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
Slide 2
Outline of topics Introduction Site evaluation Species
selection Formula for success Roots/hardscape conflicts
Trees/sidewalk solutions Parking lot/buffer strip solutions
Structural soils
Slide 3
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
Solutions
Slide 4
Parking lot construction Parking lots are carved out of
existing soil or are developed after soil has been removed or added
to the area; in either case, severe soil disturbances almost always
occur This makes it tough to grow trees in islands and other small
soil spaces constructed in the parking lot design
Slide 5
Parking lot design--we can do better Parking lots are typically
located between the street and the building with signs placed on
the building about 20 feet from the ground This places tree
canopies and signs in immediate conflict with each other because
they occupy the same vertical space---it is difficult for customers
to view signs and the building design
Slide 6
Parking lot design--we can do better There are many ways to
design parking lots differently, resulting in more sustainable
urban areas place signs closer to the ground place signs out by the
street and not on the building build the parking lot on the back
side of the building install trees with a dominant single trunk and
train the lowest branches so they are at least 25 feet off the
ground move the trees to the side of the buildings and property
borders design
Slide 7
Signs and trees sharing the same space 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
multi- trunked 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
design
Slide 8
Sustainable parking lot design Trees are located only in the
buffer strips surrounding this parking lot--no trees are placed in
islands. This keeps lights away from trees and ensures that trees
will not have to be pruned to make way for the lights Locating
trees in large buffer strips around the parking lot provides roots
with adequate space to grow design
Slide 9
Most trees in buffer strips-- one large island 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
Slide 10
Large island supporting trees and shrubs The parking stalls are
located over porous pavers that support grass growth This design
allows water and air to enter the soil and should reduce the amount
of water running off the parking lot This large island is
supporting many trees and shrubs design
Slide 11
Non-sustainable buffer strips under wires Trees must be pruned
to keep them small This makes for unnecessary work and prevents the
urban forest from developing 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 Buffer strips at the edge of parking lots are
often located under utility wires. THIS DOES NOT WORK! design
Slide 12
Sustainable buffer strip on side of property Buffer strips on
the side of the property make more sense Often, there are no
utility lines on the side of the property Trees in large soil
spaces with no utility lines nearby can grow to a large size.
design
Slide 13
Porous pavers for parking stalls Traditional pavement was used
for the travel lanes cars stalls were constructed of porous paver
blocks This should allow for better root growth because air and
water is more likely to enter the soil under the porous pavers This
parking lot for a large zoo in the southern U.S. was designed to
reduce runoff and support good tree growth design
Slide 14
Porous pavers near existing trees 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
Slide 15
Porous pavers near existing trees-detail A close up of the
previous photo shows that a large portion of the root system was
apparently protected during construction Be sure soil is not
removed from the root zone prior to laying the sub-base and the
pavers because this would remove a large portion of the root system
This photo shows what happens when some of the construction detail
was left to chance--note the huge damage to the trunk on the tree
in the foreground design
Slide 16
Gravel as a parking lot surface 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
Slide 17
Porous asphalt as a parking surface Porous asphalt allows water
to run through the surface to a layer of gravel below the surface
Forty percent of the volume below the surface is air space that can
fill with runoff water This reduces the volume running into
adjacent retention ponds and streams design
Slide 18
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
Solutions
Slide 19
Buffer strip too narrow to sustain trees 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
Buffer
Slide 20
Buffer strip too narrow-- tree breaks hardscape 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
Slide 21
Buffer strip suitable to sustain trees The fifteen foot wide
buffer strip at the left edge of this parking lot is more suitable
for tree growth than in many designs 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 Buffer
Slide 22
Buffer strip suitable to sustain trees 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
Slide 23
Buffer strip suitable to sustain trees for long time 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
Slide 24
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
Solutions
Slide 25
Large trees are not suited for small islands Planting
large-maturing trees in small parking lot islands is not
sustainable something will break Nonetheless we continue to see
landscape architects specify large- maturing trees for small
parking lot islands Islands
Slide 26
Standard planting in a parking lot island 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 Islands
Slide 27
Where are the roots? 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 Islands
Slide 28
Big island means healthier trees Here is a parking lot
constructed on a compacted clay soil The two trees on the left and
center (blue arrows) are larger and darker green than the tree on
the right The stressed tree on the right was planted in a tiny
island whereas the healthier ones were in a larger island.
