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Pruning is the most common tree maintenance procedure.
Although forest trees grow quite well with only nature's pruning,
landscape trees require a higher level of care to maintain their
safety and aesthetics. Pruning should be done with an
understanding of how the tree responds to each cut. Improper
pruning can cause damage that will last for the life of the tree, or
worse, shorten the tree's life.
Reasons for Pruning
Since each cut has the potential to change the growth of the tree,
no branch should be removed without a reason. Common reasons
for pruning are to remove dead branches, to remove crowded or
rubbing limbs, and to eliminate hazards. Trees may also be pruned
to increase light and air penetration to the inside of the tree's
crown or to the landscape below. In most cases, mature trees are
pruned as a corrective or preventative measure.
Routine thinning does not necessarily improve the health of a tree.
Trees produce a dense crown of leaves to manufacture the sugar
used as energy for growth and development. Removal of foliage
through pruning can reduce growth and stored energy reserves.
Heavy pruning can be a significant health stress for the tree.
Yet if people and trees are to coexist in an urban or suburban
environment, then we sometimes have to modify the trees. City
environments do not mimic natural forest conditions. Safety is a
major concern. Also, we want trees to complement other landscape
plantings and lawns. Proper pruning, with an understanding of tree
biology, can maintain good tree health and structure while
enhancing the aesthetic and economic values of our landscapes.
When to Prune
Most routine pruning to remove weak, diseased, or dead limbs can
be accomplished at any time during the year with little effect on the
tree. As a rule, growth is maximized and wound closure is fastest if
pruning takes place before the spring growth flush. Some trees,
such as maples and birches, tend to "bleed" if pruned early in the
spring. This may be unsightly, but is of little consequence to the
tree.
A few tree diseases, such as oak wilt, can be spread when pruning
wounds allow spores access into the tree. Susceptible trees
should not be pruned during active transmission periods.
Heavy pruning just after the spring growth flush should be
avoided. This is when trees have just expended a great deal of
energy to produce foliage and early shoot growth. Removal of a
large percentage of foliage at this time can stress the tree.
Making Proper Pruning Cuts
Pruning cuts should be made just outside the branch collar. The
branch collar contains trunk or parent branch tissue and should
not be damaged or removed. If trunk collar has grown out on a
dead limb to be removed, make the cut just beyond the collar. Do
not cut the collar.
If a large limb is to be removed, its weight should first be reduced.
This is done by making an undercut about 12-18 inches from the
limb's point of attachment. A second cut is made from the top,
directly above or a few inches further out on the limb. This
removes the limb leaving the 12-18 inch stub. The stub is removed
by cutting back to the branch collar. This technique reduces the
possibility of tearing the bark.
Pruning Techniques
Specific types of pruning may be necessary to maintain a mature
tree in a healthy, safe, and attractive condition.
Cleaning:
the removal of dead, dying, diseased, crowded, weakly attached,
and low-vigor branches from the crown of a tree.
Thinning:
the selective removal of branches to increase light penetration
and air movement through the crown. Thinning opens the foliage of
a tree, reduces weight on heavy limbs, and helps retain the tree's
natural shape.
Raising:
removes the lower branches from a tree in order to provide
clearance for buildings, vehicles, pedestrians, and vistas.
Reduction:
reduces the size of a tree, often for clearance for utility lines.
Reducing the height or spread of a tree is best accomplished by
pruning back the leaders and branch terminals to lateral branches
that are large enough to assume the terminal roles (at least 1/3 the
diameter of the cut stem). Compared to topping, this helps maintain
the form and structural integrity of the tree.
How Much Should Be Pruned?
The amount of live tissue that should be removed depends on the
tree size, species, and age, as well as the pruning objectives.
Younger trees will tolerate the removal of a higher percentage of
living tissue than mature trees. An important principle to remember
is that a tree can recover from several small pruning wounds faster
than from one large wound.
A common mistake is to remove too much inner foliage and small
branches. It is important to maintain an even distribution of foliage
along large limbs and in the lower portion of the crown. Over-
thinning reduces the tree's sugar production capacity and can
create tip-heavy limbs that are prone to failure.
Mature trees should require little routine pruning. A widely
accepted rule of thumb is never to remove more than 1/4 of a tree's
leaf bearing crown. In a mature tree, pruning even that much could
have negative effects. Removing even a single, large-diameter
limb can create a wound that the tree may not be able to close. The
older and larger a tree becomes, the less energy it has in reserve
to close wounds and defend against decay or insect attack. The
pruning of large mature trees is usually limited to the removal of
dead or potentially hazardous limbs.
Wound Dressings
Wound dressings were once thought to accelerate wound closure,
protect against ins ects and diseases, and reduce decay. However,
research has shown that dressings do not reduce decay or speed
closure and rarely prevent insect or disease infestations. Most
experts recommend that wound dressings not be used. If a
dressing must be used for cosmetic purposes, then only a thin
coating of a non-toxic material should be applied.
Hire an Arborist
Pruning large trees can be dangerous. If pruning involves working
above the ground or using power equipment, it is best to hire a
professional arborist. An arborist can determine what type of
pruning is necessary to improve the health, appearance, and safety
of your trees. A professional arborist can provide the services of a
trained crew, with all of the required safety equipment and liability
insurance.
There are a variety of things to consider when selecting an arborist:
• Membership in professional organizations such as the
International Society of Arboriculture (ISA), the Tree Care Industry
Association (TCIA), or the American Society of Consulting Arborists
(ASCA).
• Certification through the ISA Certified Arborist program.
• Proof of insurance.
• A list of references (Don't hesitate to check.)
• Avoid using the services of any tree company that:
- Advertises topping as a service provided. Knowledgeable
arborists know that topping is harmful to trees and is not an
accepted practice.
- Uses tree climbing spikes to climb trees that are being pruned.
Climbing spikes can damage trees, and their use should be limited
to trees that are being removed.
For Additional Information
This brochure is one in a series published by the International
Society of Arboriculture as part of its Consumer Information
Program.
Why Hire an Arborist
Tree Benefits
Tree Values
Tree Selection
Buying High Quality Trees
Avoiding Tree & Utility Conflicts
New Tree Planting
Mature Tree Care
Plant Health Care
Trees and Turf
Proper Mulching Techniques
Pruning Young Trees
Pruning Mature Trees
Why Topping Hurts Trees
Insect & Disease Problems
Avoiding Tree Damage During Construction
Treatment of Trees Damaged by Construction
