How to prune trees

Historic, Archive Document

Do not assume content reflects current
scientific knowledge, policies, or practices.

HOW to

m *

1

United States
Department of
Agriculture

Prepared by
Forest Service

Northeastern Area

State & Private
Forestry

NA-FR-01-95

How to

Prune Trees

Contents Page

Introduction 1

Reasons for pruning 1

Pruning approaches 4

Crown thinning 4
Crown raising 7
Crown reduction 8

Pruning cuts 10

Pruning living branches 10

Pruning dead branches 1 2

Drop crotch cuts 13

Pruning practices that harm trees 14

When to prune 16

Pruning tools 17

Treating wounds 20

Pruning guidelines 21

General 2 1

Crown thinning 22

Crown raising 22

Crown reduction 23

Glossary 23

References 26

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Introduction

The objective of pruning is to produce
strong, healthy, attractive plants. By under-
standing how, when and why to prune, and
by following a few simple principles, this
objective can be achieved.

Reasons For Pruning

The 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. 1 A)
involves removing branches that could fall
and cause injury or property damage, trim-
ming 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. IB) involves re-
moving 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.

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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.

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.

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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.

Pruning Approaches

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 new
node is formed. The length of a branch
between nodes is called an internode.

The most common types of pruning are:

1 . Crown Thinning (Fig. 2)

Crown thinning, primarily for hardwoods,
is the selective removal of branches to

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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.

Figure 2. 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.

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A. U-shaped strong union B. V-shaped weak union

Figure 3. Types of branch unions

Branches with strong U-shaped angles of
attachment should be retained (Fig 3 A).
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 a codominant stem can reduce
its growth enough to allow the other stem to
become dominant.

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

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sunscalding, 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.

2. Crown Raising (Fig. 4)

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
pines can 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 the stem to encourage taper
and protect trees from vandalism and sun
scald. Less vigorous shoots should be
selected as temporary branches and should
be about 10 to 15 cm apart along the stem.
They should be pruned annually to slow their
growth and should be removed eventually.

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Figure 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.

3. Crown Reduction (Fig. 5)

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

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Figure 5. Crown reduction - branches to be removed
are shaded in blue; pruning cuts should be made where
indicated with red lines. To prevent branch dieback,
cuts should be made at lateral branches that are at least
one-third the diameter of the stem at their union.

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

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.

1 . Pruning living branches (Fig. 6)

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.

A proper cut begins just outside the branch
bark 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.

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B. Cutting a small branch C. Cutting a larger branch

D. Crown reduction cut

Figure 6. Pruning cuts

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

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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

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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.

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.

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.

To prevent the abundant growth of
epicormic sprouts on the stem below the cut,
or dieback of the stem to a lower lateral

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branch, make the cut at a lateral branch that
is at least one-third of the diameter of the
stem at their union.

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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Figure 7. Practices that harm trees
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When to Prune

Conifers may be pruned any time of year,
but pruning during the dormant season may
minimize sap and resin flow from cut
branches.

Hardwood trees and shrubs without showy
flowers: prune in the dormant season to
easily visualize the structure of the tree, to
maximize wound closure in the growing
season after pruning, to reduce the chance of
transmitting disease, and to discourage
excessive sap flow from wounds. Recent
wounds and the chemical scents they emit
can actually attract insects that spread tree
disease. In particular, wounded elm wood is
known to attract bark beetles that harbor
spores of the Dutch elm disease fungus, and
open wounds on oaks are known to attract
beetles that spread the oak wilt fungus.
Take care to prune these trees during the
correct time of year to prevent spread of
these fatal diseases. Contact your local tree
disease specialist to find out when to prune
these tree species in your area. Usually, the
best time is during the late fall and winter.

