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.
Reasons for Pruning
Because 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
preventive 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 our landscapes.
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. It may
be unsightly, but it 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. At that time, trees
have just expended a great deal of energy to produce foliage and early shoot growth.
Removal of a large percentage of foliage at that time can stress the tree.
Making Proper Pruning Cuts
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 the trunk
collar has grown out on a dead limb to be removed, make the cut just beyond the collar.
Do not cut the collar.
If a large limb is to be removed, its weight should first be reduced. This is done by
making an undercut about 12 to 18 inches from the limb’s point of attachment. Make a
second cut from the top, directly above or a few inches farther out on the limb. Doing so
removes the limb, leaving the 12- to 18-inch stub. Remove the stub by cutting back to
the branch collar. This technique reduces the possibility of tearing the bark.
Pruning Techniques
Specific types of pruning may be necessary to maintain a mature tree in a healthy, safe,
and attractive condition.
Cleaning is the removal of dead, dying, diseased, crowded, weakly attached, and low-
vigor branches from the crown of a tree.
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 shape.
Raising removes the lower branches from a tree in order to provide clearance for
buildings, vehicles, pedestrians, and vistas.
Reduction reduces the size of a tree, often for clear- ance 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, reduction helps
maintain the form and structural integrity of the tree.
How Much Should Be Pruned?
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 tolerate the removal of a
higher percentage of living tissue better than mature trees do. 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. Overthinning 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-quarter 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
removal of dead or potentially hazardous limbs.
| Pruning Mature Trees |
| US TREE SERVICE |
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