How to Prevent Construction Damage
Dennis S. Schrock
Department of Horticulture, University of Missouri-Columbia
Shortcuts
Types of construction damage
Preventing damage
Tree care after
construction
Tolerance of trees
to root damage
Trees are a valuable asset in home landscapes. Their shade
makes summer living more pleasant and can significantly reduce air
conditioning costs. In winter, they shield against wind and snow, reducing
heating costs. Leaves act as air cleaners, filtering dust and removing
airborne pollutants. Trees shelter wildlife, slow rainfall runoff, muffle
noise and provide privacy. But perhaps the main reason people value trees
highly is simply the beauty and grace they add to a community.
Because it takes many years for a tree to grow to
maturity, wooded building sites command premium prices. All too
frequently, however, the trees that make the site attractive are damaged
during construction. Trees may decline and die soon after construction
work is finished. The homeowner then faces the cost of tree removal in
addition to the expense and time needed to grow replacement trees.
Trees around older, established homes suffer from
construction damage, too. Remodeling projects, sidewalk replacement,
landscaping projects or utility work may cause injury to trees.
Damage to trees occurs directly from physical wounding or
indirectly through change of environment around the tree.
Physical wounds
Careless movement of construction equipment causes wounds
to tree trunks and root collars, the area of the tree at ground line where
the roots begin to spread out. A healthy tree is capable of sealing off
small wounds, localizing injury. However, large wounds and those on
stressed trees will not readily seal off, allowing decay to begin.
Improper pruning to create clearance for construction
equipment and tree removal techniques are other sources of physical injury
to branches and trunks. As trees are removed for placement of a new
building or driveway, they may scrape bark off trunks or break branches of
trees that are to be saved. These wounds serve as entry points for
diseases. Improper pruning leaves branch stubs that die and begin to
decay. Make clean cuts with a sharp pruning saw just outside the swollen
branch collar.
Below ground, root damage is common from excavation and
grade changes. Roots may be torn by improper excavation, opening wounds
for disease organisms to enter. Fine, absorbing roots are lost by topsoil
removal, putting the tree under stress. Structural support is lost by
trenching too close to major roots, creating a potential hazard. Bruising
or crushing of roots by heavy equipment may not be apparent from above
ground.
Environmental changes
Soil compaction is a serious problem on many construction
sites. Even when care is taken to avoid trunk and branch injury from
equipment, trees may be damaged by equipment driving over root systems.
The weight of the equipment compacts soil, reducing air space in the root
zone. Limited oxygen availability to roots is also a problem when soil is
stockpiled at the base of trees or paving is put over existing roots.
Excessive thinning of tree stands or removal of underbrush
causes increased exposure to sun, wind and heat. Sunscald may develop on
trees previously acclimated to shade. Increased wind and heat exposure
increases moisture stress.
Also, moisture stress may develop from grade changes that
lower the water table or divert drainage patterns away from the site. On
the other hand, excess soil moisture may develop from grade changes, as
well. A rise in the water table, puddling from improper grading, or an
increase in water flow through the area will decrease the amount of oxygen
in the root zone and lead to tree decline.
Adding fill soil or cutting away excess soil alters the
environment around tree roots. Hauling in fill reduces oxygen to the
roots. Adding as little as 1 or 2 inches of heavy clay soil on top of the
existing grade may damage sensitive trees such as oaks. The soil profile
and soil pH are also altered. Topsoil is often more acidic than excavated
subsoils spread on the surface. Trees adapted to growing in acidic topsoil
will be stressed when forced to develop new roots in soil of a different
pH and texture. Ability of roots to take up many micronutrients is reduced
in high pH soils, leading to decreased growth rate and yellowing leaves.
Construction material buried on-site also often raises soil pH.
Symptoms of damage
Symptoms of construction damage to trees appear over a
period of several months to several years after the damage occurs. Because
of the delay in development of symptoms, it is often difficult for people
to understand the relationship between the earlier injury and the current
symptoms.
The first symptoms to develop may be a slight wilting or
shedding of a few leaves at the time of construction. Fall coloration
often develops early and leaves drop prematurely. In later years, leaf
size and shoot growth may be reduced. Twigs and branches die, and in the
case of conifers, excessive needle drop occurs. General growth of the tree
is slowed and resistance to diseases and insects is weakened.
