life Sciences, The University of
Arizona
Written by
Elizabeth Davison, Lecturer, Department of Plant Sciences
Trees in the wild are never pruned, yet they often have long healthy
lives. In a natural setting, their branches develop a balance and form
typical of the species. It is when trees are brought into an urban situation
that correct pruning becomes so important. Pruning is both a skill and
an art.
Why Prune A Tree?
There are only a few valid reasons to prune a tree in the urban landscape:
- Promote tree’s health
- Reduce hazards to people
- Train a young tree
- Increase or decrease flowers/fruit
Identify the purpose for pruning your deciduous shade tree. Resist
pruning because the tree is growing rapidly, or because the neighbors
are working on their trees. Understand that pruning always causes a
wound and always results in some response in the tree:
- Loss of foliage and ability to create food from sunlight
- Potential entry points for decay organisms
- An increase in sprouting
- Reduced vigor
- Susceptibility to insect problems
Decide whether the desired benefit will override the negative effect
on the tree.
When to Prune A
Deciduous Tree
In general, pruning has a greater benefit if it is performed while
the tree is young. Wounds are smaller and recovery is more rapid. This
document will discuss pruning young trees and older trees separately.
Deciduous trees are those that lose their leaves and go through a dormant
period in response to cold (and sometimes drought). The best time of
year to prune is during this dormancy — when all of the foliage
has dropped. Generally this occurs from mid-December to the latter part
of January (or beyond, in higher elevations). All pruning should be
done before buds show evidence of swelling. Why?
- Less likelihood of tearing bark
- Less likelihood of dripping sap
- More time for recovery before bud break
- Easier to see branch structure
- Less likelihood of stimulating unwanted growth
Equipment Needed for
Pruning
Limbs of various sizes will be removed during pruning. Matching the
limb size to the hand tool will make the job go more easily. Buy the
best tool you can afford. In general, you get what you pay for.
For limbs over one inch (2.5 cm) in diameter, use a pruning saw. There
are several available styles. High quality forged stainless steel is
strongest. Those with blades on both sides of the teeth will cut efficiently
on both forward and backward strokes. Models with blades that narrow
at the end will allow easy access to tight spots.
Pruning loppers are convenient for limbs that are about the size of
a finger. Blades that cut like scissors (“bypass” type) will
make cleaner cuts. Models are available that have gears for efficient
movement, hollow handles for reduced weight, and high quality steel
blades.
Hand pruning shears are best for branches less than 1/2 inch (1.25
cm). Left handed-versions are available. Those with stainless steel
bypass blades, and handles that fit the hand will cut efficiently with
less effort.
There is always the possibility of spreading a disease from one plant
to another. When pruning several plants, it is wise to disinfest your
tools between plants. Dip or coat the blades with a 10% bleach solution
or with alcohol. After using pruning tools, thoroughly wash, dry, and
coat the blades with a film of light oil to prevent rusting. If kept
sharp and clean, good tools will provide years of service.
Proper Technique
 Rapid sealing off of
pruning wounds depends upon where the pruning cut is made. Most trees
produce a corky ridge at the juncture or crotch of a branch; this is
called the Branch Bark Ridge. At the base of a limb is a ring
of growth; this is called the Branch Collar. The pruning cut
should be made just outside a line connecting these two points. This
technique is called Natural Target Pruning, and it minimizes
problems in recovery. See Figure 1.
Figure 1: Natural Target
Pruning.
If a tree does not produce the characteristic ring of bark, make the
cut so that it forms an angle to the line of the Bark Ridge that’s
identical to the angle formed between the Bark Ridge at the crotch and
a vertical line to the ground. See Figure 2.
Figure 2: Angle to cut if Branch Collar is not
evident.
Large heavy limbs over 2" in diameter should be cut in a
3-step process as shown in Figure 3. Sharper tools make cleaner cuts.
Figure 3. Three cuts are necessary to remove
large
limbs. This procedure prevents bark from stripping down the trunk.
Do not apply pruning paint; wounds have been found to recover
more rapidly in the open air if the relative humidity is low.
Pruning/training young trees
(first 3 years on the site)
Proper pruning when the tree is young will result in a mature tree
that is structurally stronger, lives longer, and is less costly to maintain.
A. Pruning your Tree at Planting Time:
There is no need to prune a newly-planted tree unless branches have
been damaged. It has been found that removing tips and buds of young
trees slows root growth. If trees are left unpruned, expanding buds
and new leaves help root expansion and tree establishment. Damaged branches
can be removed at their point of origin, or they can be cut back to
a lateral branch that will provide foliage and help establishment the
first season.
