It is all together
too common for the natural growth characteristics of arid region
trees to be completely ignored when these species are pruned.
Trees native to mixed conifer and hardwood forests have a
generally upright growth habit, typically dominated by a strong
central leader. In these forests, competition is primarily for
sunlight. The ability to grow straight and tall give those trees
a significant competitive advantage over trees with other growth
habits. In the southwestern deserts, competition is primarily
for water. In this habitat, trees are typically highly dispersed
and tend to grow nearly as wide as they are tall, producing
branches that often extend to the ground. This unique growth
habit must be considered when developing pruning practices for
desert region landscape trees. The goals of pruning desert trees
must be to promote tree vigor and health and to enhance and
compliment the natural form of these native species.
Successful pruning requires an understanding of the growth
habits and unique horticulture of the tree being pruned. Most
popular desert landscaping books dedicate at least one section
to pruning. These books are excellent references that cover
subjects like crossing branches, proper cutting techniques and
tools. They also provide helpful diagrams and detailed
descriptions.
Poor pruning can ruin landscape
trees and cause damage that cannot easily be repaired. If you
are not sure what to do, get a certified arborist's professional
advice. Fortunately, extensive pruning is not required for most
desert tree species when they are carefully located in landscape
designs. Proper tree placement and growth management can
significantly reduce the amount of pruning desert adapted trees
require. When placing trees in the landscape consider the mature
height and spread of the tree. If in doubt, mildly exaggerate
the mature size rather than underestimate and risk having trees
ultimately too close. Plant trees strategically around
structures, play areas, pedestrian traffic and surrounding trees
so that mature trees can provide maximum shade with minimal
interference. Also, consider how mature trees will interact with
other components and uses of the landscape.
Desert adapted trees with light
colored trunks typically have small leaves that allow filtered
light through the canopy while species with dark trunks develop
heavier shade. When under story plantings are located beneath
dense canopy trees (like mesquites), regular thinning of the
trees will be needed to admit adequate sunlight for flowering
and lush growth. Annually evaluating pruning of mature trees can
reduce potential storm damage, simplify pedestrian access,
improve seating and better accommodate play areas.
In the process of developing
proper structure, caliper, branch and trunk taper nurseries
encourage the growth of "temporary branches". These branches,
usually removed when they are around a quarter to half inch in
diameter, promote the development of caliper and branch
thickening but are not a part of the mature tree. In the
nursery, the process of shaping and structure development begins
and must continue once the tree is installed in the landscape.
Care is required in selecting the correct limbs to prune during
every growing season to determine the appropriate amount of wood
to remove. Such pruning promotes proper growth of both feeder
and stabilizing roots, keeps leaf and root mass in balance, and
forms well defined branching structure.
Beyond this point, all that is required is maintenance pruning
and thinning to compliment and reinforce the established form
and remove potentially problematic branches. It follows then
that, in locations where single leader, upright trees are
required, the desired ultimate form will need to be established
at the time trees are purchased and installed. To develop
adequate taper and caliper, for single trunk trees to support
the canopy, requires keeping lower temporary limbs on the tree
perhaps for the first few growing seasons. This time frame
depends on the tree size when planted and the rate of growth
following installation. Rushing to raise the canopy height on
single trunk trees by remove temporary or major limbs too soon
will often encourage S or C shaped trunks.
Pruning always has some adverse effects on trees and should be
evaluated from the prospective of its impact on the tree's
growth and vigor. Pruning immediately following planting can
delay or prevent successful establishment of transplanted trees.
Studies on desert species, like Blue Palo Verde, suggest that as
much as 60% of all photosynthesis occurs on the surface of young
branches. Removing branches serves to limit the total surface
area available for photosynthesis, lowering the plants capacity
to manufacture "food". It also reduces the total amount of
stored carbohydrates (particularly with trees transplanted while
dormant) and results in less vigorous growth. Damaged or broken
branches can and should be removed but pruning for shaping or
form should be minimized at the time of transplanting.
A significant portion of the
structure and form of nursery-grown trees is usually firmly
established in the three to four years before they are offered
for sale. For these trees all that is required is maintenance
pruning and thinning to compliment and reinforce the established
form and the removal of potentially problematic branches. With
younger, underdeveloped trees, branching form, removal of
co-dominant branches, and many other pruning decisions will need
to be made. Where single trunk, upright trees are required, this
form should be well established at the time trees are purchased
and installed. In these instances it maybe necessary to purchase
larger box size trees. Attempting to substantially modify the
structure of desert species (e.g. pruning a low branching
multiple trunk tree into a more standard trunk form) is never
successful. The typical result is trunks that have a corkscrew
or highly twisted shape with large gaps between branches and
numerous, large and unsightly pruning scars.
