Title: Urban%20Design%20to%20Accommodate%20Trees:%20Parking%20lot%20Solutions
1Urban 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
2Outline of topics
- Introduction
- Site evaluation
- Species selection
- Formula for success
- Roots/hardscape conflicts
- Trees/sidewalk solutions
- Parking lot/buffer strip solutions
- Structural soils
3Solutions 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
4Parking 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
5Parking 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
6Parking 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
7Signs 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
8Sustainable 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
9Most 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
10Large 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
11Non-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
12Sustainable 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
13Porous 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
14Porous 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
15Porous 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
16Gravel 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
17Porous 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
18Solutions 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
19Buffer 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
20Buffer 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
21Buffer 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
22Buffer 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
23Buffer 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
24Solutions 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
25Large 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
26Standard 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
27Where 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
28Big 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.
29Small 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
30Large 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)
31Large 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)
32Large 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
33Very 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
34Roots 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
35Large 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
36Large 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
37A 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
38A 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
39Solutions 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
40Linear 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)
41Linear 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
42Alternatives 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
43Linear 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
44Long 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
45Trees 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
46Trees 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
47Success 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
48Wider 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
49Wide 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
50Root 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
51Root 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
52Outline of topics
- Introduction
- Site evaluation
- Species selection
- Formula for success
- Roots/hardscape conflicts
- Trees/sidewalk solutions
- Parking lot/buffer strip solutions
- Structural soils
53Many 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
54Root 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
55Root 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
56Outline of topics
- Introduction
- Site evaluation
- Species selection
- Formula for success
- Roots/hardscape conflicts
- Trees/sidewalk solutions
- Parking lot/buffer strip solutions
- Structural soils
57Typical 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
58Structural 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
59Structural 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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61Root 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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67Tree 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
68Tree 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
69Tree 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
70Tree 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
71Tree 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
72Many 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
73Urban 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