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Street Trees

Converted from 2_Sidewalks.pdf on 2019-11-25 by Nat Taylor <nattaylor@gmail.com>
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Trees, shrubs, grasses, and other landscape plantings, or “greenscape,” play an important role in making streets comfortable, delightful, memorable, and sustainable. Used appropriately, they can help define the character of a street or plaza, provide shade and cooling, reduce energy consumption, and absorb and cleanse stormwater. They also absorb greenhouse gases and help filter airborne pollutants. In proximity to other green spaces, street trees can contribute to native wildlife systems.

In addition to providing environmental benefits, a healthy greenscape provides psychological and social benefits. People are attracted to places that have well-maintained plantings. Healthy greenscapes are good for city life and for business. The changing light and color along a tree-lined street reminds us of the changing seasons. By connecting us with nature in its beauty and complexity, plants help reduce stress and restore a sense of calm and focus.

Maintaining landscape plantings on Boston’s dense streets is challenging. Sidewalk space is at a premium and the hard surfaces required to support concentrated activity can be hostile to trees and other plantings. Soil compaction, lack of rooting space, poor soils, road salt, temperature fluctuations, physical damage, and even air pollution and litter all put stress on plants. These guidelines seek to balance the benefits of a healthy greenscape with the realities of limited space and the ongoing need for care and maintenance.

The guidelines in this section are intended to enable street trees and plantings to thrive, and to use stormwater as a resource to support plant life and replenish groundwater. The following sections provide a discussion of the benefits of street trees, plantings, and vegetated stormwater management along sidewalks, as well as the importance of soil selection and management in cultivating plant life.

Greenscape elements in the public right-of-way must be approved by the Boston Parks Department and PWD. Enhanced and pilot treatments will require special maintenance agreements.


Phytoremediation or phytotechnologies—the use of plants to clean, remove, and stabilize contaminates—should be strongly considered in the design of greenscape elements. Many common organic contaminates, including petroleum hydrocarbons, can be easily processed and degraded by plants and associated soil biology. Contaminates are found in stormwater, air, existing site soils, and groundwater; it is encouraged that plantings not only be designed to treat stormwater, but other pollution sources as well. Greenscape can be designed to prevent the spread of contamination spills before they occur, or to remediate areas where a previous contamination is suspected. This is especially important on Industrial Street Types, in maintenance yards, brown fields, and other areas where high concentrations of pollutants may be of concern. For additional information on phytoremediation, please refer to the Environmental Protection Agency’s website page on Contaminated Site Clean-Up Information (CLU-IN) and phytotechnologies.

Benefits of Street Trees


Reduced energy use and heat island effects: Trees reduce energy use by shading buildings and cooling the air through transpiration. A study on heat island effects in New York City concluded that trees and green roofs substantially reduce air temperatures, with street trees providing the most cooling per unit area.*



Benefits of Vegetated Stormwater Management


Boston’s streets and sidewalks are one of the city’s most valuable resources, and they offer tremendous opportunities to improve stormwater management. New green strategies for managing runoff along streets and sidewalks can reduce flooding, increase groundwater recharge, and reduce pollution to our rivers and streams as well as to Boston Harbor. Capturing rainfall before it flows into the city’s drainage and sewer system can also help reduce sewer overflows and save the city money on upgrading and repairing infrastructure. Many of the best techniques for managing stormwater runoff use trees and other vegetation to capture rainwater as it falls, and to collect and filter runoff from streets, sidewalks, and other paved areas. Increasing vegetation also helps keep streets cooler, both by the shade from large trees, and by evaporation and plant transpiration, which cool the air just as perspiring cools the skin.

The City of Boston encompasses just over thirty-one thousand acres of land, over half of which is paved over with streets, buildings, and parking lots. Stormwater runoff from Boston flows into four major watersheds: the Charles River, the Mystic River, the Neponset River, or directly into Boston Harbor. Boston also has a major challenge maintaining groundwater levels, mainly in areas that are on filled land that was previously open water and marsh. In these areas, wood pilings that support many buildings may rot if groundwater levels drop. Recharging stormwater rather than directing runoff into pipes is one strategy for maintain groundwater levels.