PRUNING PHILOSOPHY: It is ALWAYS preferable to regularly
remove many small branches than to periodically remove a few
larger ones. Removal of crossing and parallel branches and
branches that pose hazards to foot traffic should be removed
first. Both crossing and obstructing branches are best removed
when they are relatively small twigs. By starting with "clean-up
pruning" (the removal of small branches) the general form of the
tree is more apparent. Stop periodically and step back from the
tree, like an artist working on a painting, and take in an
overall view of the tree from ALL sides. Identify problem areas
then begin pruning again. Repeat this process several times
while pruning. Recall that excessive branch removal and improper
or inappropriate pruning can act to stimulate additional
unwanted growth. Corrective pruning can stimulate desired growth
or reduce and better control growth and form.
The most common
point where tree branches fail is at the junction of two or more
co-dominant or adjacent branches. This failure usually is from
an included bark branching juncture or from lion tailing the
trees branching structure, over burdening the branching
connection points. Included bark is bark embedded or a bark
ridge turning inward between opposing branches, a branch and a
main trunk or two co-dominant branches creating a structurally
weak point in the tree Included bark prevents strong attachments
of branches, often causing a crack at the point where branches
meet. An inward bark ridge line usually develops where they join
and, more importantly, the included area declines or dies from
to the cambium of both branches being squeezed and killed,
weakening the branch or trunk. Trees with co-dominant leaders
tend to have included bark and are more likely to split and
ultimately fail. Included bark may be
remedied by removing the smaller of the two branches or the one
supporting less of the overall mass. Branches with wider
or U-shaped angle of attachment should be retained. Good branch
attachments have a raised ridge line or collar at the point
where branches meet.
Periodic light thinning is the
most desirable method of pruning. Such light thinning is
especially important in June, July and August. Defining the
trees landscape goal and outlining a growing season pruning plan
synchronizes desirable light monthly thinning instead of an
annual or bi-annual heavy pruning session. This managed pruning
can reduce the number of wind-damaged branches and prevent
uprooting of trees. Removal of large portions of the tree canopy
(more than 30%) during any one pruning session in the summer
growing season, can lead to aggressive, unwanted re-growth,
limited root development and increased vulnerability to sunburn
injuries that can be colonized by wood boring insects. No more
than 20% of the tree foliage should be removed at any one time
with 80% of this pruning concentrated on the new growth on the
outer third of the canopy. The remaining 20% should be removed
from the two thirds of the canopy removing succulent (“water”)
growth and crossing branches (Figure A).
No more
than 20% of the tree foliage should be removed at any
one time with 80% of this pruning concentrated on the
new growth on the outer third of the canopy.
PRUNING METHODS: It is well documented that sharper
pruning tools make cleaner cut that generally heal rapidly. Keep
pruners and saws sharpened and clean. Use the appropriate tool
for the size of branch being removed. "Fine toothed" saws can be
used on larger branches to finish the pruning process, leaving a
smooth cut surface that will quickly heal. Removal of small
branches can be done almost any time of year. Fall and winter
have the advantage of giving the individual a better view of the
structure of branches when leaves are shed.
The primary goals of tree pruning are to compliment the natural
shape of the tree and to promote healthy, vigorous growth.
Pruning can have a number of effects on trees. It can compensate
for root loss, aid in maintaining health and appearance, control
plant growth, influence vigor and re-invigorate stagnant trees.
Two things should always be appreciated when pruning: 1)
improper pruning can serve to stimulate additional unwanted
growth; and 2) each pruning cut creates a wound in the tree's
protective bark. Proper and selective cuts will enhance the
appearance and health of tree and leave wounds that will heal
rapidly. The angle and position of pruning cuts greatly
influences the overall success of a pruning program and dictate
how quickly wounds will "heal". Pruning cuts should be made
close to, but not beyond, the branch bark ridge and the collar
at the base of the branch (Figure 1).
Figure 1
Proper
Pruning and Healing
A
clean, angled pruning cut (A) leaves a smooth cut
surface with intact edges allowing callus formation
(B) to proceed rapidly. The result (C) is a pruning
wound that closes quickly, preventing invasion by
insects or die back of tissue as the result of
drying.
Sharp tools that make clean,
smooth edged wounds will heal the quickest. These wounds don't
"heal" like animal wounds. Instead trees produce callus tissue
that essentially "re-cover" the open area (Figure 2).
