Urban Design to
Accommodate
Trees: Parking lot
Solutions
by Dr. Edward F. Gilman, professor
Department of Environmental Horticulture
University of Florida, Gainesville
http://hort.ufl.edu/woody/planting
Solutions
Solutions for trees in parking
lots and buffer strips
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The idea is to create a
system that can
accommodate tree roots
while minimizing
interference and damage
to the infrastructure
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design and structure
edge buffer strips
islands
interior linear strips
design
Signs and trees sharing the
same space
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When tree canopies grow in the
same space as signs, store owners
respond by either topping or
rounding over the trees, or
inappropriately raising the canopy
Raising the canopy on single
trunked trees results in less
maintenance and a longer-lived tree
than raising the canopy on the multitrunked trees pictured here
Installing trees with one dominant leader and one trunk in parking
lots makes it easier to remove lower branches as they get in the way
of the signs
Sustainable parking lot
design
Locating trees in large buffer
strips around the parking lot
provides roots with adequate space
to grow
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design
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Trees are located only
in the buffer strips
surrounding this
parking lot--no trees are
placed in islands.
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This keeps lights away
from trees and ensures
that trees will not have
to be pruned to make
way for the lights
design
Most trees in buffer strips-one large island
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Trees are located primarily in the buffer strips
surrounding this parking lot, not in islands
There is only one island (left side) and it is quite large
Large parking lot islands and wide buffer strips support
large trees
design
Non-sustainable buffer
strips under wires
Buffer strips at the edge of parking lots are often located under
utility wires. THIS DOES NOT WORK!
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Trees must be pruned to
keep them small
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This makes for
unnecessary work and
prevents the urban forest
from developing
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Move this large soil space
from under the wires to a
different location such as
the side of the property as
in the next slide
design
Sustainable buffer strip on
side of property
Trees in large soil spaces with no utility lines nearby can grow
to a large size.
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Buffer strips on
the side of the
property make
more sense
Often, there are
no utility lines on
the side of the
property
Porous pavers for
parking stalls
design
This parking lot for a large zoo in the southern U.S. was designed
to reduce runoff and support good tree growth
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Traditional pavement was
used for the travel lanes –
cars stalls were constructed
of porous paver blocks
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This should allow for better
root growth because air and
water is more likely to enter
the soil under the porous
pavers
design
Porous pavers near existing
trees
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This parking lot for a large botanical
garden was constructed near
existing large trees
Porous pavers were used to help
preserve the root system and soil
structure
Be sure that the soil grade is not
lowered during the construction
process because this will damage
roots
Soil can be added around the roots
to prepare a base for the pavers, but
be sure it is coarser than the existing
soil
design
Gravel as a parking lot
surface
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Gravel has been used
successfully for two
decades in this large
employee parking lot
Trees are very happy
and have grown to
form a closed canopy
If roots grow to the
soil surface forming
an irregular surface,
more gravel is added
design
Porous asphalt as a parking
surface
This reduces the volume running into
adjacent retention ponds and streams
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Porous asphalt
allows water to run
through the surface
to a layer of gravel
below the surface
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Forty percent of the
volume below the
surface is air space
that can fill with
runoff water
Solutions
Solutions for trees in parking
lots and buffer strips

The idea is to create a
system that can
accommodate tree roots
while minimizing
interference and damage
to the infrastructure




design and structure
edge buffer strips
islands
interior linear strips
Buffer strip too narrow to
sustain trees
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Buffer
Installation of buffer strips
around the edge of parking
lots are typically mandated
by municipal ordinances
and codes
Narrow buffer strips are
fine for shrubs but they are
too small to accommodate
root growth needed to
support trees
The large maturing honeylocust trees are likely to struggle; if roots
somehow find suitable soil under the walk and pavement, the
hardscape is likely to become dislodged and damaged
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Buffer
Buffer strip too narrow-tree breaks hardscape
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The original space allocated
for the tree roots was much
too narrow
Roots somehow found
suitable soil under the
pavement and walk where
they grew very well in the
20 to 30 years after planting
The trunk flare lifted and
disintegrated the curb (see
arrow) as roots expanded in
diameter
Buffer
Buffer strip suitable to
sustain trees
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The fifteen foot wide buffer
strip at the left edge of this
parking lot is more suitable for
tree growth than in many
designs
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It will allow for root flare
expansion and provide open
soil for good root growth for a
decade or two
One key to success in this design will be preventing compaction by
keeping pedestrians off the soil and mulch in the strip
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Buffer strip suitable to
sustain trees
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Buffer
The fifteen foot wide buffer strip between building and street is
more suitable for tree growth than many other designs
It will allow for root flare expansion and provide open soil for
good root growth for a decade or two
Roots can share soil in the long strip of open soil space
One key to success in this design will be to prevent compaction by
keeping pedestrians off the soil and mulch in the strip
Buffer strip suitable to
sustain trees for long time
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The thirty foot wide
buffer strip at the edge
of the parking lot above
is more suitable for tree
growth than in most
other designs
That is the reason why
these trees have grown
so large
Large soil space in buffer strips equates to
healthy, vigorous trees
Buffer
Solutions
Solutions for trees in parking
lots and buffer strips