B. Young Tree Health and Structure:
A word of caution: different tree species grow at different rates.
Your tree may not seem to be adding any height or spread for the first
year or so. Check the irrigation design and frequency, keep the soil
mulched, and avoid compacting the root zone. Resist the urge to prune
a young tree simply to “force” new growth. Such growth is
weakly attached, rarely looks good, and will require additional maintenance
later in the tree’s life. Conversely, overwatering and overfeeding
a young tree may result in fast growth that appears to need pruning
each year. Cutting back on irrigation after the first summer, particularly
if the species is adapted to your climate, can slow growth and maintain
a more balanced tree that requires less maintenance.
Know the species’ natural form. Many deciduous trees, including
most desert trees, have a rounded spreading shape with no single central
leader. See Figure 4. The lateral branches grow as fast or faster than
the main trunk. Do not try to force a rounded species into a tall narrow
shape (or a tall species into a rounded shape!)
Rounded spreading deciduous tree species may naturally form multiple
trunks. These can be very attractive and provide much needed shade.
To develop a multi-trunk tree in a species that naturally has low branching,
allow the secondary trunks to arise close to the ground with wide branch
angles. If several trunks all arise from the soil, choose 3 or 4 of
the largest, removing the smaller ones.
 Early in the tree’s
life, decide between closely spaced scaffold branches (large branches
that form the main structure). Try to visualize how the tree will look
as it thickens in years to come. Know the natural form of the species.
Remember — branches do not slowly rise above the ground! As a tree
grows, branches retain their position on the trunk, though they increase
in diameter and become more crowded. Spacing scaffold branches radially
and vertically allows growth to be channeled where it will be more effective.
See Figure 5. No more than 1/4 of the canopy should be removed at any
one time. Depending on the tree’s growth rate and branching structure,
this training make take several years.
During the tree’s first 3-4 dormant seasons, keep an eye on the
water sprouts (vigorous upright shoots from a branch), suckers
 (vigorous
upright shoots from below the soil level), and crossing branches. See
Figure 6. These should be pruned at their points of origin. Again, cut
cleanly just outside the branch collar.
Should small branches covering the lower trunk be removed? Not completely.
Cutting off lower trunk branches does not “force” growth of
the top, and may result in more sprouting from the cut area. It has
been found that small branches along the lower trunk provide hormones
and food for trunk expansion and stability. Leaving them intact, or
only trimming back slightly will help the trunk develop taper and strength
during early years. See Figure 7.
Most tree species form secondary branches at regular intervals along
the trunk and scaffold branches. Removing branch ends or tips usually
causes a proliferation of buds to break open down the stem, resulting
in a “bushier” form that will need more attention in future
years. If one branch is longer than the others, prune it back to a side
branch that’s headed in the right direction. Do not shear the tips
of tree branches.
Likewise, “topping” or severely heading back large branches
causes large wounds and severe stress to the tree. This practice results
in a proliferation of sprouts that all originate from the same area,
are weakly attached, and will need extensive care in future years. Topping
is extremely detrimental to any tree species.
Some trees that are planted in pedestrian areas may more easily survive
in the sites if their lower scaffold branches are at least 8 ft above
the  ground. See Figure
8. This is called raising the crown, and is best done gradually; i.e.,
prune one scaffold branch each year over several dormant seasons, rather
than removing them all at once. The lowest permanent branches should
have wide angles and be less than half the width of the trunk at the
point of attachment. Clean cuts outside the branch collar will allow
more rapid recovery.
Pruning Older Trees
(longer than 3 years on the site)
A. Form and Health of the Tree:
Trees growing in native habitat develop natural forms even though they
were never pruned. There may be some defects: large limbs may be rubbing
against each other, limbs may be crossing over to the opposite side
or there may be sprouts arising on the main trunk or near ground level.
Even with defects present, these trees have stabilized branch structures
and they normally withstand strong winds. Their graceful forms are more
attractive than severely pruned trees in home yards.
If your older tree has never been pruned, this doesn’t mean it
needs to be. Walk around the tree and carefully observe carefully limb
placement. Look for obvious defects on larger size limbs: narrow angles
that have included bark (a buildup of bark within the angle), branches
originating too close together, branches that parallel each other, or
rubbing or crossing branches.
Branch angles are important. Narrow angles are more prone to splitting
because included bark cells do not join together. Wider angles are more
stable. See Figure 10. Remove the smaller or least desirable branch
by cutting outside the Branch Collar.
 Large scaffold branches
should be evenly spaced throughout the tree. If two branches originate
from points less than about 12" (30 cm) from each other, they will
only put stress on the trunk as they increase in girth. Decide if two
or more branches are originating from the same point on the trunk. Choose
the one with the narrowest crotch angle, or the most internal one and
remove it. If there are several troublesome branches, take the one that
will leave the smallest void if removed.