Diagnosing compaction or root smothering damage can be
difficult because it may take five to seven years for symptoms to appear.
The speed and severity of symptom development depends on the amount of
damage, the species of tree and soil type.
Deciding which trees to save
The first step in deciding which trees to save is to
accurately mark out placement of proposed buildings, driveways, parking
areas and utility routes. After marking these features, stand back to look
over the site. A small shift in the position of the building, a change in
driveway location, or altering the proposed utility line could make the
difference between saving or cutting a valuable tree.
After deciding on building placement, prioritize trees for
saving. Trees directly in the way of construction; undesirable, weedy tree
species; trees already in a state of decline; or structurally hazardous
trees should be marked for removal. If in doubt, consult a trained
arborist, horticulturist, forester or nursery person to determine tree
condition.
Next, determine which trees can be saved with little or no
protection. Desirable trees located away from construction or traffic
areas will survive if reasonable care is taken.
The final group of trees to examine are those that may
survive construction, but only if proper measures are taken. Examine them
more closely to determine if one of the techniques described below will
effectively minimize damage. If severe damage is likely, it will be less
expensive to remove the tree before construction begins than afterward.
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Control traffic around trees
Tree roots are not mirror images of the tree top. Roots
are concentrated in the top 12 to 18 inches of soil and spread two to
three times the width of branches (see Figure 1). Protecting roots within
the dripline of the tree is most critical, but damage to roots outside the
dripline on only one side of the tree may remove one-third or more of the
tree's roots (see Figure 2).
 |
Figure 1. Tree roots are not mirror
images of the top branch structure (lower figure). While some trees
do have deep tap roots, most do not. Roots spread to where soil
conditions provide adequate moisture, aeration and nutrients.
Typically, about 85 percent of a tree's roots are within the top 18
inches of soil (upper figure). |
 |
Figure 2. Roots typically spread
out from two to three times the width of the branches. However, the
essential roots are usually considered to be within the dripline,
which is the area underneath the tree's branches. |
Erect a fence at the dripline or farther out, if possible, to prevent
damage from excavation, soil compaction or stockpiling of soil over roots.
It is easier to save groups of trees than individual ones. Build a fence
around the dripline of the outside trees to keep construction machinery
away from the grove. Remove protective fences only after all construction
work is done, including final grading and smoothing of the site.
Carefully remove unwanted trees
Be careful removing unwanted trees. A tree being removed
might fall on and injure one of the trees you plan to save. If possible,
remove unwanted trees when none of the trees have leaves. When trees are
in full leaf, sudden removal of nearby trees is a shock and can cause
sunburn to other trees.
Make clean cuts
Clean cuts to roots seal off quickly and help prevent
entry of disease-causing organisms. Ragged, rough wounds from dull or
improper equipment allow decay to progress to the rest of the tree.
Sharply cut ends promote a flush of new roots, helping the tree recover
from injury. Bulldozers tend to tear roots apart, leaving wounds that will
not seal readily. Trenchers and backhoes make cleaner cuts through the
soil. When cutting roots larger than 2 inches in diameter, use a pruning
saw to make a smooth cut.
Do not automatically prune the top of a tree that has been
root pruned. As long as moisture is not limiting, leaves in the crown of
the tree manufacture food to help roots grow and recover from being cut.
Prune out only weak or dead branches.
Tunnel rather than trench
Trenching near a tree kills a large portion of the tree's
roots. Tunneling under the tree does virtually no damage. Since most roots
live in the top 18 inches of soil, a tunnel 2 feet deep often does little
damage. However, placing the tunnel 3 to 4 feet deep is safer.
It is best to tunnel at least 1 to 2 feet away from the
tree's center to avoid a tap root (see Figure 3). For trees under 6 inches
in diameter at breast height, trenching should come no closer than the
dripline of the tree. See Table 1 for larger trees.
 |
Figure 3. Trenching near a tree can
kill almost half its roots. A tunnel in the same place will do
virtually no damage to the tree. |
Table 1. Minimum distance to trench trees. (Adapted from Municipal
Foresters of Northeastern Illinois guidelines.)
|
Tree diameter at breast height
|
Minimum distance from tree to start tunneling
|
|
less than 6 inches
|
dripline of tree
|
|
6 to 9 inches
|
5 feet
|
|
10 to 14 inches
|
10 feet
|
|
15 to 19 inches
|
12 feet
|
|
more than 19 inches
|
15 feet
|
Prevent soil compaction
Where fencing to prevent construction traffic is not
possible, use temporary wood chip mulch, gravel mulch or bridges to
prevent soil compaction around tree roots. Place wood chips or gravel
mulch 6 to 12 inches deep on top of a geotextile landscape fabric placed
over the root zone of the trees to be protected.
