HOW to Prune Trees
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Table of Contents
Introduction
Reasons for pruning
Pruning approaches
Crown thinning
Crown raising
Crown reduction
Pruning cuts
Pruning living branches
Pruning dead branches
Drop crotch cuts
Pruning practices that harm tree
References
Authors |
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Introduction |
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The objective of pruning is to produce
strong, healthy, attractive plants. By understanding how, when
and why to prune, and by following a few simple principles,
this objective can be achieved. |
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Figure 1
Reason for pruning |
Reasons for pruning The
main reasons for pruning ornamental and shade trees include
safety, health, and aesthetics. In addition, pruning can be
used 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 encourage 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.
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All woody
plants shed branches in response to shading and competition.
Branches that do not produce enough carbohydrates from
photosynthesis to sustain themselves die and are eventually shed;
the resulting wounds are sealed by
woundwood
(callus).
Branches that are poorly attached may be broken off by wind and
accumulation of snow and ice. Branches removed by such natural
forces often result in large, ragged wounds that rarely seal.
Pruning as a cultural practice can be used to supplement or replace
these natural processes and increase the strength and longevity of
plants.
Trees have many forms, but the most common
types are pyramidal (excurrent) or
spherical (decurrent ). Trees with
pyramidal crowns, e.g., most conifers, have a strong central stem
and lateral branches that are more or less horizontal and do not
compete with the central stem for dominance. Trees with spherical
crowns, e.g., most hardwoods, have many lateral branches that may
compete for dominance. To reduce the need for pruning it is best to
consider a tree's natural form. It is very difficult to impose an
unnatural form on a tree without a commitment to constant
maintenance.
Pollarding and
topiary are extreme examples of pruning to create a desired,
unnatural effect. Pollarding is the practice of pruning trees
annually to remove all new growth. The following year, a profusion
of new branches is produced at the ends of the branches. Topiary
involves pruning trees and shrubs into geometric or animal shapes.
Both pollarding and topiary are specialized applications that
involve pruning to change the natural form of trees. As topiary
demonstrates, given enough care and attention plants can be pruned
into nearly any form. Yet just as proper pruning can enhance the
form or character of plants, improper pruning can destroy it.
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Producing strong
structure should be the emphasis when pruning young trees. As
trees mature, the aim of pruning will shift to maintaining
tree structure, form, health and appearance.
Proper pruning cuts
are made at a node, the point at which one branch or twig
attaches to another. In the spring of the year growth begins
at buds, and twigs grow until a newnode is formed. The length
of a branch between nodes is called an internode.
The most common types of pruning are:
1.Crown thinning,
primarily for hardwoods, is the selective removal of branches
to increase light penetration and air movement throughout
the crown of a tree. The intent is to maintain or develop a
tree's structure and form. To avoid unnecessary stress and
prevent excessive production of epicormic sprouts, no more
than one-quarter of the living crown should be removed at a
time. If it is necessary to
remove more, it should be done over successive years. |
Fig 2. Crown thinning -
branches to be removed are shaded iblue; pruning should be
maid at the red lines. No more than one forth of the living
branches should be removed |
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Branches with strong
U-shaped angles of attachment should be retained
(Fig 3A). Branches with narrow, V- shaped angles of
attachment often form included bark and should be removed
(Fig. 3B). Included bark forms
when two branches grow at sharply acute angles to one another,
producing a wedge of inward-rolled bark between them. Included
bark prevents strong attachment of branches, often causing a
crack at the point below where the branches meet. Codominant
stems that are approximately the same size and arise from the
same position often form included bark. Removing some of the
lateral branches from codominant stem can reduce its growth
enough to allow the other stem to become dominant. |
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Lateral branches
should be no more than one-half to three-quarters of the
diameter of the stem at the point of attachment. Avoid
producing "lion's tails," tufts of branches and foliage at the
ends of branches, caused by removing all inner lateral
branches and foliage. Lion's tails can result in sun scalding,
abundant
epicormic sprouts,
and weak branch structure and breakage. Branches that rub or
cross another branch should be removed.
