are attached to the tree trunk by interlocking branch and trunk
tissue. A new layer of interlocking tissue is produced each year
over the previous layer. A branch collar, produced by the trunk,
holds the branch base. When the branch/trunk union has a narrow
angle, the branch and trunk bark become overgrown and is
referred to as included bark (Figure 1). This results in a weak
union that is likely to split.
Trees have a natural defense response to wounds and pruning
cuts. They form four types of walls to compartmentalize the area
thus preventing the spread of decay organisms (Figure 2). The
decay or injury remains but is sealed off and does not increase
in size if the walls are stronger than the decay organisms. The
storage capacity and function of the injured wood is lost
an injury, Wall 1 is formed by plugging the vertical vessels of
the vascular system above and below the injury. This is the
weakest wall but can slow the vertical spread of decay. Wall 2
is formed at the outer edge of a growth ring. It is a stronger
barrier and provides some resistance to inward spread of decay.
Each growth ring is subdivided into compartments by the rays
(Wall 3). Rays provide resistance to lateral spread by way of a
maze of physical obstacles and a chemical barrier. Wall 4 is
formed by cambium growth after an injury. It is the strongest of
the four walls. Internally, it separates the wood present at the
time of injury from new wood formed after the injury.
Externally, wood develops around the injury and should
eventually cover it by growing over the dead wood at the site of
A tree branch has
a branch bark ridge, which denotes where the upper side of the
branch meets the tree trunk (Figure 3). The bark collar is the
swelling located at the base of a branch where the lower side of
the branch joins the trunk. The natural decay of a dead branch
usually does not spread beyond the collar. When pruning a dead
branch, do not create a new wound by cutting into the collar of
live wood that forms around the dead branch.
The old practice of pruning tree branches was to cut flush with
the tree trunk. When this is done the ability of the tree to
stop decay is greatly decreased. Flush-cut pruning cuts into the
wood of the trunk cause a wound that can allow decay organisms
to infect the main trunk of the tree. The current recommendation
is to prune the branch to just outside the collar. Properly
pruned branches will have a circular closure around the wound.
When the branch is cut too close to the trunk, the closure will
be oval or distorted.
When to Prune
Pruning and training should start when trees are quite
young. This will prevent many serious problems before they
develop. Older, neglected trees are more difficult, dangerous,
and expensive to prune. Most of the pruning on older trees
should be done when they are dormant; there is less weight on
the limbs. At this time, it is easier to see the framework of
the branches. Pruning of young trees should be done when
problems can be observed.
Some trees, such
as birch, dogwood, elm, honey locust, maple, and walnut exude
excessive sap from the wound when pruned in late winter or early
spring. Sap flow does not hurt the tree. Prune these trees in
late spring, summer, or fall to minimize sap flow.
The time of
pruning should take into account the life cycle of insects and
diseases. Plants in the genus Prunus (flowering
cherry, cherry laurel) are prone to develop bacterial cankers.
The spores for the diseases, which are released in fall and
early winter, can enter plants through fresh pruning cuts and
wounds. Prunus trees
do not initiate new cankers during late spring or summer.
Dogwood borers are most active in May, June, and July. Thus,
dogwoods should not be pruned during these months.
The training of a young tree should begin the first year
after transplanting. Training should take place gradually over
several years and no more pruning should be done than is
necessary to enhance the natural shape or structural strength of
the tree. The objective in the first few years is to identify
and correct problems with the main framework of the tree. Lower
branches should be left on the trunk to manufacture food and to
shade the lower bark. The height of a branch does not increase
as the tree grows but remains at the same height for life. With
time some of the lower branches will need to be removed as they
increase in length, create unwanted shade, and/or interfere with
gardening activities or traffic.
Most trees are grown with one central leader (the top most
vertical branch). Exceptions are trees, such as crapemyrtle and
river birch, that are grown for their interesting bark color and
have been pruned to develop multi-trunks. A tree that will grow
to more than 40 feet should have a single trunk well up into the
canopy, but the trunk does not have to be perfectly straight.
