PDDG Chapter 13 - Landscape Design

Chapter 13 of the MassDOT Project Development and Design Guide

13.1 Introduction

This includes an overview of landscape planning and design, an integral part of transportation project development. This includes guidance on landscape objectives, corridor considerations, protection and preservation techniques, landscape design, and plant material section as well as the project documentation typically required for landscape elements.

13.2 Landscape Design Objectives

The goal of landscape design is to integrate the newly constructed site into the surrounding context. This goal is guided by three objectives: (1) protection of natural and cultural resources, (2) restoration and rehabilitation of landscapes damaged or compromised by transportation improvements, and (3) enhancement of the corridor such that it becomes not merely a functional facility, but a community asset. Achieving these objectives involves evaluating the features of the specific site and of the corridor as a whole to determine project impacts and how to best integrate the facility into the larger community and environmental context.

Protection and Preservation

Some features, such as wetlands, waterways, historic landmarks and districts, and parks, are protected by regulation. Yet there may be locally significant elements – pathways, architectural details, monuments, groves of trees – that may not meet regulatory thresholds but support adjacent ecosystems or define the character of a village or town.

Plants, especially trees, provide cooling and shading, air filtration, erosion control and slope stability, storm water retention, and natural habitat. They also enhance the visual quality of the roadway corridor. Project planning and design should seek to preserve and protect existing plants, important cultural features, and ecosystems that make up the corridor.

Restoration and Rehabilitation

When impacts to the landscape are unavoidable, project design should, to the extent feasible, seek to restore or repair landscapes damaged by construction. For natural systems, the intent of such restoration is not merely to replace lost elements, but to replace specific lost functions. This may mean restoring habitat, water buffers, edge systems, hydrologic connections, and soil functions. Streetscape restoration, like ecological restoration, involves more than simply replacing individual elements that have been lost. Therefore, the overall character of the street context must be evaluated in project planning and restored to the extent feasible.

Enhancement

Often landscaping is part of a civic improvement, environmental improvements, or a beautification initiative that has been incorporated into a transportation project. The local community is typically involved in design decisions for these projects and is committed to the future care of the streetscape elements. Project enhancements may include sidewalk improvements, furnishings, lighting, pollinator gardens, rain gardens, and street trees. New elements should be properly integrated with and linked to existing streetscape elements and should contribute to creating a sense of place and town identity.

13.3 Corridor Considerations

There are many requirements and considerations involved in safely accommodating the multiple uses of roadway corridors while respecting environmental conditions and meeting public expectations.

Public Expectations

The roadway landscape is the interface between the functional area of a road and the community through which it passes. Project planning and design must address a broad constituency with sometimes competing interests. For example, vehicles, including bicycles, require certain minimum widths for travel; neighborhood residents may desire street trees; and pedestrian accommodation requires minimum vertical as well as horizontal clearances. The design must recognize, evaluate, and balance competing interests.

Safety

The overriding spatial design consideration for roadside landscaping is the safety of the users.  The principal considerations are:

• Sight Lines must be maintained near intersections and driveways. No obstructions, such as plant material, signs, lights, signals, and other furnishings should block sight lines. Chapter 3 provides guidance as to the necessary sight distances.
• Clear Zones provide a recovery area adjacent to the roadway. Trees that will attain trunk diameters of 4 inches (measured 2 feet above ground) or more at maturity should not be located within the clear zone. See Chapter 5 for specific clear zone guidance.
• Buffers provide separation between higher speed vehicular traffic and pedestrian traffic. They serve to increase the comfort and safety of pedestrians walking on sidewalks. Measures, including landscape, to provide buffers and improve walkability are discussed in Chapter 5 and Chapter 16.

Utilities

Roadsides often serve as utility corridors. Underground and overhead wires and conduits are among the most common features. However, electric boxes, manholes, fire hydrants, lighting, and other fixtures must also be evaluated for clearance and access requirements. Underground utilities may require setbacks for the purpose of access and maintenance, although excavation of underground service is typically infrequent.

With the exception of the occasional high voltage utility corridor, overhead clearances are primarily a concern in urban and other developed areas. Utility poles, wires, and streetlights present special challenges for landscape design.

Roadway Maintenance

Landscape design must anticipate the type of maintenance that will be required. Responsibility for maintenance varies depending on roadway jurisdiction. The Design to Accommodate Roadway Maintenance section provides design guidance to accommodate maintenance for urban/suburban corridors and naturalized areas typically maintained by MassDOT. Maintenance for streetscape areas, as discussed in the Urban/Suburban Corridors section, is typically the responsibility of local municipalities.

13.4 Landscape Design General Guidance

This section provides general design guidance. Design decisions are ultimately based on specific site conditions and may, in certain instances, require different solutions than those recommended. For this reason, this section classifies roadside landscapes into naturalized context and urban/suburban. Projects in naturalized contexts typically include interstates, bridges (especially stream crossings), and trail/shared use paths. Projects in urban/suburban contexts include urban arterial improvements, including streetscapes, and roundabouts.