The City of Boston owns and controls about one quarter of the land area of the city, and over half of city-owned property is streets and roads. The streetscape is one of the city’s best areas for controlling and managing stormwater runoff.




Soils Selection and Management


Proper soil selection and management is one of the best ways to support healthy vegetation and to improve stormwater management in urban areas. Healthy soils—soils that have a high organic content and plenty of pore space—support healthier trees and plants and promote more groundwater recharge and better filtration of stormwater. Heavily compacted soils act almost like pavement, absorbing little water, and supporting less biological activity than well aerated soils.

Existing trees and planted areas that have become compacted and degraded can be significantly improved with aeration to restore porosity and/or the addition of soil amendments, such as weed-free compost, to help retain soil moisture. Soil improvements can make a significant difference in the health and longevity of trees and other vegetation. They can also improve stormwater management. Soil maintenance should be part of an operation and maintenance plan for urban vegetation.

New street trees and plantings present an opportunity to use engineered soils to grow a much larger and healthier greenscape and to clean and recharge significant volumes of stormwater runoff. Design details for planting street trees and implementing vegetated stormwater management techniques are found in the following sections. In all of these applications, careful selection of soil type and providing maximum soil volume should be priorities.

In constrained situations where existing street trees cause sidewalk heaving or where space is limited, consider using structural soils. Structural soils are a type of engineered soil that is designed to meet the load bearing requirements of urban streets while still maintaining adequate porosity and organic content to support healthy vegetation. Some structural soils also contain materials that specifically retain moisture. In urban contexts, structural soils allow the placement of ample, healthy soil beds beneath sidewalks and parking areas. Trees and plantings can be grown in dense urban settings with paved surfaces above the root systems, provided there is a way for water to enter the structural soil mixture.

Structural soils require irrigation (passive or active) to support a variety of plant types. Overflow drains may be necessary depending on the characteristics of the surrounding soils. Structural soil applications can both provide a healthier environment for plants and better capture, filter, and recharge of stormwater.

As an alternative to structural soils, soil cell systems can be used to provide appropriate soil volumes. See Covered Tree Trenches later in this chapter for more information about structural soils.

Street Trees

Street trees help define many of Boston’s best-loved streets and are a critical component of Boston’s urban forest ecosystem. This section describes how and where to plant street trees to achieve both environmental and urban design benefits.

Any resident of Boston can request to have a street tree planted in front of their home or business, provided the sidewalk is wide enough, by calling the Park Line at 617-635-PARK (7275). An arborist must inspect the site to determine if a tree can be planted.

The Boston Parks Department oversees maintenance and planting of trees in the public right-of-way. The maintenance program includes pruning, disease control, removal, and storm damage repairs. The Department’s oversight includes review and approval of trees to be planted by others and the planting of new trees throughout Boston’s neighborhoods. Tree selection and planting design in the public right-of-way must be approved by the Boston Parks Department and PWD.

Street Trees and Urban Design

Street trees can be used to serve a variety of urban design functions. Based on their location, arrangement, and spacing trees can:

Iconic plantings of street trees associate neighborhoods with seasons, and contribute to a unique sense of place. Red oaks in autumn on the Jamaicaway embody the essence of New England. Magnolias in bloom on Commonwealth Avenue mark the arrival of spring.

Trees are an ideal form of shade, providing protection on hot summer days while allowing heat and light to penetrate during cold winter months. They can also calm traffic by narrowing the apparent width of the roadway.

Street trees should be used in thoughtful compositions that respect the overall street context, local environment, and adjacent land uses.

Street Trees and Street Types

Street trees should be considered in every street design project; however, on some Street Types, trees are essential. For example, Boulevards and Parkways are defined in large part by the presence of trees. Below are guidelines for using trees on Boston’s Street Types.

Parkways are lined with continuous green spaces for trees, either on the sides or in the median. If sufficiently wide, green spaces provide an excellent rooting environment for largestature shade trees. Trees should be planted no more than 40’ apart to help create a continuous canopy. Species of a similar size, scale, and form should be planted along the length of the road for consistency and to maximize visual impact. Avoid monocultures, as disease and insects may destroy street trees along an entire street.