When a young tree has competing leaders, the weaker ones should
be removed. If they are essentially equal, either can be
removed. Trees with several trunks often develop included bark
in the crotch that can cause one of the trunks to split from the
rest of the tree during a storm.
branches (primary branches that will make up the tree's
framework) emerging from the trunk should have a 45 degree angle
of attachment (Figure 4). Generally, branches with less than 45
degree angles becomes weaker as they grow longer and increase in
diameter. Branch angles of less than 30 degrees result in a high
percentage of limb breakage while those between 60 and 70
degrees have a small breakage rate.
should look like ascending spokes around a central axle (Figure
5). This will provide a structurally strong tree that is
attractive, balanced, and allows sunlight to penetrate and wind
to pass through the canopy. Major scaffold branches should have
at least 8 inches and preferably 20 inches of vertical
separation. Closely spaced scaffolds will produce less lateral
branches than widely spaced branches. The result will be long,
thin branches with poor structural strength. Good radial spacing
prevents one limb from overshadowing another. Branch arrangement
and spacing is more critical for large shade trees than for
small flowering trees.
Laterals that have
grown taller than the terminal leader or beyond the canopy of
the tree should be headed back. Laterals that have grown inward
towards the center of the tree should be removed back to their
origin. Branches that are less than half the diameter of the
trunk have a stronger branch/trunk union than those that grow
larger than half the trunk diameter. Water sprouts that result
from extensive pruning should be removed because they are
structurally weak and can lead to overly dense growth in the
interior of the tree.
In many parts of the state, trees are topped (also called
sheared, headed back, or dehorned) because of the perceived fear
that the tree is too large and could damage property during a
storm. The practice of topping trees should not be undertaken.
This type of tree abuse involves cutting back main branches and
the central leader to a specific height without regard to the
location of lateral branches. The result is wounds that do not
close, thus, allowing decay organisms to enter the tree. New
growth from cuts usually have weak angles and are easily damaged
during storms. Thinning would be a possible way to reduce wind
resistance without damaging the tree.
- will not
significantly reduce the overall size of a tree but will open up
its canopy to allow more sunlight to penetrate the interior of
the tree and to reach plants growing below (Figure 6). Thinning
can be used to reduce resistance to wind and to reduce stress
during drought periods or following construction damage.
weak, or diseased branches should be removed. Then, selectively
remove limbs from the perimeter of the canopy, especially those
growing close together or beyond the desired canopy size. Also,
remove branches with narrow angles of attachment. Branches
should be taken back to their point of origin or to laterals
that are at least one-third the diameter of the limb being
removed. Trees vary in the amount of thinning they can tolerate
without creating undesirable effects. An over thinned tree will
respond by producing numerous watersprouts and suckers. Sunscald
can occur on trees with thin bark. Never remove more than 25
percent of the total foliage at one time.
back - will
reduce the overall size of a tree. Cut back to good lateral
branches and possible head the tips of the laterals (Figure 7).
Its best to cut back over several years than to attempt dramatic
pruning in one year. When cutting back to an intersecting
lateral branch, choose a branch that forms an angle of no more
than 45 degrees with the branch to be removed. Also, the branch
that is cut back should have a diameter at least half that of
the branch to be removed.
branches over 1 inch in diameter, use the three-cut method
(Figure 8) to avoid tearing the bark of the trunk. Make the
first cut on the underside of the branch, 1 to 2 feet out from
the trunk and about half-way through the branch. The second cut
is made on the top of the branch, about 3 inches further out
from the first cut. As this cut is made, the weight of the
branch will cause it to break between the two cuts. If there is
danger of the branch damaging other limbs below or objects on
the ground, it must be properly roped and supported, then
carefully lowered to the ground after the second cut. The
remaining stub can then be cut back to the branch bark ridge.
slanting cuts when removing limbs that grow upward; this
prevents water from collecting in the cut and speeds closure.
Large branches should be removed just outside the collar ----
not flush with the trunk.
Treating Pruning Cuts
Historically wound dressings have been mistakenly applied to
pruning cuts to block out microorganisms, keep moisture in or
out, and speed the "healing" process. However, research has
shown that treated wounds do not close quicker than untreated
wounds. In most cases, sealed wounds actually give wood
inhabiting microorganisms an environment favorable for growth
development. Some wound dressings kill cambial cells and cause
the wound to remain open for years longer than if no treatment
had been applied.