Wounds heal from the edges. This healing is easily seen by
observing the ring of raised or swollen bark surrounding the
edge of a healing wound. The bark tissue forms the callus,
giving the edges of the cut this raised appearance. Over time,
with growth and the subsequent increase in the branch diameter,
the old wound is closed completely. Dull pruners and saws leave
ragged edged cuts. Such cuts develop callus more slowly,
delaying the healing process. Wounds that are slow to heal can
be sources of oozing sap (that can stain hardscape elements and
patio furniture) and serve as points of entry for insects,
bacteria and fungi. These pests can cause additional damage and
further delay healing.
Figure
2:
Pruning Cut Angles Dictate the Pattern of Wound Tissue
Formation
Improper pruning cut angles result in
incomplete distribution of wound healing
tissue that will slow the healing process and can lead
to die-back, infestation by
insects or infection by fungi and bacteria.
Correct
Cut Results with Good Wound Wood Pattern
Cuts that preserve the branch collar form a complete
ring of callus tissue, heal rapidly, help prevent
invasion by pests and ultimately promote healthy growth.
Another common mistake is leaving
short stumps instead of pruning branches off just above the
collar. Aside from being extremely unsightly (and unprofessional
looking) these stubs can sunburn, dry out, cause die back and
serve as entry sites for some wood boring insects, bacteria and
fungi. Tree borer damage is often misidentified as being caused
by tree stress or general decline when in fact it is the result
of stub pruning. Secondly stubs can snag clothing or the skin of
pedestrians. Most importantly, such cuts generate additional
unwanted branches by stimulating both lateral and adventitious
buds (bud arising from previously woody tissue) to produce
numerous new branches. These additional branches must ultimately
be removed by additional pruning (Figure 3).
Figure 3:
Other Common Pruning
Mistakes
Flush cuts cause
underlying tissues to dry out slowing callous formation, are subject
to insect invasion and promote tissue die-back.
Pruning cuts that
leave a branch stump can result in tissue die-back (A) or
proliferation of unwanted branches (B). Stumps may also be a hazard
to pedestrians by snagging clothing or causing injury to passersby.
Removing the branch at the collar
serves to direct the subsequent growth towards the terminal of
the remaining branch without excessively stimulating other buds
to produce additional branches. Similarly, when heading back a
branch, pruning back to a lateral bud seems to direct the branch
growth through that bud making it the new terminal bud (Figure
4).
Figure 4:
Pruning Cut Angle and
Placement
1. If the cut angle is
too steep (A) the tissue can dry before healing and cause die back.
Too flat an angle (B) leaves a blunt surface that can collect water
which slow wound healing and can promote decay. Cutting too far (C)
from a lateral branch or bud can over stimulate new growth resulting
in undesirable multiple, tightly packed branches, die-back and
defeat the original purpose of pruning.
2. Make pruning cut
directly above a bud or lateral branch at about a 45 degree angle.
Such cuts result in controlled growth of the cut branch and healing
of the pruning wound.
Growth Management: Growth
management is the least exploited strategy to avoid pruning.
Many established desert natives can be naturalized to where they
can survive with little or no supplemental irrigation. The
practice of limiting water and fertilizer serves to
significantly slow growth and reduce the need for pruning and
thinning. For species that cannot be entirely naturalized,
limiting irrigation and nitrogen can reduce growth and the need
for pruning.
Seasonal Pruning: Periodic
thinning is the most desirable method of pruning. Thinning trees
before the monsoon season can reduce wind damage to branches and
uprooting of trees. Removal of large portions of the tree canopy
(more than 30%) during the summer can lead to sunburn injuries
that can later be colonized by wood boring insects. Avoid
hedging or heading back desert species, as this will only
stimulate excessive branching. For trees less than 7 years old,
monthly light pruning (removal of no more than 20 to 30% of the
leaf mass) during the growing season will keep the root to
foliage mass ratio equal, not stimulate excessive flush growth,
allow optimal photosynthesis to occur, initiate quicker root and
tip branch growth, prevent sun burn injury and lessen the
likelihood of wind damage. Always use clean, sharp tools that
are regularly cleaned in a 10% solution of bleach.
Some desert species are slow to
exhibit the effects of freeze damage. Branches that do not
immediately bear leaves in spring may not be damaged. Delay
pruning suspect damaged branches until the tree has begun
producing new leaves. By then residual freeze damaged branches
should be obvious and easily removed. Remember Spring pruning
can reduce spring and summer flowering.