Air spaces in the mulch cushion the impact of machinery or
foot traffic and disperse the weight over a larger area. The landscape
fabric makes cleanup easy. Temporary bridges built from steel plates
placed on railroad ties distribute the weight of equipment over larger
areas. What compaction does occur will be limited to areas directly below
the railroad ties (see Figure 4).
 |
Figure 4. Use of bridges to protect
tree roots. |
Handle grade changes
Cutting away soil from a tree removes a portion of the
root system and changes the soil moisture level. For reasonable survival
of trees, make no cuts within the dripline. Construct a retaining wall at
the dripline or farther out to preserve the tree's roots (see Figures 5
and 6).
|
|
Figure 5. This change in grade has
killed half the tree's roots. Most trees will die from this extent
of injury. |
 |
Figure 6. The tree on this cut has
been protected by a curved retaining wall at the dripline. The tree
should recover nicely. |
Filling around trees smothers roots. Build a retaining wall at the
dripline to keep soil from burying roots within the dripline. It is
possible to completely cover the roots by using expensive drainage and
venting techniques that require professional design and installation. It
is easier, cheaper and more practical to alter grading plans than it is to
protect a tree from fill over the entire root zone.
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With proper care, trees moderately affected by
construction damage will recover. Homeowners can help trees recover by
practicing annual tree care as outlined in the following:
Aerate the soil. Pull out cores of soil 12 to 18
inches deep to aid movement of oxygen and moisture into soil and help
combat compaction. Fertilizer may be placed in the holes. Use compost to
backfill the holes. New roots quickly fill in the cores.
Water deeply. During dry periods, moisten the root
zone of stressed trees. Let the water soak in 12 to 18 inches deep by
applying a slow trickle throughout the spread of the tree's roots.
Inspect the tree for damage. Stressed trees are
more susceptible to disease and insect attack. Check them frequently to
catch pest problems before they become severe. Treat with a pesticide, if
necessary. Prune out any dieback that develops.
Mulch. A 2- to 4-inch layer of mulch around the
base of trees helps prevent soil compaction, reduces weed competition and
conserves soil moisture. Keep the mulch away from the tree trunk to
prevent collar rot from developing.
Fertilize. Severely stressed trees should not be
fertilized until they become re-established a year or two later. Young,
rapidly growing trees should be fertilized annually. Mature trees may be
fertilized every two or three years. Using a complete fertilizer, apply 2
pounds of actual nitrogen per thousand square feet. Fertilizer may be
spread over the soil surface or distributed among aeration holes punched
12 inches deep every 2 feet under the tree's canopy.
Not all trees are equally sensitive to soil-related
construction injury (see Table 2). Some can generate new roots quickly
when conditions become unfavorable for the old roots. This adaptation
occurs primarily in species that grow in river bottom flood plains. Tree
species native to upland sites are less likely to adapt to soil grade
changes or construction damage.
Table 2. Tree tolerance to root damage, by species.
|
Very sensitive
|
Moderately sensitive
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Fairly tolerant
|
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Oaks
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Sugar maple
|
Silver maple
|
|
Hickories
|
Ash
|
Basswood
|
|
Honeylocust
|
Walnut
|
Cottonwood
|
|
Kentucky coffeetree
|
Sycamore
|
Poplar
|
|
Horse chestnut
|
Hackberry
|
Willow
|
|
All conifers
|
Red maple
|
River birch
|
|
Redbud
|
Hawthorn
|
|
|
Serviceberry
|
Ironwood
|
|
Trees are a valuable asset in the landscape and should be
protected during construction. With careful planning and forethought,
desirable trees can be saved and the disappointment of delayed tree
decline prevented.
Copyright 2000 University of
Missouri. Published by University Extension, University of
Missouri-Columbia
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