3.6 Street Trees
Street trees are required by municipal code and standards set by SDOT designed to provide maximum public benefit and compatibility with other infrastructure in the street right-of-way. Street trees are to be protected during construction and otherwise managed directly, or indirectly, by SDOT Urban Forestry for optimum health, longevity, and public safety. SDOT Urban Forestry advises applicants on tree selection, installation and protection measures to preserve the functional, environmental, social, and economic benefits of trees and to support the citywide urban forestry goal to increase canopy cover. For more information on design standards for downtown, see the Downtown Design Section.
The SDOT Urban Forest Landscape Architect’s Office reviews SDCI building permits and SIP projects and helps the team to:
- Identify existing street trees to be retained and protected to guide site planning and building design.
- Identify the size category of new street trees to meet objectives.
Street Tree Preservation and Species Selection
Like other street design standards, street trees are reviewed and approved by SDOT.
Tree species selection for optimum, sustainable canopy cover is based on four primary factors:
- Right-of-way width for planting (relative to the street type standard 6′ width)
- More than standard with no overhead powerlines
- More than standard with overhead powerlines
- Does not exceed standard with no overhead powerlines
- Does not exceed standard with overhead powerlines
- Corridor, community and project objectives
- Corridor identity – specific arterial streets and businesses or historical district frontages
- Community interests/input from Design Review, Design Commission, and/or Community Councils
- Unique cultural influence relevant to the neighborhood and/or project
- Compatibility with drainage and greenfactor ordinance requirements
- Green Stormwater Infrastructure – Bioretention facility design
- Green Factor in the ROW compliance
- Compatibility with public utility infrastructure in the ROW and/or inadequate ROW to accommodate trees
- Underground Public Utilities less than 5′ clear — Site specific selections coordinated with Utility
- Use of 5′ setback on private property — Site specific selections coordinated with Project Design Team
The street tree design criteria in this section are based on transportation safety requirements and on minimum requirements for soil volume to achieve a mature canopy for effective reduction of both storm water runoff and reflected heat from paved street and sidewalk surfaces.
The SDOT Recommended Street Tree List for spacing recommendations and standard plan details including Standard Plan #030 for clearances are resources to inform permit applicants and to expedite the process of over-the-counter permits issued to property owners by SDOT Street Use. However, because trees are living infrastructure, growing in size over time, each project is reviewed and inspected on a site-specific basis to ensure the optimum outcome for the project, the neighborhood, and the city as a whole.
Links to Standard Plans and Specifications
Std Specification 1-07.16(2) Tree Vegetation and Soil Protection
Std Specification 8-01.3(2)B Tree Vegetation and Soil Protection Plan (TVSPP)
Std Specification 8-02 Landscape Construction
Std Specification 8-14 Cement Concrete Sidewalk
Standard Plans 030 through 142
Standard Plans Section 100: Landscape Planting (includes standard plans for trees, shrub and ground cover, irrigation, tree protection, soil preparation and grading)
Standard Plan 424a and b: Tree Pit Detail
Clearances from street trees: Street trees require access to air and water, space for growth and must be located, installed and managed for compatibility with the built environment. Standards for layout of trees and other infrastructure within a transportation corridor described in this manual are established to establish corridors, setbacks and depths for various elements including utilities (i.e. storm drains, sanitary sewer lines, water lines, etc.) to ensure that minimum clearances from trees and other objects in the right-of-way can be met.
Clearances from street trees—at grade: With limited right-of-way, SDOT will evaluate site conditions and may permit one or both of the following:
- Deviations from the standard clearances from face of curb and/or sidewalk edge based on street types, tree species, curb lane use (parking or travel lane) and recorded or projected information about pedestrian volumes for the corridor; and
- Deviations from the typical planting strip and/or tree pit location within the street cross section (i.e. tree planting in bulb outs within a parking lane, behind the sidewalk, etc.)
Clearances from street trees—below grade: The design of street improvements must consider underground utilities in relation to standard utility corridors. The location of private service connections must also consider clearances from street trees. Though less than optimum for both utilities and trees, a minimum standard of 5’ lateral clearance is required. Where right-of-way width allows allocation of more than 5’, the investment to provide additional space up front often provides a long-term benefit through reduced impact on trees due to utility line maintenance and repair.
- Where both utilities and street improvements are proposed by a project, the design must follow standards, including the 5’ clearance standard between service connections and street trees.
- Where street improvements are proposed in a ROW with existing utilities that do occupy standard corridors, the standard 5’ clearance between street trees and service connections is required.
- Where utilities are proposed within a ROW with existing street trees, the design must follow standards with regard to placement of utilities within designated utility corridors and the 5’ standard clearance for service connections.
- Where street improvements are proposed within a ROW with existing service connections and the 5’ clearance standard conflicts with proposed street trees in standard planting strips or tree pits, Street trees will still be required with additional mitigation measures to help protect both the trees and the service connection. The mitigation measure must be approved by SDOT.
- Where street improvements are proposed within a ROW with existing utilities that do not occupy standard corridors, Street trees will still be required with additional mitigation measures provided to protect the street tree and public utility. The mitigation measures are subject to approval by SDOT and the public utility.
In some cases, depending upon the age, depth, and material of the utility, mitigation may not be possible, and the utility may be required to relocate if trees are required.
Possible Mitigation Measures: Other mitigation measure may be considered as new technologies become available and are assessed as to their feasibility for the project.
- Vertical Root Barrier
- Horizontal Root Barrier
- Ductile Iron Pipe
- Concrete Pipe with Rubber Gaskets (post 1960) after review by utility owners.
