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As aspiring homesteaders
contemplate a wooded lot, they usually imagine their dream home surrounded
by healthy, vigorous trees that shade the summer sun and whisper with every
breeze. After the home is built, however, the trees that made the location
desirable often are either gone or dying, the victims of damaging
construction work.
The conflict between land development and tree protection seems to
be a losing battle. Often times, a building site has been chosen because of
the presence of mature trees. These trees, however, have difficulty
surviving the construction process. Although most developers would prefer to
save trees on a property, they are often discouraged by past failures or
regulations that force them to remove trees to locate utilities.
Communication and cooperation among all participants involved in the
building process (landowner, contractors, architect, landscape architect,
arborist, etc.) is essential to ensure a successful tree-protection plan.
No matter how good our
intentions, any construction work near trees will have some impact on them,
because of the close relationship between a tree and the soil surrounding
it. Trees are much more than the visible trunk, branches and leaves.
How Trees Are Harmed
by Construction
Trees can be harmed by construction work in several ways. Any
break or tear in a tree's bark disrupts the flow of vital fluids and exposes
wood to invasion by disease and decay microorganisms, which the tree must
then expend energy to deal with. A trunk wound does not always cause
corresponding loss of branches or foliage, so the consequences may not be
fully apparent. But a large wound in the trunk of a tree is serious-it
cannot be repaired and will almost certainly result in future decay and loss
of stem strength.
Just as serious, although not as visible, is damage to the root
system. Roots can be severed by excavation or smothered by earth fill or
compacted soil.
Compaction, the loss of tiny air spaces within the soil from foot
or vehicle traffic, is especially insidious. Not only will existing tree
roots be affected, but future root growth also will be impaired. Symptoms of
root damage from compaction include slow growth and branch dieback in the
top of the tree. Soil compaction may kill trees, although no other damage
occurs. New trees, shrubs or ground covers planted in the dense soil also
will suffer.
Injuries are cumulative.
Construction work will compound problems trees may have received from
earlier drought, insects or other natural causes. This means that trees in
poor condition before construction work are not as likely to tolerate
further damage. It also means that trees that do not succumb to construction
disturbance may be left weakened after the work is finished.
How to
Protect Trees During Construction
Are we just lucky if trees
survive construction? Not at all. We can greatly increase the chances of
keeping trees healthy by using these strategies:
Survey the entire construction
site well before work begins to determine where trees are and what condition
they are in. It is best to plot all trees on a scaled drawing, but this also
can be done on the ground, especially for small construction projects.
Healthy, vigorous trees with solid stems and a full complement of live
branches are the best candidates for saving. Large, old trees may not
tolerate much disturbance. Don't overlook smaller, under-story trees, such
as dogwood, redbud, serviceberry and ironwood. If you are working with
builders, inform them of your desire to save trees.
Develop a construction plan
that blends the buildings with the trees on the site. This is the time to
decide which trees will be removed and which will remain. Consider alternate
locations for footings, walks, drives and changes in the ground line to save
the best trees.
Keep in mind that it may
be necessary to remove some good trees simply because there is not
sufficient space for them. It is much easier to
make these choices before the construction work begins. The plan can be
drafted on a layer that overlays the survey plan, or mark the ground with
stakes or flags. Remember that you need to keep a relatively large
undisturbed area around each tree to help protect it. The larger the tree,
the larger the area needed surrounding it.
Establish tree protection
zones around individual trees or groups of trees to be saved. Exclude any
type of construction disturbance, including grade changes, vehicle parking
or storage of materials around protected trees. Set steel fence posts with
flexible, snow-fence-type fabric around the perimeter of each protection
zone.
Route trenches as far away
from trees as possible. Utilities that may require trenching include
sanitary sewer, water, gas, electricity and telephone or television cable.
Some utilities may be advantageously placed in the same trench. Placement of
some utilities is flexible, while others are not. If a trench cannot be
placed by the builder to avoid coming close to a valuable tree, consider
going under it. Dig the trench directly up to a tree trunk on both sides.
Then bore or force a tube or line through the soil below the tree. Rerouting
or tunneling for utilities may add to the cost of the project, but also will
increase the chances of saving trees.