Flowering trees and shrubs: these should
also be pruned during the dormant season
for the same reasons stated above; however,
to preserve the current year's flower crop,
prune according to the following schedule:

> Trees and shrubs that flower in early
spring (redbud, dogwood, etc.)
should be pruned immediately after

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flowering (flower buds arise the year
before they flush, and will form on
the new growth).

> Many flowering trees are susceptible
to fireblight, a bacterial disease that
can be spread by pruning. These
trees, including many varieties of
crabapple, hawthorn, pear, mountain
ash, flowering quince and pyracan-
tha, should be pruned during the
dormant season. Check with your
county extension agent or a horticul-
turist for additional information.

> Trees and shrubs that flower in the
summer or fall always should be
pruned during the dormant season
(flower buds will form on new twigs
during the next growing season, and
the flowers will flush normally).

Dead branches: can be removed any time
of the year.

Pruning Tools

Proper tools are essential for satisfactory
pruning (Fig. 6). The choice of which tool to
use depends largely on the size of branches
to be pruned and the amount of pruning to
be done. If possible, test a tool before you
buy it to ensure it suits your specific needs.
As with most things, higher quality often
equates to higher cost.

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Generally speaking, the smaller a branch is
when pruned, the sooner the wound created
will seal. Hand pruners are used to prune
small branches (under 2.5 cm diameter) and
many different kinds are available. Hand
pruners can be grouped into by-pass or anvil
styles based on the blade configuration.
Anvil style pruners have a straight blade that
cuts the branch against a small anvil or
block as the handles are squeezed. By-pass
pruners use a curved cutting blade that
slides past a broader lower blade, much like
a scissors. To prevent unnecessary tearing
or crushing of tissues, it is best to use a by-
pass style pruner. Left- or right-handed
types can be purchased.

Slightly larger branches that cannot be cut
with a hand pruner may be cut with small
pruning saws (up to 10 cm) or lopping
shears (up to 7 cm diameter) with larger
cutting surfaces and greater leverage. Lop-
ping shears are also available in by-pass and
anvil styles.

For branches too large to be cut with a hand
pruner or lopping shears, pruning saws must
be used. Pruning saws differ greatly in
handle styles, the length and shape of the
blade, and the layout and type of teeth.
Most have tempered metal blades that retain
their sharpness for many pruning cuts.
Unlike most other saws, pruning saws are
often designed to cut on the "pull-stroke."

Chain saws are preferred when pruning
branches larger than about 10 cm.
Chainsaws should be used only by qualified

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individuals. To avoid the need to cut
branches greater than 10 cm diameter, prune
when branches are small.

Pole pruners must be used to cut branches
beyond reach. Generally, pruning heads can
cut branches up to 4.4 cm diameter and are
available in the by-pass and anvil styles.
Once again, the by-pass type is preferred.
For cutting larger branches, saw blades can
be fastened directly to the pruning head, or a
separate saw head can be purchased. Be-
cause of the danger of electrocution, pole
pruners should not be used near utility lines
except by qualified utility line clearance
personnel.

To ensure that satisfactory cuts are made
and to reduce fatigue, keep your pruning
tools sharp and in good working condition.
Hand pruners, lopping shears, and pole
pruners should be periodically sharpened
with a sharpening stone. Replacement
blades are available for many styles. Prun-
ing saws should be professionally sharpened
or periodically replaced. To reduce cost,
many styles have replaceable blades.

Tools should be clean and sanitized as well
as sharp. Although sanitizing tools may be
inconvenient and seldom practiced, doing so
may prevent the spread of disease from
infected to healthy trees on contaminated
tools. Tools become contaminated when
they come into contact with fungi, bacteria,
viruses and other microorganisms that cause
disease in trees. Most pathogens need some
way of entering the tree to cause disease,

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and fresh wounds are perfect places for
infections to begin. Microorganisms on tool
surfaces are easily introduced into
susceptible trees when subsequent cuts are
made. The need for sanitizing tools can be
greatly reduced by pruning during the
dormant season.