The idea is to create a
system that can
accommodate tree roots
while minimizing
interference and damage
to the infrastructure




design and structure
edge buffer strips
islands
interior linear strips
Islands
Large trees are not suited
for small islands

Planting large-maturing
trees in small parking lot
islands is not sustainable
– something will break
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Nonetheless we continue
to see landscape
architects specify largematuring trees for small
parking lot islands
Islands
Standard planting in a
parking lot island
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Here is a group of
standard-issue parking
lot islands, each with
two red maple trees
The space is too small
to sustain tree growth
for very long
Red maple can grow
OK in this situation in
northern climates but
struggles in the South
Where are the roots?
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Islands
Occasionally trees can
grow in places that seem
to defy reason.
In the parking lot pictured
here trees were able to
explore the soil below the
pavement and become
quite large in relation to
the tiny soil space at the
base of the trunk
This example should not be used as a model for successful
design since it is so atypical
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Big island means
healthier trees
Islands
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Here is a parking lot
constructed on a compacted
clay soil
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The two trees on the left and
center (blue arrows) are
larger and darker green than
the tree on the right
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The stressed tree on the
right was planted in a tiny
island whereas the healthier
ones were in a larger island.
Islands
Small trees for small
islands
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Small-maturing trees
such as crape myrtle are
the best ones to choose
for small islands
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They cause less damage
to hardscape than trees
that grow to be large
Islands
Large tree running out of
soil space
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Large trees planted in
small parking lot islands
may grow well for 20
years; when they
completely fill the soil
space with roots, they
begin to decline
This sawtoothed oak grew here for about 20 years before
beginning to show decline as indicated by die-back in the
canopy (arrows)
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Islands
Large tree cracking curb in
small space
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Large trees planted in
small parking lot islands
constructed over coarser
soil types can explore
the soil beneath the
pavement
As roots grow and
expand in diameter they
often begin to lift the
curbs and crack the
pavement
Islands
Very large tree cracking
curb in narrow island
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The root system on this tree
managed to find a way to
explore the soil directly under
the pavement
Two large roots can be seen
lifting the pavement (blue
arrows); research shows that
water and air are abundant on
the underside of the pavement
surface
When a tree manages to get
large in a small space, roots
typically destroy hardscape
Islands
Roots lifting curbing in
narrow island
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Trees can grow large in
small spaces but the
hard surfaces nearby
often suffer serious
damage
There is no way to
replace this curbing
back to its original
location and save the
tree
Large island with largematuring oak tree
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Islands
Designing parking lots with large islands such as the
one pictured above will allow large-maturing trees to
be planted with less risk of them disrupting hardscape
Large diameter buttress roots that can raise curbs and
walks are farther away from curbs
Islands
Large islands with largematuring oak trees
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Designing parking
lots with large
islands such as the
two pictured here
allow largematuring trees to
be planted with
less fear of them
disrupting
hardscape
Islands
A peninsula can increase
available soil space
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Designing parking lots
with peninsulas is a good
method of supplying trees
with much more root
space than the traditional
island
An island is surrounded
on all sides by curbing
Recent research shows
that the fewer the curbs
surrounding the tree the
better its growth
Islands
A peninsula can increase
available soil space
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Trees can grow to be
quite large in a
peninsula because roots
have access to ample
soil space
Solutions
Solutions for trees in parking
lots and buffer strips

The idea is to create a
system that can
accommodate tree roots
while minimizing
interference and damage
to the infrastructure