If branches or roots must be
severed, cut them with care. Generally, remove entire limbs or branches at
their origin. Use the 3-cut method to avoid stripping bark below the limb
and to promote proper wound closure. Roots should be cleanly cut with a saw
to maximize root regeneration and minimize chances for decay. Do not leave
ragged ends. Dig carefully around large roots and allow them to pass through
a trench. Place utility pipes or lines below the roots. Backfill trenches
with loose soil placed on top.
Because
leaves manufacture food for a tree, removal of more than one fourth of the
live branches threatens a tree. Weak trees, or trees with root damage, for
example, may tolerate less. Removing dead limbs will not hurt a tree.
Pruning your
trees before construction could be the difference between saving that beauty
and losing it! These recommended guides will give you the instructions you
need to keep those trees healthy!
Use wood chips as a protective
blanket over the ground. A layer 4 inches or more deep will help prevent
soil compaction, especially where construction work near trees cannot be
avoided. Chips help protect soils anywhere on the site where new trees,
shrubs or turf will be planted. Replenish chips as they deteriorate or wear
thin.
Avoid post-construction
activities which could further stress weakened trees. Refrain from adding
topsoil around trees, installing underground irrigation pipes or using
herbicides within tree rooting areas. Do not prune trees heavily, until
normal growth rate returns.
Plan for new trees, shrubs,
and ground covers, which are compatible with a wooded environment. Plant
shade tolerant shrubs and small trees around saved trees to maintain a
wooded appearance and help preserve the original root environment. Retain
and expand the natural forest floor with bark mulch. Plant turf grasses in
more open, sunny areas where they will grow better and compete less with
tree roots.
Building homes or other
buildings on wooded sites requires taking precautions to preserve the trees.
Consider tree needs before construction begins. Find the best trees and
concentrate on saving them. You may wish to protect small trees that have
the potential to grow into shade trees.
During construction, protect
as much undisturbed area around each tree as possible, remembering to take
into account both the visible and the fragile underground parts of the
trees. Finally, continue to care for your trees after construction is
finished. Your efforts on behalf of the trees will make your dream home in a
woodsy setting a reality.
No two trees will
react the same to disturbance because of differences in soil type, species,
age and condition. Healthy trees generally can tolerate limited injury if
they have a good growing environment for recuperation. The more severe the
damage and adverse the growing conditions, the higher the risk.
Understanding where roots are.
How close to the
tree can a tree's roots be cut?
An easily recognizable
limit for root disturbances is the ground outside the branch spread, or
drip-line. Soil excavation inside this point may result in some root loss.
But if damage is not done on all sides, a healthy tree can likely tolerate
it. If roots are exposed, cut them off cleanly with a saw to promote better
re-growth.
Understanding where roots are and
the roots' need for oxygen can help us properly protect trees during
construction. To prevent construction damage, a sturdy barrier fence must
be erected around the root protection zone before any construction activity
begins. Furthermore, the fence must be monitored to assure that it remains
standing during construction.
Once you have selected the
trees to remain on the property, consider their location in deciding
placement of the house, garage, driveway, walks and patio. Simply changing
the angle of a building or curving a walk can preserve the essential root
space of a prized tree. It is important at this point to be in close
communication with your architect, who can help by locating buildings to
harmonize with the natural terrain.
The key to the survival
of trees in the years following construction is protection of the roots
during construction. The three main causes of tree death during construction
are soil compaction, grade changes and root severing. Decline and tree
mortality usually show up two to three years after the home is completed.
Many people still believe tree roots grow deep and
extend only to the drip line. We now know roots do not grow that way.
Tree
roots can grow three times beyond the drip line .
Tree Basics
- the root of most tree problems......Out
of sight...out of mind
But continuing
research has demonstrated time and again that 95 % of
most tree
 roots
are in the top 12-16 inches of soil. And tree roots extend far out beyond
the drip line usually about as far as the tree is tall. That could
extend three times beyoud the drip edge. Feeder roots are shallow because
living roots require oxygen as well as moisture. Tree roots want to receive
moisture and nutrients as soon as possible following a rain. Imagine... the
area under a tree is like a fresh piece of baked bread. The capillaries
contain all the gases, moisture and nutrients the tree needs, Flatten this
root zone out like a tortilla and materials are no longer available to the
tree. The consequences of this treatment can cause mortality within two to
three years. The unseen killer. Imagine if someone stood on you foot all
day! That has got to HURT!.