If sanitizing is necessary it should be
practiced as follows: before each branch is
cut, sanitize pruning tools with either 70%
denatured alcohol, or with liquid household
bleach diluted 1 to 9 with water (1 part
bleach, 9 parts water). Tools should be
immersed in the solution, preferably for 1-2
minutes, and wood particles should be
wiped from all cutting surfaces. Bleach is
corrosive to metal surfaces, so tools should
be thoroughly cleaned with soap and water
after each use.

Treating Wounds

Tree sap, gums, and resins are the natural
means by which trees combat invasion by
pathogens. Although unsightly, sap flow
from pruning wounds is not generally
harmful; however, excessive "bleeding" can
weaken trees.

When oaks or elms are wounded during a
critical time of year (usually spring for oaks,
or throughout the growing season for elms)
— either from storms, other unforeseen
mechanical wounds, or from necessary
branch removals — some type of wound
dressing should be applied to the wound.

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Do this immediately after the wound is
created. In most other instances, wound
dressings are unnecessary, and may even be
detrimental. Wound dressings will not stop
decay or cure infectious diseases. They may
actually interfere with the protective
benefits of tree gums and resins, and prevent
wound surfaces from closing as quickly as
they might under natural conditions. The
only benefit of wound dressings is to
prevent introduction of pathogens in the
specific cases of Dutch elm disease and oak
wilt.

Pruning Guidelines

To encourage the development of a strong,
healthy tree, consider the following guide-
lines when pruning.

General

> Prune first for safety, next for health,
and finally for aesthetics.

> Never prune trees that are touching
or near utility lines; instead consult
your local utility company.

> Avoid pruning trees when you might
increase susceptibility to important
pests (e.g., in areas where oak wilt
exists, avoid pruning oaks in the
spring and early summer; prune trees
susceptible to fireblight only during
the dormant season).

> Use the following decision guide for
size of branches to be removed:

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1) under 5 cm diameter - go ahead,

2) between 5 and 10 cm diameter -
think twice, and 3) greater than 10
cm diameter - have a good reason.

Crown Thinning

y Assess how a tree will be pruned
from the top down.

> Favor branches with strong, U-
shaped angles of attachment. Re-
move branches with weak, V-shaped
angles of attachment and/or included
bark.

Ideally, lateral branches should be
evenly spaced on the main stem of
young trees.

Remove any branches that rub or
cross another branch.

> Make sure that lateral branches are
no more than one-half to three-
quarters of the diameter of the stem
to discourage the development of co-
dominant stems.

Do not remove more than one-
quarter of the living crown of a tree
at one time. If it is necessary to
remove more, do it over successive
years.

Raising

Always maintain live branches on at
least two-thirds of a tree's total
height. Removing too many lower
branches will hinder the
development of a strong stem.

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>

>

Crown
>

> Remove basal sprouts and vigorous
epicormic sprouts.

Crown Reduction

> Use crown reduction pruning only
when absolutely necessary. Make
the pruning cut at a lateral branch
that is at least one-third the diameter
of the stem to be removed.

> If it is necessary to remove more
than half of the foliage from a
branch, remove the entire branch.

Glossary

Branch Axil: the angle formed where a
branch joins another branch or stem of a
woody plant.

Branch Bark Ridge: a ridge of bark that
forms in a branch crotch and partially
around the stem resulting from the growth of
the stem and branch tissues against one
another.

Branch Collar: a "shoulder" or bulge
formed at the base of a branch by the annual
production of overlapping layers of branch
and stem tissues.

Crown Raising: a method of pruning to
provide clearance for pedestrians, vehicles,
buildings, lines of sight, and vistas by
removing lower branches.

Crown Reduction Pruning: a method of
pruning used to reduce the height of a tree.

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Branches are cut back to laterals that are at
least one-third the diameter of the limb
being removed.

Crown Thinning: a method of pruning to
increase light penetration and air movement
through the crown of a tree by selective
removal of branches.

