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Urban Design to Accommodate Trees: Introduction by Dr. Edward F. Gilman, professor Department of Environmental Horticulture University of Florida, Gainesville.

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Presentation on theme: "Urban Design to Accommodate Trees: Introduction by Dr. Edward F. Gilman, professor Department of Environmental Horticulture University of Florida, Gainesville."— Presentation transcript:

1 Urban Design to Accommodate Trees: Introduction by Dr. Edward F. Gilman, professor Department of Environmental Horticulture University of Florida, Gainesville

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

3 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

4 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

5 Poor design leads to failure 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 Introduction

6 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 Introduction

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

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

9 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 evaluation

10 Other important site evaluation criteria 1. What is the average annual rainfall in the area? 2. Will the tree be planted:in the ground, in containers or in above ground planters, or near the coast 3. What is the distance between the top of the water table and the soil surface? 4. How will the site be irrigated?. 5. Will the tree be planted in a tree lawn or streetscape (the grassy strip between the curb and the sidewalk)? 6. Will the tree be planted along a street without a sidewalk. 7. Will the tree be planted in a sidewalk cutout? 8. Will the tree be planted in a parking lot? 9. Will the tree be planted in an open lawn area or in a shrub bed? What is the approximate size of this area? 10. Will the tree be planted within 8 feet of a sidewalk, driveway or other hard surface? 11. Will an adjacent sidewalk or roadway receive deicing salts? 12. Is there a swimming pool, vegetable garden, masonry wall or septic tank or drain field within 50 feet of the planting site? 13. Are overhead wires within 30 feet of the planting site? 14. Is there a street light or security-type light within 35 feet of the planting hole? 15. Is the planting site within 35 feet of a building? 16. Would you care to eliminate trees that could drop messy fruit, large leaves or twigs during an extended period? 17. Would you like to eliminate trees that are known to be susceptible to breakage? 18. What is your budget for pruning trees? 19. Would you care to plant only native trees? 20. Please list any other attributes that you would like your trees to have? 21. Other considerations: 1) soil salt levels, 2) soil contamination, 3) soil layering, presence of construction debris, 4) health of and type of existing plants, 5) presence of underground utilities, 6) ordinance restrictions, 7) species diversity index, 8) politics, 9) community expectations, 10) design life of site, 11) tree life expectancy, 12) forest vs. former agricultural field, 13) location of existing tree roots, 14) recent construction activities. evaluation

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

12 The dilemma 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 Selection …the design…

13 The dilemma continued 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 …the trees…

14 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,

15 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 Selection

16 Examples of right tree in the right place 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 Small planting pit = hardscape damage and poor growth so pick small, low O 2 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 Selection

17 Diversity can be the key to adversity 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 Selection

18 Species selection for hardscape compatibility 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 Selection

19 Species selection for hardscape compatibility 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 Selection

20 Species selection for hardscape compatibility Avoid planting trees with fleshy fruit to reduce the likelihood of people slipping and falling on walks and pavement Selection

21 Species selection for hardscape compatibility Avoid trees with long sharp thorns or spines unless the thorns will be well above the ground and out of the way of pedestrians Selection

22 Species selection for hardscape compatibility Trees tolerant of low soil oxygen conditions often perform better than other trees when placed in small restricted soil spaces with poor drainage Selection

23 Other selection criteria 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…. Selection

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

25 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

26 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

27 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

28 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

29 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 Success Trees gained tremendous size due to the almost unlimited access roots had to soil space

30 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

31 No canopy closure Trees were planted about 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 Success

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

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

34 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

35 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

36 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

37 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 Conflicts Flare commonly 2.5 to 3.5 times trunk diameter

38 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

39 Misplacement of large maturing trees 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 Conflicts

40 Young trees likely to grow to disrupt hardscape 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 Conflicts

41 Damage can result Large maturing trees located too close to walks can cause structural damage that is costly to repair Conflicts

42 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 Conflicts

43 Picking slow growing trees can help 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 Conflicts

44 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

45 Urban Design to Accommodate Trees: Introduction by Dr. Edward F. Gilman, professor Department of Environmental Horticulture University of Florida, Gainesville


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