Figure 1
Scaffold branches should have good vertical as well as radial spacing.
Pruning and Care of Shade Trees
Christopher J. Starbuck
Department of Horticulture
A healthy tree planted where it has room to grow to its natural size and form
may need little or no pruning after it becomes established. Yearly pruning during
the first few years after planting will often help the tree to become established
and to develop a structure that is both strong and aesthetically pleasing. Occasional
pruning may be required throughout the life of the tree to remove broken, dead
or dying branches or those interfering with pedestrian or automobile traffic.
However, once established, most shade trees need not be pruned on a regular
schedule.
Pruning will not cure problems causing branches to die back. Causes of dieback
may include insect or disease damage, poor soil drainage, drought stress, excessive
shade or competition with other trees. The apparent invigorating effect of pruning
is simply the result of forcing growth into a smaller number of buds. Excessive
pruning may weaken a tree by removing too much leaf area and food reserves stored
in the stem.
Pruning new trees
Planting time is a good time to correct minor structural problems a tree may
have. Remove branches that are broken, rubbing or crossing each other. Cutting
a few of the smallest branches back to the trunk will help direct growth into
the terminal leader and stronger lateral branches, promoting the development
of a well-spaced branch scaffold system. For plants dug from a nursery (not
grown in a container), moderate thinning at planting time will also help compensate
for the loss of roots due to digging. Removing more than about 20 percent of
the branches may, however, reduce the tree's ability to produce food through
photosynthesis, thereby slowing the establishment process. In most cases, it
is best to avoid cutting off the terminal leader, as it may be difficult to
restore a straight, central stem or trunk once the influence of the terminal
bud has been removed. Occasionally it is necessary to remove competing leaders
that form narrow branch angles.
One or two years after planting, start pruning the tree to improve its form.
Select the main (scaffold) branches at this time. They should be well spaced
along the main stem, smaller in diameter than the main stem, and distributed
around the tree so that a full crown will develop. Properly selected scaffold
branches have good vertical and radial spacing on the trunk (Figure 1).
Choose branches with wide-angle crotches (Figure 2). Narrow crotches
will be weak and subject to storm damage as the tree grows.
Figure 2
Choose branches with strong wide-angle crotches, right. Branches with
narrow-angle crotches, left, are weaker and should be removed.
The position of the tree limb on the trunk remains essentially the same throughout
the life of the tree. The height of the lowest permanent branch will depend
on the use underneath the tree. The height needed should be determined and allowed
for when selecting the main branches. Since branches contribute to the development
of the trunk, allowing lower branches to remain for a year or two makes a stronger
tree. A young tree should be pruned up gradually so that the unbranched trunk
is never more than one-third the total height of the tree. Thus, a tree should
not be pruned up to provide six feet of clearance until it is about 18 feet
tall. If it is necessary to limb up to 6 feet sooner than this for sidewalk
clearance, the tree was planted too close to the sidewalk.
Pruning young established trees
A regular pruning schedule should be developed to care for young trees. Eliminate
undesirable branches while they are young. Many of the problems that require
extensive repair work in old trees can be prevented by early corrective pruning.
Look for and prune out the following problems:
- Dead, dying or unsightly twigs and branches.
- Sprouts growing near the base of the trunk.
- Crossed branches and branches growing toward the center of the tree.
- Narrow crotches. If it is possible to do so without ruining the appearance
of the tree, remove one of the members forming the weak crotch. This helps
to prevent future storm damage.
- Multiple leaders. If more than one leader develops on a tree that normally
has a single stem, cut out all but one to encourage the tree to develop its
normal shape (Figure 3).
- Remove branches that may develop into potential hazards such as low-hanging
branches that interfere with traffic and weakened or dead branches that may
break.
- Single branches other than the leader sometimes develop faster than the
rest of the tree. They need to be cut back to allow the tree to develop its
normal shape.
Figure 3
Eliminate competing leaders that form narrow branch angles.
Pruning mature trees
Mature trees may need occasional pruning for reasons already noted. However,
working in large trees is dangerous. When pruning is needed, it is best to obtain
the services of a competent arborist. Make sure that the arborist is insured
against personal injury and property damage.
Large old trees are subject to sunscald if they are heavily top-pruned. The
bark may be killed when it is suddenly exposed to full sunlight. To prevent
sunscald, prune only part of the tree top in any one year. If it is necessary
to remove a large limb, the trunk may be protected against sunscald by painting
it with light colored water-base paint. Oil-base paints may damage the bark.
Topping or dehorning is sometimes done to old trees (Figure 4). Major
branches are drastically cut back and all smaller ones are removed. Although
topping has been a widespread practice, it is not recommended, and its use should
be restricted to a few extreme cases. The shape of the tree will be ruined for
several years, and the large wounds left in the topping operation will not heal
properly. This leaves the tree more susceptible to disease, insect attack and
storm damage.
Figure 4
Topping hurts the appearance of a tree for several years and increases
its vulnerability to disease, insects and storms.