Callus: see woundwood.

Decurrent: a major tree form resulting
from weak apical control. Trees with this
form have several to many lateral branches
that compete with the central stem for
dominance resulting in a spherical or glo-
bose crown. Most hardwood trees have
decurrent forms.

Epicormic Sprout: a shoot that arises from
latent or adventitious buds; also know as
water sprouts that occur for on stems and
branches and suckers that are produced from
the base of trees. In older wood, epicormic
shoots often result from severe defoliation
or radical pruning.

Excurrent: a major tree form resulting
from strong apical control. Trees with this
form have a strong central stem and pyrami-
dal shape. Lateral branches rarely compete
for dominance. Most conifers and a few
hardwoods, such as sweetgum and tuliptree,
have excurrent forms.

Flush Cuts: pruning cuts that originate
inside the branch bark ridge or the branch
collar, causing unnecessary injury to stem
tissues.

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Included Bark: bark enclosed between
branches with narrow angles of attachment,
forming a wedge between the branches.

Pollarding: the annual removal of all of the
previous year's growth, resulting in a flush
of slender shoots and branches each spring.

Stub Cuts: pruning cuts made too far
outside the branch bark ridge or branch
collar, that leave branch tissue attached to
the stem.

Tipping: a poor maintenance practice used
to control the size of tree crowns; involves
the cutting of branches at right angles
leaving long stubs.

Topping: a poor maintenance practice
often used to control the size of trees;
involves the indiscriminate cutting of
branches and stems at right angles leaving
long stubs. Synonyms include rounding-
over, heading-back, dehorning, capping and
hat-racking. Topping is often improperly
referred to as pollarding.

Topiary: the pruning and training of a plant
into a desired geometric or animal shape.

Woundwood: lignified, differentiated
tissues produced on woody plants as a
response to wounding (also known as callus
tissue).

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References

ANSI Z133.1. 1994. Safety standards.

American national standard for tree care
operators. Washington, DC: American
National Standards Institute.

ANSI A300. 1995. Standard practices for
tree, shrub, and other woody plant
maintenance. Washington, DC: Ameri-
can National Standards Institute.

Fazio, J. R. ed. 1992. Don't top trees. Tree
City USA Bulletin No. 8. Nebraska City,
NE: The National Arbor Day Founda-
tion.

Harris, R.W. 1994. Clarifying certain prun-
ing terminology: thinning, heading,
pollarding. Journal of Arboriculture
20:50-54.

ISA Performance Guidelines Committee.
1994. Tree-pruning guidelines. Savoy,
IL: International Society of
Arboriculture.

Ryan, H.D.P. III. 1994. Arboricultural
pruning methodologies. Arborist News
Volume 3(4):33-38.

Shigo, A. 1991. Modern arboriculture.
Durham, NH: Shigo & Trees, Associ-
ates.

Shigo, A. 1989. Tree pruning: a worldwide
photo guide. Durham, NH: Shigo &
Trees, Associates.

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Notes:

27

Notes:

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"How to Prune Trees" was written to help
people properly prune the trees they care
about. If you doubt your ability to safely
prune large trees, please hire a professional
arborist. Information in this publication can
be used to interview and hire a competent
arborist.

Northeastern Area State & Private
Forestry Offices:

Headquarters Office

Northeastern State & Private Forestry

USDA Forest Service

100 Matsonford Road

5 Radnor Corporate Center, Suite 200

Radnor, PA 19087-8775

Durham Field Office

Northeastern Area State & Private Forestry
USDA Forest Service

Louis C. Wyman Forest Sciences Laboratory

P.O Box 640

Durham, NH 03824-9799

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, MN 55108-1099

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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 Special-
ist, USDA Forest Service, Northeastern
Area State and Private Forestry

Illustrations by:

Julie Janke, Scientific Illustrator, Afton,
Minnesota

For Further Information, Contact:

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