design and structure
edge buffer strips
islands
interior linear strips
Linear strips
Linear planting strips in a
parking lot
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Trees can grow very well in long planting strips in parking lots
Combined with the traditional island at the end of the strip as
shown above, this design can lead to many shaded parking
spaces. Security lights will conflict (arrow)
Linear strips
Long strips support healthy
trees
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These three trees in a long
planting strip are growing
well
The shrubs and ground
cover planted in the strip
help keep people off the
soil, thereby preventing
soil compaction
Compacted soil can
dramatically reduce tree
growth
Linear strips
Trees are growing into
lights
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These trees are growing nicely
due to good site design; long
linear strips allow roots to
share soil space
However, since the security
lights were installed way too
high (blue arrows) the tree
canopy is beginning to reach
the lights
This conflict often leads to
poor tree pruning choices
resulting in unhealthy trees
Linear strips
Wider is better
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Very wide soil
strips in
parking lots
allow for
optimal tree
growth
This strip is forty feet across and will support trees for
decades
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Linear strips
Wide strip supporting a
double tree row
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Very wide soil strips in
parking lots provide the
best trees
This strip is fifty feet
across and has
supported this double
row of oaks for decades
This is sustainable
urban design
Outline of topics
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Introduction
Site evaluation
Species selection
Formula for success
Roots/hardscape conflicts
Trees/sidewalk solutions
Parking lot/buffer strip solutions
Structural soils
Many tools at your
disposal
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Other professionals – engineers,
planners, architects, landscape
architects, urban foresters, arborists
Species selection and spacing
Creative design solutions
Ordinance and code changes
Solutions
Root barriers can deflect
roots
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Barriers have been placed vertically in the soil to deflect roots
away from hardscapes
Place the barriers sufficiently away from the structure (about six
inches) to be protected so that as the roots grow wider they will
not touch the curb or walk
Be sure the top of the barriers reaches above the top of the soil so
roots do not grow over it
Solutions
Root barriers can deflect
roots
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Roots are deflected horizontally and down by most of the
barriers on the market
In compacted soils and soils with a high water table, roots
grow under the barrier and up the other side
In well drained soil, roots may remain at deeper depths
longer
Outline of topics








Introduction
Site evaluation
Species selection
Formula for success
Roots/hardscape conflicts
Trees/sidewalk solutions
Parking lot/buffer strip solutions
Structural soils
Structural soil
Typical root growth under
pavement
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Photo credit: Jason Grabosky, Rutgers University
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Roots (blue arrow)
typically grow directly
under the sidewalk slab
as shown here because
that is where air and
moisture is present
The sidewalk slab has
been remove in the
photo
Roots lift the walk as
they increase in
diameter
Structural soil
Structural soil design
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Structural soil is designed
to support the weight of
walks, roads, pedestrians
and vehicles as well as
provide a well-aerated
soil substrate for tree root
growth
Weight is transferred
from aggregate to
aggregate then to the soil
under the aggregate; no
weight is borne by the
soil between aggregates.
Illustration credit: Jason Grabosky, Rutgers University
This allows roots to grow
well in the soil between the
aggregates
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Structural soil
Structural soil installed
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Structural soil is
composed of small
aggregate material
(angular rocks about
one inch diameter) with
enough soil to almost
fill the space between
the rocks
Photo credit: Jason Grabosky, Rutgers University
Structural soil
Root growth in structural
soil after three years
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Roots grew well in
structural soil under a
sidewalk (walk has been
removed-blue arrow) in
the first three years after
planting
Roots grew down and
out from the tree
It is not known if all
trees will grow like this
one
Photo credit: Jason Grabosky, Rutgers University
15’
9’
Structural soil
Tree growth in structural
soil
Photo credit: Jason Grabosky, Rutgers University
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These trees were
planted into a strip of
structural soil installed
in this retrofitted
parking lot between the
blue arrows
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They have performed
quite well
Structural soil
Tree growth in clay vs.
structural soil
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Trees on the left were planted
in clay soil that was fairly
compacted-typical of many job
sites; many of these trees were
performing poorly as indicated
by the die-back (blue arrow)
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Trees on the right were planted
in structural soil installed
beneath the sidewalk and they
looked great
Photo credit: Jason Grabosky, Rutgers University
Structural soil
Tree growth in well-drained
good soil vs. structural soil
Photo credit: Jason Grabosky, Rutgers University
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Trees on the left
were planted in
well-drained good
soil while those on
the right were in a
type of structural
soil
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All trees were
planted at the same
time
Structural soil
Tree growth in well-drained
good vs. structural soil
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This is a photo of the
same site as shown in
the previous slide about
ten years later
Canopies have closed to
form a nice shaded
sidewalk
Although trees on the
right are growing slower
than those planted in
soil, all have grown
acceptably and none
have been replaced
Tree in structural soil
around parking lot island
Structural soil
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Structural soil was
placed in this parking lot
island and under the
pavement around it
Roots should grow under
the pavement without
difficulty because they
will be growing in the
uncompacted soil that is
between the aggregates
This is likely to allow the
tree to grow to a large
size and provide more
benefits to the site than a
tree that remains small
and unhealthy
Many tools at your
disposal




Other professionals – engineers,
planners, architects, landscape
architects, urban foresters, arborists
Species selection and spacing
Creative design solutions
Ordinance and code changes
Urban Design to
Accommodate
Trees: Parking lot
Solutions
by Dr. Edward F. Gilman, professor
Department of Environmental Horticulture
University of Florida, Gainesville
http://hort.ufl.edu/woody/planting