If two limbs are paralleling each other too closely, the least desirable
one should be cut at its point of origin. Try to visualize the appearance
of the tree if one or the other is removed. Which one is best placed?
Which one will leave the smallest void if removed? Which one will leave
the smallest wound? If the branch is larger than about an inch, use
the 3-step method as shown in Figure 3.
The same procedure should be used if two large limbs are rubbing against
each other. Study the placement of each and then remove the least desirable
one. Look for limbs that are growing in the wrong direction. These include
limbs that are growing either downward through the tree or those that
are crossing over to the opposite side.
 After removing these
undesirable limbs, STOP and re-evaluate. If you are tempted to severely
prune, remember this: trees with natural forms are beautiful and healthy.
By removing only those limbs that threaten sound structure, you will
have a tree that surpasses those in nature. Severe pruning destroys
the natural form and causes trees to develop shapes that are completely
different than nature intended . More importantly, from a health standpoint,
removing more than 1/4 the total amount of the live wood at any one
time causes significant stress and reduced photosynthetic ability as
the tree recovers.
Remove only limbs that cause problems and allow the tree to develop
a natural growth habit. Few pruning cuts will be made; sunlight will
filter through the foliage, reducing leaf drop, the bark will not burn
from sudden exposure to sun, and wind will pass through the tree without
threatening breakage.
B. Reduction of Height and Width
Reduction of height or width may be necessary, but it is also possible
that the tree has been planted in the wrong site. If the natural form
of the species includes a wide spreading canopy, it was the wrong choice
for a narrow walkway or close to a building. The landscape might be
improved by removing the tree and replacing it with one that has a more
narrow upright form. Likewise, if the species typically makes a tall
narrow canopy, it is the wrong tree for under power lines or roof overhangs.
As a one time effort, however, a limb or the central leader can be
removed to reduce the height or redirect growth from interfering with
a building. Be aware that, depending on the species, the tree may respond
to the cut by putting forth more growth than what was originally there.
This is particularly true in attempts to reduce the height. A professional
tree care expert should be consulted for help with reduction in height.
See AZ Cooperative Extension Publication # AZ1003 — How to Hire
a Tree Expert.
Before removing lateral branches to reduce a tree’s width, remember
your goal is to prevent regrowth from the wound. Always attempt to cut
back to a side branch, or to the point of origin. See Figure 9. Either
of these choices will discourage development of numerous new limbs that
are weakly attached. Generally the branch that is left should be about
1/3 the diameter of the part that’s removed. Heavy branches should
be cut in 3 steps as shown in Figure 3.
Trees often suffer limb breakage during severe wind storms. Removal
of heavy, large limbs should be left to professionals. Cut limbs do
not always fall in the intended places and serious damage to home or
body is always a hazard. Smaller broken limbs resting on the ground
can be removed with little danger.
Can a fractured limb be lifted and wrapped and heal itself? NO. Trees
do not heal the way bones do.
C. Pruning to increase flowering or fruiting
Older trees of low vigor can be induced to produce more flowers. When
problem branches are thinned out, the branches remaining have more available
nutrients. Typically, flowers on wood produced after the pruning will
be larger or more numerous. Species that flower early in the spring
on last season’s wood should be pruned after the bloom season.
Species that flower later in the season, on wood formed the current
year, should be pruned during the dormant season.
Pruning or ruining?
There is an old saying among professional arborists in connection with
tree pruning, “If in doubt, don’t prune.” Nature has
a unique way of allowing trees to develop natural forms that balance
shoot growth with root zone health. When this natural form is destroyed,
trees will react accordingly — by producing rank, weakly-attached
growth, elongation of existing shoots, reduced vigor, increased susceptibility
to insect or disease problems, or consumption of stored carbohydrates.
There is always a reaction. If even one limb is removed from a tree,
there should be a reason for doing so. Anyone who prunes for the sake
of pruning “because everyone else is” will undoubtedly cause
problems that will be difficult to correct in later years.
On the other hand, careful selective pruning for any of the valid reasons
listed will produce a healthy, beautiful tree that will enhance your
landscape and improve the quality of life in your neighborhood for years
to come.
The University of Arizona is an Equal Opportunity/Affirmative
Action Employer. Any products, services, or organizations that are mentioned,
shown, or indirectly implied in this publication do not imply endorsement
by the University of Arizona.
Document located http://cals.arizona.edu/pubs/garden/az1139/
Published October 1999
Return to College publication list
|