Conifers that
have branches in whorls and pyramidal crowns rarely need crown
thinning except to restore a dominant leader. Occasionally,
the leader of a tree may be damaged and multiple branches may
become codominant. Select the strongest leader and remove
competing branches to prevent the development of codominant
stems. |
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Fig 4. Crown raising - branches to be
removed are shaded
in blue; pruning cuts should be made where indicated with red
lines.
The ratio of live crown to total tree
height should be at least two-thirds. |
Crown raising
is the practice of removing
branches from the bottom of the crown of a tree to provide
clearance for pedestrians, vehicles, buildings, lines of site,
or to develop a clear stem for timber production. Also,
removing lower branches on white pine scan prevent blister
rust. For street trees the minimum clearance is often
specified by municipal ordinance. After pruning, the ratio of
the living crown to total tree height should be at least
two-thirds (e.g., a 12 m tree should have living
branches on at least the upper 8 m).
On young trees "temporary" branches may
be retained along stem to encourage taper and protect trees
from vandalism and sun scald. Less
vigorous shoots should be selected
as branches and should be about 10 to 15 cm apar tlong the
stem. They should be pruned annually slow their growth and
should be removed
eventually |
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Fig 5. Crown reduction - branches to be
removed are
shaded in blue;pruning cuts should be made where
indicated with red lines The ratio of live crown to total
height should be at least tow-thirds
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Crown reduction pruning is most often used when a tree
has grown too large for its permitted space. This method,
sometimes called drop crotch pruning,
is preferred to topping because it results in a more natural
appearance, increases the time before pruning is needed again,
and minimizes stress (see drop crotch
cuts in the next section).
Crown reduction pruning, a method of last resort, often
results in large pruning wounds to
stems that may
lead to decay. This method should never be used on a tree with
a pyramidal growth
form. A better long term solution is to
remove the tree and replace it with a tree that
will not grow beyond the
available space. |
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Pruning cuts should be made so that
only branch tissue is removed and stem tissue is not damaged.
At the point where the branch attaches to the stem, branch and
stem tissues remain separate, but are contiguous. If only
branch tissues are cut when pruning, the stem tissues of the
tree will probably not become decayed, and the wound will seal
more effectively. |
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1. Pruning living
branches (Fig. 6 A.B) |
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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 not damage either the
branch bark ridge or the branch collar. |
Fig 6 A. Targeting the cut |
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A proper cut
begins just outside the branchbark ridge and angles down away
from the stem of the tree, avoiding injury to the branch
collar (Fig. 6B). Make the cut as
close as possible to the stem in the
branch axil,
but outside the branch bark
ridge, so that stem tissue is not injured and the wound can
seal in the shortest time possible. If the cut is too far from
the stem, leaving a branch stub, the branch tissue usually
dies and woundwood forms from the stem tissue. Wound closure
is delayed because the woundwood must seal over the stub that
was left. |
Fig
6 B-C. Branch Collar |
The quality of pruning cuts can be
evaluated by examining pruning wounds after one growing
season. A concentric ring of woundwood will form from proper
pruning cuts
(Fig. 6B). Flush cuts made inside
the branch bark ridge or branch collar, result in pronounced
development of woundwood on the sides of the pruning wounds
with very little woundwood forming on the top or bottom
(Fig. 7D). As described above,
stub cuts result in the death of the remaining branch and
woundwood forms around the base from stem tissues.
When pruning
small branches with hand pruners, make sure the tools are
sharp enough to cut the branches cleanly without tearing.
Branches large enough to require saws should be supported with
one hand while the cuts are made. If the branch is too large
to support, make a three-step pruning cut to prevent bark
ripping
(Fig. 6C).
1.The first cut is a shallow
notch made on the underside of the branch, outside the branch
collar. This cut will prevent a falling branch from tearing
the stem tissue as it pulls away from the tree.