Soil Compaction
Their root systems, close to the surface and
wide spreading, are easy to damage, even far from the trunk. Trees cannot be
repaired or restored to their original condition after a construction
project is finished. Therefore, it is better to prevent construction
injuries to trees, rather than attempt to treat them after the fact.
Soil compaction cuts off air and water to the
tree roots. The damage caused by soil compaction occurs slowly, sometimes
not becoming evident for several years. To prevent vehicular and foot
traffic around the roots of protected trees, erect physical barriers beyond
the dripline of individual trees, or better yet, groups of trees. When this
is not possible, other protective methods can be used: (1) spreading several
inches of wood chips in the root zone area; (2) bridging root areas with
plates of steel. Work with the builder to locate and mark (with signs or
flagging) all parking places for workers, construction roads, and areas
for storage of building materials, soil and gravel.
Raising
The Existing Grade
How much soil can be added
over the roots? Preferably,
none. Added soil can suffocate roots from lack of oxygen. If soil must be
added, use the thinnest possible layer of loose soil over the smallest
possible area. Think in terms of inches rather than feet. Willows or
cottonwoods can tolerate more fill, ashes less and white oaks little, if
any, added soil.
Grade changes are often
necessary during construction of a new building. When the grade around an
established tree is being raised, consider methods of preventing injury to
the tree before the fill is made rather than attempting to take corrective
measures after the damage has been done. While the initial cost may be high,
prevention is always cheaper and more effective than attempting to correct
the situation after damage has been done.
Remove all vegetation,
including underbrush and sod, beneath the branch spread of the tree. Break
up the top 3 to 6 inches of soil carefully so as to disturb the least
possible amount of roots. This allows better contact between the fill and
soil surface. Apply fertilizer at recommended rates.
Construct an open-joint wall of shell, brick, rock or masonry in a circle
around the tree trunk, with at least 1 to 2 feet between the wall and trunk.
This wall should be as high as the top of the new grade. This opening is
commonly referred to as a tree well. Construct an aeration system using
4-inch agricultural clay tile or 4-inch perforated plastic pipe arranged in
five to six horizontal lines radiating from the tree well like spokes in a
wheel to a point beyond the branch spread. Allow excess moisture to drain
away by installing the radial lines so they slope away from the trunk.
Connect the outer ends of the radiating system with a circle of tile or
perforated plastic pipe. (See Figures
1 and 2.)
To provide vents, place 4- or
6-inch plastic pipe or bell tile upright over the junction of the radial
lines with the circle. They should extend to the surface of the planned
grade level. Extend the lower end of the aeration system to a curb or storm
drain to carry excess moisture away from the root system.
Cover the exposed soil and
tile system with rock or coarse gravel to a depth of 6 to18 inches,
depending on the amount of fill. Follow this with a covering layer of
gravel. Place a thin layer of straw, woven plastic or other porous material
over the gravel to prevent soil from filtering into the gravel and stone.
Fill with good topsoil to the desired grade.
To discourage rodents, fill the tree well with enough coarse
gravel to cover the ends of the lines opening into the well. Also fill the
upright bell tile and cover with a screen or grill. The tree well can be
left open, covered with a metal grill or wooden deck, or filled with a
mixture of coarse sand and charcoal (50 percent each, by volume) to within
several inches of the top. If filled with the sand/charcoal mixture, cover
with pea gravel, decorative bark or other attractive material to allow air
circulation through the tile system. An alternate method can be used if 30
inches or less fill will be used. No tile or pipe is used - only gravel.
Again, remove all sod and underbrush, break up the soil surface above the
roots and apply fertilizer at recommended rates.
Starting
at the dripline, apply from 3 to 6 inches of crushed stone or coarse gravel.
