Urban Forest Hurricane Recovery Program

Wind-resistant urban design Dr. Edward F. Gilman and Traci Partin

Could this have been prevented? Photo credit: Chuck Lippi

The answer is yes!

Wind-resistant urban design 1. Design conflicts that can cause tree failure 2. Design solutions that promote wind resistance - existing design situations - new design/construction 3. How to take action

How often do trees fall down due to poor design?

What design conflicts cause trees to fail? Large-maturing trees within 10 feet of a paved surface, without space designed for root growth Lack of open soil space Large roots cut during construction Poor soil conditions Single specimens (versus grouping trees) Wrong species selected for the site NOT ENOUGH ROOT SPACE!

Large maturing tree near curb Photo credits: Brent Marable

Space is needed for roots to spread Fine roots can be found well beyond the canopy of the tree at full maturity These roots help anchor the tree under high winds roots

Soil depth is necessary for root stability Roots need adequate soil depth to anchor the tree under high winds. Soil should be at least 3 feet deep for large maturing trees

Lack of open soil space Root flare is interrupted by curb and sidewalk

Open soil space allows root flare to develop 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 The root flare provides balance and stability for the massive weight of a tree Flare commonly 2.5 to 3.5 times trunk diameter

Roots cut during construction 3 x trunk rule Step one Step two Step three Photo credit: Andy Kittsley

Poor soil conditions Compacted Shallow soil – rocky; high water table Drainage issues High clay content Alkaline soil/ Lime rock - inhibits uptake of essential nutrients Little to no organic matter (i.e. builder’s sand) Photo credit: Jim Urban

This is a familiar site for many who live in South Florida. What can be done with shallow, rocky soil?

Single specimens vs. grouping trees

Larger groupings are less damaged This design did not suffer any tree damage when a hurricane blew through

Consider the natural setting for a tree

Notice the trees still standing

Selecting the wrong species for the site Choose small trees for sites where soil space is limited.

What design conflicts cause trees to fail? Large-maturing trees within 10 feet of a paved surface, without space designed for root growth Lack of open soil space Large roots cut during construction Poor soil conditions Single specimens (versus grouping trees) Wrong species selected for the site NOT ENOUGH ROOT SPACE!

Where do we grow now?

Wind-resistant urban design 1. Design conflicts that can cause tree failure 2. Design solutions that promote wind resistance - existing design situations - new design/construction 3. How to take action

Existing design situations Problem: Mature tree with large roots interfering with hardscape. Solutions: Install different surface material Add fill and re-pour walk Bridge over roots Re-route walk CUTTING ROOTS IS NOT AN OPTION!

Install different surface material: rock dust Remove slabs Spread rock dust Pack dust tightly Photo credits:

Final product: looks attractive and is no longer a tripping hazard

Install different surface material: porous pavers To protect the root zone around the existing trees in this parking space at a botanical garden, porous pavers were selected

Add soil then re-pour over roots

Bridging over roots

Re-routing walk around tree When damaged sidewalks are repaired they can be re-routed around the tree trunks This can eliminate the need to prune roots that caused the walk to lift

Do not cut roots!

New design situations How do we make trees fit? Parking lots Planting islands Road medians Sidewalks Streets Buildings

Good design happens in two ways… Choose the right tree for the conditions of the site Design the right place to fit the trees you want

Designing the right place 1.Plant trees in the open soil space available 2.If this is not possible, direct roots toward the open soil 3.If there is not open space for root growth, design appropriate soil space 4.Consider groupings vs. individual plantings

“Appropriate” soil space At least 1000 to 2000 cubic feet of soil for each healthy, large maturing tree Open soil space 3 X wider than trunk diameter at maturity (dbh) to allow root flare development

Planting strips in successful designs

. Structural soil Small aggregate material (angular rocks ~ 1 in. diameter) with enough soil to almost fill the space between the rocks. Roots grow well in the soil between the aggregates. Illustration credit: Jason Grabosky

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

Suspended sidewalk Suspending the sidewalk avoids issues with soil compaction so that roots can spread without interrupting the hardscape

This particular system provides structure to support the hardscape, filling in the remaining space with quality soil. Suspended sidewalk: Soil cells Structure Soil Pavement

Use alternate surface materials Materials other than concrete can be used as a wearing surface: crushed granite gravel wood decking brick-in-sand porous pavers porous asphalt These materials allow oxygen to penetrate the soil and may encourage root growth

Porous paving surfaces Porous surfaces are a good design idea for areas prone to flooding: reduce runoff provide aeration to the soil

Designing the right place 1.Plant trees in the open soil space available 2.If this is not possible, direct roots toward the open soil 3.If there is not open space for root growth, design appropriate soil space 4.Consider groupings vs. individual plantings

The goal: A healthy urban forest Canopy cover Diversity of tree species grouped together

Plant trees closer for canopy closure Trees planted feet apart grow large lower limbs due to wide spacing, and require pruning to allow clearance. Group trees 30 ft. apart to encourage an upright form.

Grouping vs. individual planting This is a sustainable design that will last many years. Larger soil space shared by more trees = healthier trees! In a few years, this tree will outgrow the space.

Example of sustainable parking lot design Trees are located only in the buffer strips surrounding this parking lot – trees are not in small islands scattered throughout parking lot. This provides adequate space for tree roots to grow.

Remember: small spaces do not support large trees!

Solutions can happen in two ways… Choose the right tree for the conditions of the site Design the right place to fit the trees you want

Small trees for small spaces When planting within 10 ft. of curbing Plant small or medium sized trees (no greater than about 35 ft. at maximum height)

Wind resistant species To get a comprehensive list of wind-resistant species, refer to our website: ( ifas.ufl.edu/index.html

Do not over-plant one species Restrict one genera or species to < 20% for a few years Develop a list of alternatives for each commonly planted tree Examples of alternatives to live oak: Swamp chestnut, redbay, trident maple, sugarberry, ash, sweetgum, american elm, cedar elm, overcup oak

Wind-resistant urban design 1. Design conflicts that can cause tree failure 2. Design solutions that promote wind resistance - existing design situations - new design/construction 3. How to take action

Where to begin… 1.Get the right people involved 2.Set new building ordinances for the community 3.Become a Tree City USA

Who should be involved in the planning process? City engineer Landscape architect Urban forester/arborists Builders & developers Planners Parks and Recreation Power companies Homeowner’s Association City or county commissioners

What is Tree City USA? Program started by The National Arbor Day Foundation to promote communities that take care of their trees Provides a framework for starting a tree management program. More information:

Contact your local county extension! Get information specific to counties across the state i.e. tree species that grow well in the area, local soil conditions, etc. Cuts time spent on looking up information in half!