Urban Design to Accommodate Trees: Parking lot Solutions

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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
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Outline of topics

• Introduction
• Site evaluation
• Species selection
• Formula for success
• Roots/hardscape conflicts
• Trees/sidewalk solutions
• Parking lot/buffer strip solutions
• Structural soils

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Solutions for trees in parking lots and buffer
strips
Solutions

• 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

4
Parking lot construction
design

• 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

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Parking lot design--we can do better
design

• 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
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Parking lot design--we can do better
design

• 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

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Signs and trees sharing the same space
design

• 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
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Sustainable parking lot design
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

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Most trees in buffer strips--one large island
design

• 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

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Large island supporting trees and shrubs
design
This large island is supporting many 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

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Non-sustainable buffer strips under wires
design
Buffer strips at the edge of parking lots are
often located under utility wires. THIS DOES NOT
WORK!

• 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

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Sustainable buffer strip on side of property
design
Trees in large soil spaces with no utility lines
nearby can grow to a large size.

• Buffer strips on the side of the property make
  more sense
• Often, there are no utility lines on the side of
  the property

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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

• 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

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Porous pavers near existing trees
design

• 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

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Porous pavers near existing trees-detail
design

• 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

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Gravel as a parking lot surface
design

• 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

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Porous asphalt as a parking surface
design

• 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

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Solutions for trees in parking lots and buffer
strips
Solutions

• 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

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Buffer strip too narrow to sustain trees
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 strip too narrow--tree breaks hardscape
Buffer

• 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

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Buffer strip suitable to sustain trees
Buffer

• 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

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Buffer strip suitable to sustain trees
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

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Buffer strip suitable to sustain trees for long
time
Buffer

• 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
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Solutions for trees in parking lots and buffer
strips
Solutions

• 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

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Large trees are not suited for small islands
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

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Standard planting in a parking lot island
Islands

• 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

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Where are the roots?
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

• 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.

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Small trees for small islands
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

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Large tree running out of soil space
Islands

• 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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Large tree growing well in small space
Islands

• 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)

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Large tree cracking curb in small space
Islands

• 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

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Very large tree cracking curb in narrow island
Islands

• 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

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Roots lifting curbing in narrow island
Islands

• 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

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Large island with large-maturing oak tree
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

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Large islands with large-maturing oak trees
Islands

• 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

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A peninsula can increase available soil space
Islands

• 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

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A peninsula can increase available soil space
Islands

• Trees can grow to be quite large in a peninsula
  because roots have access to ample soil space

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Solutions for trees in parking lots and buffer
strips
Solutions

• 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

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Linear planting strips in a parking lot
Linear strips

• 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)

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Linear planting strips can conflict with signs
Linear strips

• 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

See next slide for alternatives
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Alternatives to tradition
Linear strips

• 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

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Linear planting strips supporting trees
Linear strips

• 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

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Long strips support healthy trees
Linear strips

• 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

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Trees are growing into lights
Linear strips

• 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

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Trees healthy due to lots of soil space
Linear strips

• These trees are growing nicely due to good site
  design
• Long linear strips allow roots to share soil space

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Success from long linear soil strip
Linear strips

• 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

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Wider is better
Linear strips

• 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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Wide strip supporting a double tree row
Linear strips

• 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

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Root barriers can deflect roots
Solutions

• 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

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Root barriers can deflect roots
Solutions

• 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

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Outline of topics

• Introduction
• Site evaluation
• Species selection
• Formula for success
• Roots/hardscape conflicts
• Trees/sidewalk solutions
• Parking lot/buffer strip solutions
• Structural soils

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

54
Root barriers can deflect roots
Solutions

• 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

55
Root barriers can deflect roots
Solutions

• 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

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

57
Typical root growth under pavement
Structural soil

• 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
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Structural soil design
Structural soil

• 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

59
Structural soil installed
Structural soil

• 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
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Root growth in structural soil after three years
Structural soil

• 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
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Tree growth in structural soil
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
68
Tree growth in clay vs. structural soil
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
69
Tree growth in well-drained good soil vs.
structural soil
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
70
Tree growth in well-drained good vs. structural
soil
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

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Tree in structural soil around parking lot island
Structural soil

• 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

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

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