Gradually increase the depth towards the trunk of the tree until it is 8 to
12 inches or deeper within 2 feet of the trunk. The gravel can reach the
surface of the fill in the area extending 2 feet around the trunk of the
tree. Cover the gravel with a thin layer of straw, woven plastic or other
porous material to prevent soil from filtering into the gravel and sealing
the air spaces. Spread good topsoil over the area to the desired depth. Use
good, well-drained topsoil in making the fill in order to provide adequate
aeration for normal root activity and tree growth.
(See Figure 3.)
Lowering The Existing Grade
There will likely be less damage to a tree when the grade is lowered,
unless a great amount of the root zone is exposed or removed. Removing 1 to
2 inches of soil normally will not affect the growth of a tree, especially
if steps are taken to ensure that drought damage does not result from loss
of roots. Use retaining walls or terraces to avoid excessive soil loss in
the area of greatest root growth. When possible, spread mulch over the
exposed area to help prevent soil erosion, reduce moisture loss and keep
soil temperatures lower. Provide adequate water in the event of a prolonged
drought.
Corrective Steps After A Fill Is Made
If a fill has been in place long enough
that the tree is already showing symptoms of deterioration, there is little
that can be done to save the tree. If the fill was made recently, or if
serious damage has not occurred, steps can be taken to correct the problem.
If the increase was greater than 12 inches,
it will be necessary to install a tile and gravel aeration system as
described above, excavating the soil to the original grade. If the increase
is less than 12 inches, remove the soil around the trunk, down to the
original soil level, for a radius of 2 feet beyond the tree trunk. Install a
dry well around the trunk to hold the fill soil in place. Drill or dig holes
every 2 feet beneath the branch spread, starting about 2 feet from the well.
Insert a 6-inch tile or plastic pipe and fill with coarse gravel to allow
free air and gas exchange to the roots.
Severing Roots
Although some
cutting of roots near construction is inevitable, much of it can be avoided
with good planning and cooperation. It is not necessary to route underground
utilities in a straight line from the street to the house. Careful route
selection can often avoid the root systems of important trees. If this is
not possible, reduce damage by tunneling beneath the roots. To reduce
trenching for foundations, substitute posts and pillars for footers and
walls.
OTHER PROBLEMS
Often when grade changes are made the
terrain is altered, and there may be a change in how water drains from the
land. If too much water drains into a wooded site, trees in that area may
eventually die from lack of oxygen. It may be necessary to build a drainage
system to maintain the previous amount of moisture that provided natural
growing conditions for the existing trees. If sites are deprived of water,
irrigation may be necessary to maintain existing trees.
Watch for equipment damage to limbs and
trunks, and repair promptly. Chemicals and other products that are often
dumped on a construction site can change the soil chemistry, weakening and
oftentimes killing trees on the property. To prevent adverse effects on
construction site soils:
Do Not clean paint brushed and tools over
tree roots. Dispose of chemicals wastes (paint thinner,oil, etc.) properly
Do Not drain these wastes on site. Spread
heavy plastic tarp where concrete is to be mixed or sheet rock to be
cut. These materials raise the pH, causing alkaline soils.
The Sturdy Barrier Fence
As the new homeowner
ijnsist on this with your contrasctor,it is most important to the survival
of your trees. To prevent construction damage, THE
FENCE must be erected around the root protection
zone before any construction activity begins. Furthermore, the fence must be
monitored to assure that it remains standing during construction. Work with
the builder to locate and mark all parking places for workers, construction
roads, and areas for storage of building materials, soil and gravel. Your
trees are a very valuable asset to your new home.
Boring In Utilities In
The Way To Protect Tree Roots
Trenching cuts important roots. Boring under the critical root zone causes
little or no damage to the treee roots. This methor will ensure a healthy
tree for the life of your home.
HOME & GARDEN INFORMATION CENTER
http://hgic.clemson.edu
Illustrations taken from Protecting Existing Landscape Trees from
Construction Damage
Due to Grade Changes, by Everette E. Janne and Douglas F. Welsh, Texas
Agricultural Extension Service.
http://aggie-hortculture.tamu.edu/extension/ornamentals/protect/protect.html.
Reproduced with permission.
Prepared by Debbie Shaughnessy,
HGIC Information Specialist, and Bob
Polomski ,
Extension Consumer Horticulturist, Clemson University. |
