- 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.
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
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
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
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
Specific types of pruning may be necessary to
maintain a mature tree in a healthy, safe and attractive condition.
- Crown cleaning
- is the removal of dead, dying, diseased, crowded, weakly attached
and low-vigor branches from the crown of a tree.
- Crown thinning
- is 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
- Crown raising
- removes the lower branches from a tree in order to provide clearance
for buildings, vehicles, pedestrians and vistas.
- Crown 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 one-third the diameter of
the cut stem). Compared to topping, this helps maintain the form and
structural integrity of the tree.
How much should be
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 one fourth 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
Wound dressings were once thought to accelerate wound closure, protect
against insects 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.
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 the International
Society of Arboriculture (ISA), the National Arborist Association (NAA)
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.
- This brochure is one in a series published by the
International Society of Arboriculture as part of its Consumer
Information Program. You may have additional interest in the following
titles currently in the series: Insect
and Disease Problems; Mature
Tree Care; New
Tree Planting; Trees and
of Trees; Tree
Health Care; Avoiding
Tree and Utility Conflicts; Recognizing
Tree Hazards; Why
Hire an Arborist; Buying
High-Quality Trees; Tree
Young Trees; Pruning
Mature Trees; Why
Topping Hurts Trees; Avoiding
Tree Damage During Construction; Treatment
of Trees Damaged by Construction.
Developed by the International Society of Arboriculture (ISA), a
non-profit organization supporting tree care research around the world
and dedicated to the care and preservation of shade and ornamental
trees. For further information, contact: ISA, P.O. Box 3129, Champaign,
IL 61826-3129, USA.
© 1997 International
Society of Arboriculture.
UPDATED FEBRUARY 2000