The old adage, "A craftsman never
blames his tools," actually carries two distinct but important
messages. The first is that a craftsman takes personal
responsibility for the work he produces and second that he uses
and maintains his tools in such a way that they only enhance the
quality of his work.
All the information detailed above
is of little value if the pruning tools used are not up to the
task. Research has consistently shown that smooth-edged, clean
pruning cuts heal quickest and are least prone to invasion by
insect pests or disease organisms.
When selecting pruning tools,
three key words to think about are CLEAN, SHARP
and APPROPRIATE. These three ideas share the common
purpose of producing clean, smoothed edged cuts. Pruning tools
should be kept clean, should be sharp and should be the
appropriate type for the limb size.
Cleaning pruning tool blades after
each cut reduces the chance of spreading disease-causing
organisms from one tree to another or to other parts of the same
tree. To sanitize pruners, dip the blades in a solution that is
1 part bleach and 9 parts water (10 % solution). Bleach acts as
a sterilizing agent and helps prevent the spread of diseases
within and between trees being pruned. This cleaning solution
should be refreshed by adding more bleach every 20 to 30
minutes, as it loses its disinfectant potency quickly. Remove
debris (wood fibers, bits of debris) from the cutting edge as
they serve to dull the cutting surface and prevent clean cuts.
For greatest ease of use and long life, regularly oil the hinge
and the blades of pruning tools.
Before each pruning session
inspect the cutting surface of the pruners to see that they are
clean and sharp. Consult the owner's manual or manufacturer's
instructions regarding the appropriate sharpening tools and
proper sharpening techniques. With some tools, particularly
saws, it may be advisable to have them professionally
sharpened.
Pruning and pole saws along with
hand pruners and loppers come in many sizes and forms. Most
pruning tasks can be accomplished with three basic tools. The
tool ultimately selected is determined by the diameter of the
branch being pruned. As a general rule, hand pruners (Figure
5) should be used for removing branches smaller than the
diameter of your finger (about 1/2" in diameter), loppers (Figure
6) for branches slightly larger than your finger (about 1"in
diameter) and pruning saws for anything larger (Figure 7).
In spite of these rules, several other factors influence tool
selection. Never force pruners through a cut. In these
situations the blades do more crushing than cutting and the
resulting wounds heal very slowly. If done correctly and
regularly, pruning should only rarely (wind damage or other
injuries excluded) require the removal of limbs larger than 2"
diameter. To maintain greatest vigor, health and appearance,
desert trees should be pruned lightly and regularly as opposed
to heavily and infrequently.
The pruner used should easily cut the branches being removed. In
some instances green wood (young growth) may be quite soft and
easily cut while hardened, dead, mature or dormant wood may
prove more difficult. Most popular pruners and loppers are the
"hook and blade" bypass type where both blades have sharp edges
and the cutting edge of one blade passes underneath the other.
Properly sharpened bypass pruners produce smooth cuts with
minimal hand effort.
Arborist's saws range in size from
6 to 26 inches long with varying numbers of teeth per inch along
the blade (Figure 8). Higher numbers of teeth per inch
make for smoother, cleaner cuts. Such saws generally do not cut
as quickly. Ideally, for cutting larger limbs, use a large
toothed saw for the initial cut, then make a finish cut with a
fine toothed saw to leave a smooth, clean, rapidly healing wound
surface (Figure 9).
Most straight saws tend to cut on
the forward or push stroke while most curved saws cut on the
pull or back stroke. Curved saws are ideal for cutting branches
that are above head level. Folding or fixed blade saws work
equally well and selection is generally based on the users
preference and ease of carrying. Avoid "bow" and chain saws for
making finishing cuts. Both these saws are best adapted for
felling trees or large branches, not careful pruning.
In summary
1.
Pruning can do as much harm as good to tree
growth, form and vigor.
2.
Select proper tree variety, trunk form, and tree
spacing for landscape location.
3.
Prune to compliment and reinforce the tree's
natural form and shape.
4.
Light (no more than 20-30% of leaf mass) and
regular pruning is more desirable than heavy and infrequent.
5.
Maintain temporary limbs to develop trunk caliper
and taper.
6.
Proper angle cuts accelerating healing of wound.
7.
Use pruning tools that are appropriate to the size
of branches being cut.
8.
Always use sharp tools.
9.
Clean tools with disinfectant at regular intervals
during a pruning session.
10.
Change or replenish disinfectant frequently during
pruning session.
