Urban Design to Accommodate Trees: Introduction

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Urban Design to Accommodate Trees Introduction

• 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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Urban design to encourage tree canopy
Introduction

• Trees often grow poorly in urban areas unless the
  infrastructure has been specially designed to
  accommodate tree root growth
• This presentation is designed to help guide you
  through the design and species selection process

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Few citizens want cities without trees
Introduction

• A city without trees is hotter in summer,
  receives less rainfall, has greater runoff
  following storms, has fewer shoppers, and is not
  inviting

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Poor design leads to failure
Introduction

• Trees struggle unless spaces are designed
  appropriately
• When lots of money is thrown at tree projects
  without guidance from knowledgeable
  professionals, waste occurs and no one wins

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Good design leads to success
Introduction

• Trees thrive when good designs are executed
  properly
• Healthy trees increase property value, intercept
  air pollutants, buffer temperatures, reduce wind
  speed, cool the city, reduce runoff from storms,
  encourage people to visit and spend money at
  shops, and create a more inviting community

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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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Site evaluation

• A thorough site evaluation insures that you will
  select the right tree for your planting site

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Examples of some of the components of site
evaluation
evaluation

• Above ground
• USDA hardiness zone
• Light, heat, and wind exposure
• Below ground
• Soil volume is there enough root space?
• Soil pH and drainage
• Soil texture, compaction
• Maintenance issues
• Availability of regular irrigation
• Pruning program in place or not

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Other important site evaluation criteria
evaluation

• What is the average annual rainfall in the area?
• Will the tree be plantedin the ground, in
  containers or in above ground planters, or near
  the coast
• What is the distance between the top of the water
  table and the soil surface?
• How will the site be irrigated?.
• Will the tree be planted in a tree lawn or
  streetscape (the grassy strip between the curb
  and the sidewalk)?
• Will the tree be planted along a street without a
  sidewalk.
• Will the tree be planted in a sidewalk cutout?
• Will the tree be planted in a parking lot?
• Will the tree be planted in an open lawn area or
  in a shrub bed? What is the approximate size of
  this area?
• Will the tree be planted within 8 feet of a
  sidewalk, driveway or other hard surface?
• Will an adjacent sidewalk or roadway receive
  deicing salts?
• Is there a swimming pool, vegetable garden,
  masonry wall or septic tank or drain field within
  50 feet of the planting site?
• Are overhead wires within 30 feet of the planting
  site?
• Is there a street light or security-type light
  within 35 feet of the planting hole?
• Is the planting site within 35 feet of a
  building?
• Would you care to eliminate trees that could drop
  messy fruit, large leaves or twigs during an
  extended period?
• Would you like to eliminate trees that are known
  to be susceptible to breakage?
• What is your budget for pruning trees?
• Would you care to plant only native trees?

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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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The dilemma
Selection
the design

• Certain trees grow well in tough urban sites so
  we use them often..monoculture results
• They grow well in small spaces but disrupt and
  destroy sidewalks/curbs, grow into wires
• We fix the problem by cutting roots and
  resurfacing hardscape, or cutting tops
• Trees decline or look ugly as a result and
• ..our vision of the urban forest never develops
  because trees never make it more than 20 to 40
  years
• We can do better with appropriate design

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The dilemma continued
the trees

• We could try different species or cultivars but
  they may perform poorly and besides no one else
  has tried these
• And alternative trees may be difficult to find at
  nurseries, especially in the size and quantity
  you want
• Sowe plant what we know will work i.e. what
  everyone else plants, because it is safe
• We are more or less stuck in this pattern now

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Solution be creative

• Restrict one genera or species to lt 20 for few
  years
• Develop a list of alternatives for each commonly
  planted tree
• For example alternatives to live oak
• Swamp chestnut, redbay, trident maple,
  sugarberry, ash, sweetgum, american elm, cedar
  elm, overcup oak,

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Match species to site characteristics
Selection

• Choose the right tree that will grow in the
  conditions present at the site use books,
  software, web sites, your experience
• or
• Design the right place to fit the trees you want
  this is covered in detail next

Dont try to shoehorn a tree you want into a site
not designed to support that tree, unless you are
a short term planner, in which case go for it
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Examples of right tree in the right place
Selection

• Small planting pit hardscape damage and poor
  growth so pick small, low O2 trees
• Parking lot island drought, small space, heat
  so pick urban tough, drought tolerant trees
• Park/campus/lawn plenty of soil space so pick
  large maturing trees
• Soil pH 8.2 poor growth on many trees so pick
  alkaline soil tolerant trees

• Compacted soil surface roots and hardscape
  damage likely so pick small trees tolerant of low
  oxygen
• Wires or lights overhead regular pruning so
  pick small trees or move wires and lights
• Poor drainage surface roots so pick small trees
  or design in more soil space
• Narrow planting strip deflected roots and
  toppled trees so pick small trees or design in
  more space

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Diversity can be the key to adversity
Selection

• But, once it is discovered that a certain tree
  grows well in a situation, it tends to be used
  over and over again.what is wrong with that?
• Perhaps nothing so long as a pest problem like
  Dutch elm disease, emerald ash borer, and others
  do not occur.
• How lucky do you feel? Detroit is not feeling too
  lucky.
• Your community has to make that decision

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Species selection for hardscape compatibility
Selection

• Trees that have a long life span generally
  compartmentalize decay well so they are resilient
• Trees that are free of serious pest problems are
  good

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Species selection for hardscape compatibility
Selection

• Trees that develop a prominent root flare
  (swelling at the base of the trunk) can lift
  sidewalks and curbs soon after planting in many
  circumstances if special provisions are not
  incorporated into the design

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Species selection for hardscape compatibility
Selection

• Avoid planting trees with fleshy fruit to reduce
  the likelihood of people slipping and falling on
  walks and pavement

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Species selection for hardscape compatibility
Selection

• Avoid trees with long sharp thorns or spines
  unless the thorns will be well above the ground
  and out of the way of pedestrians

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Species selection for hardscape compatibility
Selection

• Trees tolerant of low soil oxygen conditions
  often perform better than other trees when placed
  in small restricted soil spaces with poor
  drainage

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Other selection criteria
Selection

• Showy flowers/bark
• Canopy density/texture
• Attracts wildlife/or not
• Leaf size/messiness
• Nice fall color
• Single/multi-trunked
• Pruning requirement
• Canopy form/habit
• And others.

