Urban Design to Accommodate Trees: Parking lot Solutions

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 • 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

design 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 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 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 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 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 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 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 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 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

Buffer 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 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 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 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 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

Islands 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 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 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 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 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 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 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 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 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 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 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 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 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

Linear strips 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 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 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 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 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

Outline of topics • 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 • 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 • 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 • 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 • 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 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 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 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

Urban Design to Accommodate Trees: Parking lot Solutions