2.The second cut should be
outside the first cut, all the way through the branch, leaving
a short stub.
3.The stub is then cut just
outside the branch bark ridge/branch collar, completing the
operation.
2. Pruning dead branches (Fig. 6)
Prune dead branches
in much the same way as live branches. Making the correct cut
is usually easy because the branch collar and the branch bark
ridge, can be distinguished from the dead branch, because they
continue to grow (Fig. 6A). Make
the pruning cut just outside of the ring of woundwood tissue
that has formed, being careful not to cause unnecessary injury
(Fig. 6C). Large dead branches
should be supported with one hand or cut with the three-step
method, just as live branches. Cutting large living branches
with the three step method is more critical because of the
greater likelihood of bark ripping.
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3.
Drop Crotch Cuts
(Fig. 6D)
A proper cut begins just
above the branch bark ridge and extends through the stem
parallel to the branch bark ridge. Usually, the stem being
removed is too large to be supported with one hand, so the
three cut method should be used.
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1.With the first cut,
make a notch on the side of the stem away from the branch to
be retained, well above the branch crotch.
2.Begin the second cut inside the branch crotch, staying well
above the branch bark ridge, and cut through the stem above
the notch.
3.Cut the remaining stub just inside the branch bark ridge
through the stem parallel to the branch bark ridge . |
D. Crown reduction
Fig. 6 D. Pruning cuts
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To prevent the abundant growth
of epicormic sprouts on the stem below the cut, or dieback of
the stem to a lower lateral branch, make the cut at a lateral
branch that is at least one-third of the diameter of the stem
at their union.
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Pruning Practices That Harm Trees |
Topping
and tipping
(Fig. 7A, 7B)
are pruning
practices that harm trees and should not be used. Crown
reduction pruning is the preferred method to reduce the size
or height of the crown of a tree, but is rarely needed and
should be used infrequently.
Topping, the
pruning of large upright branches between nodes, is sometimes
done to reduce the height of a tree
(Fig. 7A).
Tipping is a practice of cutting
lateral branches between nodes (Fig. 7B)
to reduce crown width.
These
practices invariably result in the development of epicormic
sprouts, or in the death of the cut branch back to the next
lateral branch below. These epicormic sprouts are weakly
attached to the stem and eventually will be supported by a
decaying branch.
Improper pruning cuts cause unnecessary injury and bark
ripping
(Fig. 7C).
Flush cuts injure stem
tissues and can result in decay (Fig.
7D). Stub cuts delay wound closure and can provide
entry to canker fungi that kill the cambium, delaying or
preventing woundwood formation
(Fig. 7E). |
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Northeastern Area State and Private Forestry Offices -
Headquarters - Northeastern Area State & Private Forestry,100
Matsonford Road , 5 Radnor Corporate Center, Suite 200 Radnor,
PA 19087-4585 Durham Field Office. Northeastern Area State &
Private Forestry - USDA Forest Service .Louis C. Wyman Forest
Services Laboratory , P.O. Box 640 ,Durham, NH
03824-0640
Morgantown Field Office - Northeastern Area State & Private
Forestry USDA Forest Service 180 Canfield Street Morgantown,
WV 26505-3101 St. Paul Field Office Northeastern Area State &
Private Forestry USDA Forest Service 1992 Folwell Avenue St.
Paul, MN551081099
Authors
Peter J. Bedker, Plant Pathologist, USDA Forest Service,
Northeastern Area State and Private Forestry
Joseph G. O'Brien,
Plant Pathologist, USDA Forest Service, Northeastern Area
State and Private Forestry
Manfred E. Mielke, Forest Health
Specialist, USDA Forest Service, Northeastern Area State and
Private Forestry
Illustrations by: Julie Janke, Scientific Illustrator, Afton,
Minnesota
Project Coordinator: Gerard D. Hertel, Assistant
Director, Forest Health and Management, USDA Forest
Service, Northeastern Area State and Private
Forestry. |
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