Trees on Boulevards are planted at regular intervals in a formal pattern with street lights, emphasizing linearity and long perspective views. The pattern draws the eye to the horizon or to an important terminus, such as the State House on Beacon Street, the Public Garden on Commonwealth Avenue, or the Blue Hills on Blue Hill Avenue. Trees are planted in the Greenscape/Furnishing Zone and are usually surrounded by pavement. Modern planting techniques such as covered tree trenches should be used to provide sufficient soil volume. Large-stature shade trees of similar size, scale, and form are typically planted 30’ apart to create a continuous canopy.

Neighborhood Connector Streets are similar to Boulevards but are less formal. Trees should be planted where they can best survive, such as in open or covered tree trenches.

Neighborhood Main Streets benefit enormously from trees, as visual preference studies have found commercial districts with shade trees are consistently preferred over districts without trees. Shade trees create the sense of an outdoor room and make streets more comfortable for sitting, café dining, window browsing, and socializing. Trees should complement and not interfere with first floor uses, entryways, cafés, or other activities in the Frontage Zone. Trees should not be planted in loading zones. Limbs should be pruned to maintain sight lines and maximize visibility of the street wall. Different species can be used in clusters to highlight special areas and create a sense of place.

Large canopy shade trees are attractive and add value to homes on Neighborhood Residential Streets. They help keep homes cool in the summer while allowing light and heat to penetrate in colder months. The branches also have the benefit of tempering winter winds. Street trees should be spaced far enough apart to allow light to reach front lawns and gardens. Open tree trenches or front yards (with permission from owners) should be used where possible to maximize rooting space.

Downtown Commercial and Downtown Mixed-Use Street Types require trees that can adapt to low light depending on building heights, street width, and street orientation. Where there is insufficient rooting depth due to underground utilities, raised tree beds can be considered.

Trees in Industrial settings must to be able to withstand drought and harsh conditions resulting from heavy traffic, green-house gas (GHG) emissions, and heat island effects from surrounding lots. Where possible, trees should be set back from the street and planted in continuous filter strips between the paved lots and the sidewalks. Tree species that can uptake and remove urban contaminates and air pollutants should be considered wherever possible.

Choosing the Right Tree

Tree selection needs to address the ability of the tree to mature in a given microclimate, as well as its ability to meet design objectives. Scale and form are key design considerations.

Large canopy shade trees play a critical role in the urban forest ecosystem, and offer a unique presence on city streets. Providing sufficient rooting space is a challenge, however this does not limit plantings to smaller trees; even small trees will suffer in a limited rooting environment. Given all the uncontrollable variables in a street it is worth taking a chance that a shade tree will survive in less than ideal conditions. Appropriate details should be used to enable trees to grow without roots rising to the surface and deforming sidewalks.

Choosing a tree for the right habitat can help minimize conflicts with adjacent infrastructure. For example:

Other considerations for selecting the right tree include: the scale and form; sight line requirements; the type of microclimate; tolerance to drought and insects; inundation; resistance to vehicular emissions and salt; the ability to remediate pollutants; and the amount of maintenance. From an aesthetic perspective, spring flowers, fall color, the quality of light and shade, and the abundance of fruit, nuts, and leaf litter should also be considered.

Examples of Parks Department Approved Street Trees

Tree Siting and Spacing

Trees should be planted in locations that provide the best conditions for growth within a given design framework. This could mean planting in private yards in residential areas (with permission from owners), or clustering trees in open planting areas on wide sidewalks or in plazas. Large, contiguous planting areas should be employed where feasible to enable large canopy shade trees to reach maturity.

Street tree plantings should strive for continuity along a street while respecting adjacent uses. Each tree should complement and not interfere with first floor uses, entryways, cafés, or other activities in the Frontage Zone. Trees should not be planted in loading zones or within 10’ of bus stop landing zones. Trees limbs should be pruned to maintain sight lines and maximize visibility of the street wall.