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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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Trees can form a canopy over the street
Success

• With appropriate spacing
• Access to open soil space
• Open soil space is soil that is not covered by a
  hard surface such as a sidewalk, pavement or a
  building

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Complete canopy closure
Success

• Trees were planted 40 to 50 feet apart in a
  planting strip 10 feet wide this spacing allowed
  for the crowns of individual trees to touch,
  encouraging development of a more natural upright
  form
• The 10' wide planting strip allowed the trunk
  flare to develop appropriately

State College, Pennsylvania
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Complete canopy closure
Success

• Trees were planted about 30 feet apart this
  spacing allowed for the crowns of individual
  trees to touch when they were fairly young and
  encouraged a more natural upright form
• Trees gained tremendous size due to the almost
  unlimited access roots had to soil space

Saint Augustine, Florida
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Complete canopy closure
Success

• Trees were planted 15 to 40 feet apart this
  spacing allowed for the crowns of individual
  trees to touch when they were fairly young
  encouraging a more natural upright form
• Trees gained tremendous size due to the almost
  unlimited access roots had to soil space

Seattle, Washington
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Barely complete canopy closure
Success

• Trees were planted about 50 feet apart. Because
  trees were spaced this far apart, they began to
  grow aggressive lower limbs. Lower limbs are
  drooping, creating a more spreading habit than
  would have occurred with closer spacing

Miami, Florida
Trees gained tremendous size due to the almost
unlimited access roots had to soil space
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No canopy closurespacing too far
Success

• Trees were planted about 50 feet apart. Because
  trees were spaced this far apart, they began to
  grow aggressive lower limbs. Lower limbs are
  drooping, creating a more spreading habit than
  would have occurred with closer spacing
• Trees gained tremendous size due to the almost
  unlimited access roots had to soil space

Charleston, South Carolina
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No canopy closure
Success

• Trees were planted about 60-70 feet apart.
  Because trees were spaced this far apart, they
  began to grow aggressive lower limbs.
• The planting strip is twenty feet wide and roots
  can grow into the lawns of the homes along the
  street

Coral Gables, Florida
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The formula

• Plenty of root space
• Closer spacing for canopy closure and reduced
  maintenance

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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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Roots can destroy hardscape with improper design
Conflicts

• Tree roots grow under sidewalks and asphalt in
  many instances because that is where the soil
  oxygen and moisture are located
• The hardscape is often inadvertently designed to
  encourage roots to grow there better urban
  design can reduce the likelihood of roots
  proliferating under hardscape

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Root spread on shade trees
Conflicts

• Shade trees extend their roots way beyond the
  tree canopy
• Note the root that is growing in the lawn (two
  arrows) it is located well beyond the branch
  tips

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Roots grow well beyond canopy edge
Conflicts

• Trees that normally grow a very expansive root
  system can become stressed and grow poorly in
  urban landscapes where soil space is limited
• The result can be poor tree health, damaged
  sidewalks and curbs, and other problems

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Root flare needs room to expand
Conflicts

• The swelling at the base of the tree (where the
  large roots meet the trunk) is commonly referred
  to as the root flare or buttress
• Roots normally raise out of the ground as shown
  here
• Adequate open soil space must be designed into
  the system to accommodate expansion of the root
  flare

Flare commonly 2.5 to 3.5 times trunk diameter
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Misfits and poor design
Conflicts

• The oaks planted in this narrow soil strip have
  two choices
• grow poorly due to the limited amount of soil
  space available for root expansion, or
• grow well by sending roots under the pavement
  which will quickly crumble the curb and asphalt

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Misplacement of large maturing trees
Conflicts

• The honeylocust planted between the walk and the
  wall are capable of growing to a large size. In
  order to thrive in this site, the trees roots
  will have to grow under the wall and into the
  lawn behind the wall
• The wall is likely to be displaced as the root
  flare develops and the roots expand in diameter
  beneath the wall

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Young trees likely to grow to disrupt hardscape
Conflicts

• The trees planted in this three to four foot wide
  strip are likely to cause disruption to the curb,
  sidewalk, and driveways along this street
• These repairs cost communities in the US
  approximately 2 billion dollars annually

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Damage can result
Conflicts

• Large maturing trees located too close to walks
  can cause structural damage that is costly to
  repair

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Sidewalks lifted
Conflicts

• Roots often grow just under the slab because that
  is where moisture and oxygen are abundant
• Roots lift the walk as they grow in diameter

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Picking slow growing trees can help
Conflicts

• Trees that remain small at maturity often cause
  less damage than large trees
• More small trees will be required (at a spacing
  of approximately 25 feet) to develop a closed
  canopy than if large maturing trees were planted

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Outline of topics

• Introduction
• Site evaluation
• Species selection
• Formula for success
• Roots/hardscape conflicts
• Trees/sidewalk solutions (go to sidewalk
  solutions PP file)
• Parking lot/buffer strip solutions
• Structural soils

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Urban Design to Accommodate Trees Introduction

• by Dr. Edward F. Gilman, professor
• Department of Environmental Horticulture
• University of Florida, Gainesville

http//hort.ufl.edu/woody/planting