Urban Design to Accommodate Trees: Introduction

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

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

Introduction Urban design to encourage tree canopy • 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

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

Introduction Good design leads to success • 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

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

evaluation Examples of some of the components of site 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

Selection The dilemma …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

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 • So……we 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

Solution – be creative • Restrict one genera or species to < 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,

Selection Match species to site characteristics • 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 Don’t 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

Success Trees can form a canopy over the street • 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

Success Complete canopy closure • 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

Success Complete canopy closure • 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

Success Complete canopy closure • 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

Success Barely complete canopy closure • 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

Success No canopy closure—spacing too far • 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

Success No canopy closure • 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

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

Conflicts Roots can destroy hardscape with improper design • 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

Conflicts Root spread on shade trees • 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

Conflicts Roots grow well beyond canopy edge • 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

Conflicts Root flare needs room to expand • 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

Conflicts Misfits and poor design 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

Conflicts Sidewalks lifted • 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

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

Urban Design to Accommodate Trees: Introduction