Preferred Tree Spacing and Offsets

Item Short Medium Large
On-Center Spacing 20’ 25’ 30’
Offset form Curbs or Path Edges 2’6" 2’6" 2’6"
Offset from Light Poles 15’ 15’ 15’
Offset from Driveways, Fire Hydrants, Loading Zones 10’ 10’ 10’
Offset from Intersections 20’ 20’ to 40’ 20’ to 40’

Root Environment for Street Trees

The ability of a tree to grow beyond a certain size is directly related to the volume of soil available for roots. Providing sufficient rooting soil in a dense, urban environment can be costly, but is worthwhile given the unique benefits that mature shade trees provide.

Tree roots do not survive well in highly compacted soil because it lacks the void spaces needed for air and water to circulate. Roots in compacted soil will migrate toward the surface for air and water, causing sidewalks to crack and heave.

When the rooting space is severely constrained, the tree roots will grow to capacity, and then the tree will decline and die.

Trees in the Northeast U.S. need approximately 2 cubic feet of soil per square foot of canopy area. † For example, a tree growing in a constrained 3’ by 8’ by 4’ pit would be expected to reach about an 8’ diameter canopy before becoming stressed and showing signs of decline. If the tree has access to soil outside the pit, the canopy can grow much larger.

Landscaped areas in the Frontage Zone or on the edge of adjacent properties (with permission from owners) can be excellent places to plant trees, as they may offer open areas for roots to spread. Examples include the residential edges on Commonwealth Avenue, where most of the iconic Magnolias are planted. When open landscape areas are not available, more intensive strategies are required.

The last decade has brought several innovations in engineered soils and sidewalk designs to support root growth. Below are four strategies for planting trees in constrained sidewalk settings. These strategies are intended to increase the volume of rooting soil while maintaining accessible sidewalks, and are discussed in detail on the following pages. Methods include:

Open Tree Trenches


An open tree trench is an area of soil connecting a row of trees that is covered with mulch, groundcover, grass (or “tree lawn”), or other greenscape. Tree trenches are generally located in the Greenscape/Furnishing Zone, though they can also be located in the Frontage Zone. For stormwater benefits, the sidewalk should be pitched toward the open tree trench. Non-linear open tree areas can also be used for planting trees in clusters.

Trees planted in open tree trenches and areas with a sufficient amount of uncompacted soil have the greatest chance of surviving and thriving in an urban environment.



Covered Tree Trenches


A covered tree trench is a linear trench covered by pavement designed to support root growth while providing structural support for sidewalks. A covered tree trench makes it possible to have large canopy shade trees in even the most constrained urban environments.

Support for the sidewalk is typically provided by using engineered structural soils. Structural soil is designed to be loose enough to allow air exchange, water movement, and root growth, yet compactable enough to support pavement||. Soil cells and structural soils can be used in clusters around trees as well as in a linear trench if needed to avoid underground obstructions.

Tree trenches should be covered with pavement and permit passive irrigation to allow water to reach the soil. Provisions may include the use of pervious pavement or flexible, perforated pipes beneath the pavement. Covered tree trenches are considered an enhanced treatment and require a special maintenance agreement.



Raised Tree Beds


Raised tree beds can be appropriate for planting trees in locations where utilities or subsurface conditions prohibit planting in the ground. However, tree growth is strictly limited by the size of the raised bed. In this constrained situation, smaller stature trees should be considered. Raised tree beds can also provide seating if the height is between 16" and 2’, with 20" being the preferred height. They can also be used to define spaces and provide a sense of enclosure in plazas and other open sidewalk areas.



Tree Pits


Tree pits are used where space or resources do not permit the use of open or covered tree trenches. The tree pit should be made as large as possible to provide maximum rooting volume while maintaining the appropriate clear width for the Pedestrian Zone. The sides of the pit below the sidewalk should be open to the surrounding subgrade to allow for root penetration beyond the pit.

Tree grates require maintenance to adjust for tree growth and to correct for any settlement that may cause a tripping hazard. Tree grates are considered an enhanced treatment and will require a maintenance agreements.