The most common reasons for topping trees are to reduce the likelihood of
damage to structures from wind-thrown branches or to reduce interference with
power lines. In both cases, it is better to plant a tree not subject to wind
damage in a place where it will not outgrow its bounds. However, if an existing
tree needs to be reduced in size, this can be done by a technique called drop
crotching. In this pruning system, branches are removed by cutting back to a
point where they join with another branch or with the main trunk (Figure
3). Using this approach, the size of the tree can be reduced significantly without
leaving stubs. In some cases, it may be best to remove the tree entirely and
plant a more appropriate tree in its place.
Pruning technique
When to prune
Pruning wounds will callus over faster if pruning is done in early spring
before trees leaf out. Some trees, such as birch and maple, bleed excessively
if pruned at that time. Bleeding does little harm to the trees, although it often
causes concern to homeowners. Bleeding may be reduced by pruning in late spring
when the trees are in full leaf. When necessary, trees may be pruned at any time
of the year, but healing will not be as rapid as when pruning is done early in
the spring. Extensive pruning in late summer or fall will remove nutrients stored
in the branches before they can be translocated into the trunk and roots. Fall
pruning may also stimulate late-season growth that may be susceptible to freezing
injury.
How to prune
There is no standard method of pruning, but certain rules and procedures
should be followed to obtain the best results. First, start at the top of the
tree and work down. This makes it easier to shape the tree and saves time in
clearing the tree of pruned branches that have become lodged. Make clean cuts
as close to the remaining branch as possible. Dead branches should be cut back
to a living crotch so that healthy tissue surrounds the final cut.
It is very important to avoid tearing the bark from the trunk when pruning.
Small branches may be supported by hand to prevent tearing when they are cut
off. Branches that are too large to be supported by hand must be removed by
the three-step, stub-cut method to avoid tearing the bark (Figure 5). Make
the first cut about one-third of the way through the underside of the branch,
about a foot from the point of the final cut. Next, cut off the branch a few
inches above the first cut. The final cut should be a "collar cut" made at about
a 45-degree angle along the swollen area, or "branch collar," at the base of
the branch. A collar cut makes a much smaller wound than a cut parallel (flush)
with the trunk. Also, cutting a branch off flush with the trunk damages the
collar, interfering with the tree's ability to seal off, or compartmentalize,
the pruning wound by producing callus tissue (Figure 6).
Figure 5
To avoid tearing the bark, prune large branches in three cuts. Make
the first and second cuts about 12 inches from the trunk. Make the third cut
near the branch collar at 45 degrees to the trunk.
Figure
6
Pruning at the branch collar encourages formation of a callus that seals
the wound and protects the tree.
Treating wounds
The most recent research indicates that treating wounds created by pruning
or mechanical damage with a wound dressing has little or no effect on wound closure.
In some cases, wound dressings have actually been shown to interfere with closure.
Pruning in the spring, using the proper types of cuts is the best approach to
promoting rapid wound closure.
Mechanical injuries may be treated in two ways. If the bark is still intact
and has not dried out, it may be possible to replace it over the wound. Trim
the splintered edges, and nail the bark back in place. The wound will compartmentalize
more quickly if it is shaded and kept moist. This can be done by placing damp
sphagnum moss over the wound and covering it with polyethelene (plastic) sheeting.
Seal the edges of the polyethelene to the tree to prevent water from getting
under it. If water does get under the polyethelene, punch a hole in it to let
the water drain out.
If the bark cannot be saved, remove all of the damaged bark and trim the
wound so that it has smooth edges. Isolated islands of undamaged bark will not
live and need to be removed.
One of the most common forms of mechanical damage is caused by lawnmowers
bumping into the base of the tree. Trees are especially susceptible to this
type of injury in the spring, when the bark is easily damaged. Care should be
taken to avoid hitting the tree with a mower. Mulching reduces the liklihood
of trunk damage from mowers and string trimmers. When this damage has occurred,
it should be treated in the manner described above.
Special problems
Figure 7
Girdling roots restrict the flow of sap. Poor growth or death of the
tree may result unless they are removed.
Girdling roots
Trees are occasionally injured or killed by girdling roots (Figure
7). A girdling root is one that crosses one or more other roots and begins to
encircle the trunk. Such a root arrangement most often results from planting
container-grown trees with circling roots. It may also result when the quality
of soil in a planting hole is better than that of the surrounding soil, discouraging
outward root growth. Other possible causes of stem girdling roots are planting
too deeply or piling mulch around the base of the trunk. As the roots grow,
the flow of water and nutrients is restricted. In effect, the girdling root
strangles the roots it crosses or even the trunk. They should be cut, leaving
a large enough gap that the ends will not grow back together. It is best to
eliminate girdling roots when the tree is still young. Cutting larger rooots
on a mature tree may impair the tree's ability to take up water or it
may make the tree unstable.
Other tree maintenance practices
If a large tree appears to be hollow, has a cracked trunk or exhibits
extensive branch dieback, it is wise to consult a professional arborist to determine
whether the tree represents a hazard and should therefore be removed. Bracing,
cabling and protecting trees from lightning are all good tree maintenance practices.
However, these procedures also should be done only by an experienced arborist.
G6866, revised October 2004