Islands
Slide 29
Small trees for small islands Small-maturing trees such as
crape myrtle are the best ones to choose for small islands They
cause less damage to hardscape than trees that grow to be large
Islands
Slide 30
Large tree running out of soil space 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)
Islands
Slide 31
Large tree growing well in small space Large trees planted in
small parking lot islands constructed over certain 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
(blue arrows) Islands
Slide 32
Large tree cracking curb in small space 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
Slide 33
Very large tree cracking curb in narrow island 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
Slide 34
Roots lifting curbing in narrow island 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 Islands
Slide 35
Large island with large- maturing oak tree 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
Slide 36
Large islands with large- maturing oak trees Designing parking
lots with large islands such as the two pictured here allow large-
maturing trees to be planted with less fear of them disrupting
hardscape Islands
Slide 37
A peninsula can increase available soil space 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
Slide 38
A peninsula can increase available soil space Trees can grow to
be quite large in a peninsula because roots have access to ample
soil space Islands
Slide 39
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
Solutions
Slide 40
Linear planting strips in a parking lot 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
Slide 41
Linear planting strips can conflict with signs Trees in long
strips can conflict with building signs if the site is not designed
correctly In this design, the security lights (blue arrow) are
located away from the tree planting strips; this eliminates
conflicts between lights and trees and reduces maintenance Linear
strips See next slide for alternatives
Slide 42
Alternatives to tradition move the trees to the edge and sides
of the property select and train trees to a single leader to ease
canopy raising reduce sign height move the signs to the front edge
of the property move the building toward the street and place trees
behind building Linear strips
Slide 43
Linear planting strips supporting trees These trees in a long,
fairly wide planting strip are growing well The close spacing
between trees allowed the canopy to close quickly over the parking
spaces Linear strips
Slide 44
Long strips support healthy trees 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
Slide 45
Trees are growing into lights 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
Slide 46
Trees healthy due to lots of soil space These trees are growing
nicely due to good site design Long linear strips allow roots to
share soil space Linear strips
Slide 47
Success from long linear soil strip Long wide strips are better
able to support trees than small islands Here is a very successful
project using clonal trees that created a nice border and screen
Linear strips
Slide 48
Wider is better Very wide soil strips in parking lots allow for
optimal tree growth This strip is forty feet across and will
support trees for decades Linear strips
Slide 49
Wide strip supporting a double tree row 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 Linear strips
Slide 50
Root barriers can deflect roots 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
Slide 51
Root barriers can deflect roots 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 Solutions
Slide 52
Outline of topics Introduction Site evaluation Species
selection Formula for success Roots/hardscape conflicts
Trees/sidewalk solutions Parking lot/buffer strip solutions
Structural soils
Slide 53
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
Slide 54
Root barriers can deflect roots 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
Slide 55
Root barriers can deflect roots 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 Solutions
Slide 56
Outline of topics Introduction Site evaluation Species
selection Formula for success Roots/hardscape conflicts
Trees/sidewalk solutions Parking lot/buffer strip solutions
Structural soils
Slide 57
Typical root growth under pavement 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 Photo
credit: Jason Grabosky, Rutgers University Structural soil
Slide 58
Structural soil design 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 Structural
soil
Slide 59
Structural soil installed 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
Slide 60
Slide 61
Root growth in structural soil after three years 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
Structural soil
Slide 62
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Slide 67
Tree growth in structural soil These trees were planted into a
strip of structural soil installed in this retrofitted parking lot
between the blue arrows They have performed quite well Photo
credit: Jason Grabosky, Rutgers University Structural soil
Slide 68
Tree growth in clay vs. structural soil 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) 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
Slide 69
Tree growth in well-drained good soil vs. structural soil Trees
on the left were planted in well-drained good soil while those on
the right were in a type of structural soil All trees were planted
at the same time Photo credit: Jason Grabosky, Rutgers University
Structural soil
Slide 70
Tree growth in well-drained good vs. structural soil 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
Structural soil
Slide 71
Tree in structural soil around parking lot island 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 Structural soil
Slide 72
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
Slide 73
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