- Utility Line Relocation
Planting strip dimensions and clearances: Maintain 5’ 6″ clearance between the back of the sidewalk and inside edge of the curb or edge of the roadway (6′ is to the curb face) to allow a setback for street trees measuring 3’ 6” from the centerline of the tree to the curb face and 2’ from the centerline of the tree and sidewalk edge. Plants within 30′ of an intersection, must be maintained at a height of 2′ or shorter. Plants within 10′ of driveways must be clear of sight obstructions between 36″ and 82″ in height from the ground.
Planting strip treatments for understory: Plants in planting strips vary greatly in their potential to provide optimum pedestrian and environmental benefits. Though SDOT allows the installation of grass, the department encourages the installation of low shrubs, perennial or groundcover plantings that provide a superior degree of separation between the sidewalk and street at reduced maintenance costs. Under some conditions, a combination of the plantings and grass or plantings and pavers may be appropriate depending on the street classification and need to accommodate parking in the curb lane.
Refer to On-Site Stormwater Management BMPs for guidance on integrating stormwater conveyance and treatment into the planting plan.
Planting strips—grading: The final grade of soil surfaces in planting strips must accommodate runoff from sidewalk surfaces cross-sloped to drain toward the street. In cases where a swale or depressed planting strip is proposed, see Green Stormwater Infrastructure. In cases where a mounded planting strip is proposed to provide a more effective separation between the sidewalk and street, a centerline height of 6” above the adjacent sidewalk grade is typical and gaps between mounded areas must be provided so that backup of runoff and ponding does not occur on the paved sidewalk.
Tree pits: Typically used as an alternative to planting strips in business districts where additional sidewalk width is important to accommodate pedestrian volumes.
When permitted as an alternative to planting strips, tree pits shall be constructed per Standard Plan 424, dimensioned to meet or exceed the minimum size required to meet standards. The minimum square footage for a tree pit is 24sf of open area (typically 4′ x 6′ or 5′ x 5′). Any proposal dimensioned below minimum standards shall be subject to site-specific review to ensure that:
- Conditions justify the substandard size;
- Soil volume (uncompacted or structural soil) is maximized
- The design meets public safety standards; and
- The design provides adequate conditions to support the proposed tree species to reach maturity in good health.
Tree pits—grading: Shall be graded to provide a soil surface 2″ below the adjacent sidewalk and curb elevation and be top dressed with bark, wood chips, cinders, or crushed angular aggregate material that is routinely maintained to minimize the grade differential between the sidewalk and open pit area.
Street Tree Permit: Please refer to right of way improvement activities.
Tree grates: Often proposed as an architectural design element and/or as a means to maximize the pedestrian accessible area in the right-of-way, tree grates may be permitted by SDOT, but are not recommended. When permitted, tree grates shall be maintained routinely by the property owner to ensure a flush condition between the grate surface and surrounding pavement, to replace broken segments, and to expand the opening as appropriate to accommodate the growth of the tree. SDOT has established the use of flexible porous surface treatment as a new standard for tree pit treatment that provides improved ADA accessibility and sustainable conditions for street tree growth and longevity. See Standard Specification 8-02.3(9)B.
Tree and plant material—selection: See SDOT Recommended Street Tree List
Tree and plant materials–installation and maintenance responsibilities: It is the responsibility of the property owner to ensure that installation and maintenance of grass, plantings and related improvements in planting strips meet public safety and industry standards. Street tree and landscaping improvements required by the Land Use Code must be maintained to meet public safety standards for the life of the project. This includes:
- Mulching to minimize water use, discourage weeds and protect against erosion
- Pruning low shrubs and groundcover to control overgrowth onto sidewalks
- Pruning street trees (after first obtaining a street use permit) to ensure appropriate clearances over streets and sidewalks.
- Watering to ensure establishment of plant material
Tree protection and maintenance: Removal or pruning of street trees proposed requires a permit and is subject to review and, when approved, subject to inspection by the SDOT Urban Forestry.
Permit applications may be required to include public notification and/or mitigation for the lost value of the tree(s) proposed for removal. In cases where the applicant is not the owner of the property abutting the proposed work, applications must include signatures of adjacent property owners. The extent of notification is determined on a case by case basis to ensure public safety and awareness and/or approval of the project. Names and addresses of contacts may be submitted for approval or provided as a component of the permit review process by SDOT Urban Forestry. All permitted work must be completed within 60 working days unless otherwise defined by the permit.
Executive Order 03-05 directs all City departments to replace every tree that is removed from City-owned land in Seattle with two new trees.
Trees and related landscape architectural treatments that are strategically planned for maximum public benefit within the limited space within the street right-of-way provide a “sense of place” critical to the vitality of neighborhoods and their business districts.
The value of street trees to the urban environment is enhanced when they are combined with understory planting, specialized pavements, street furniture and public art.
Tree health and maintenance: Allocation of space is a key factor in the management of trees for compatibility with adjacent paved surfaces. To meet industry standards for street trees in urban areas, provide 1,200 cf soil volume for each tree planted. When adequate soil volumes are not achievable via planting strips, use structural soils and tree soils cells to increase soil volumes and connect root zones of planting under paving. If using structural soil, four times the volume is necessary to match that of tree soil cells in order to match usable soil amounts.
Planting strips serve a number of important functions including:
- Pedestrian safety–they provide a buffer between the sidewalk and roadway;
- Reduction of runoff by providing area for rainfall to infiltrate;
- Water quality by treating runoff from the sidewalk and/or roadway before it enters nearby waterways; and
- The growth and longevity of street trees.