Plant Protection and Preservation

During initial project development, existing vegetation should be assessed as to whether it should be preserved and if so, what steps should be taken to protect it.

Not all vegetation should be preserved. Diseased or damaged trees that may be a public risk should be removed and, if feasible, replaced with appropriate planting.

 

Plant Preservation for Naturalized Areas

Forested areas along the highway play a critical role in the functions of the larger ecosystem and provide a perceptual buffer between residential areas and the highway.  However, other landscape types provide benefit.  For instance, a dense stand of native grass not only serves a highly important function in terms of habitat and erosion control, but such a stand takes considerable time to become established, and therefore should be protected. Vegetation should be considered as to how it fits into the larger landscape and larger ecosystem: groups of trees or vegetation are often important parts of a larger forest or vegetated corridor.

At the same time, invasive plants along the roadside can present a threat to neighboring ecosystems and to built infrastructure.  Sites should be inventoried as to the presence of invasive plants and recommendations made as to the feasibility of managing them to ensure that project improvements do not exacerbate invasive plant spread.

Plant Preservation for Urban/suburban Areas

While the urban and suburban landscapes include the range of plants, trees offer particular benefit in the urban/suburban areas, and consequently their loss by decline or removal usually has the biggest impact—both ecologically and visually. In assessing whether a tree should be preserved or removed, it is important to understand the basic structure of trees and how they function, as illustrated in Figure 13‑1.

Trees

Understanding the Basic Structure of a Tree

Roots

Trees get their water, oxygen, and most mineral nutrients through their root systems. Roots are therefore critical to the health of the tree. Roots also anchor the tree, preventing it from toppling. Most tree preservation should focus on protecting the roots of the tree.

Tree roots typically grow along the top 18 inches of soil and extend well beyond the canopy of the tree. They tend to grow along the surface of the soil because that is where they have access to oxygen and water.

In order for oxygen and water to penetrate the soil and for roots to be able to grow and obtain oxygen, water and nutrients, the soil must have good structure. Good soil structure is characterized by the amount and size of pore spaces created by soil particles and organic matter. These pore spaces allow water, air and microbes to penetrate and move through the soil. Vehicular traffic, pedestrian traffic, tillage (especially when wet) and the storage of heavy equipment will damage the soil structure, compacting the pore spaces and damaging tree roots. Compacted soils - because they deprive root systems of water, air and nutrients - can threaten mature trees and make it difficult to establish newly planted and young trees.

Trunk/Stem

The trunk of a tree serves two functions: it provides the means of transporting mineral nutrients, sugars and water between the roots and the canopy of the tree, and provides the structural support to hold the tree. The bark of the trunk protects the transport and storage cells. Protecting the bark and the trunk is important for protecting the means of transporting nutrients and water so that the tree can grow and survive. Wounds to the bark, the trunk, or major stems that do not close properly can cause decay.

Canopy and Branches

The tree canopy, through the process of photosynthesis, converts water, CO2, and nutrients into energy to support the growth of the tree. Significant damage to the canopy can compromise this conversion process. Substantial loss of limbs, either through damage, pests, or poor pruning practices, can also leave the tree prone to decay and failure.

Figure 13-1: Tree Structure and Functions

Source: MassDOT

Assessing Proposed Construction Impacts on Trees

For many projects, but especially those in urban and suburban contexts, certified arborist should be consulted during the design phase and as necessary during construction.

Proposed construction impacts should be evaluated as to how they will affect the long-term health of the surrounding vegetation, particularly with regard to trees as they have the most extensive root systems. The following construction impacts should be avoided:

• Changes in grade - As little as 2 inches of soil placed on top of the root zone can smother roots by preventing the flow of water and oxygen into the soil. Removing soil may damage and expose root systems.
• Damage to a significant portion of the root system - If more than one-third of roots will be removed or otherwise significantly impacted, the tree should be considered for removal.
• Soil compaction - Vehicle and excessive foot traffic, as well as equipment or materials storage, will damage soil by compacting it and closing soil pore spaces. This prevents water and air from moving through the soil, destroying roots and vital soil microbes. Protective measures should be taken to protect soil during construction. Storing equipment or material over root zone areas can cause moisture or aeration deprivation.
• Damage close to tree trunk - Removal or damage to the large roots close to the base of the trunk will have a significant impact on the tree. Damage to the trunk will interfere with the flow of nutrients and leave the tree susceptible to disease and decay.
• Excessive pruning - Loss of too much tree canopy will interfere with photosynthesis and nutrient uptake, weakening the tree. Excessive loss of canopy also reduces the visual quality of trees and lessens its value in terms of providing shade and screening.
• Excavation outside of tree root zones that alters subsurface water availability - Changes to the surrounding area may cause changes in moisture levels creating conditions that no longer are appropriate to the existing tree species.
• Increased exposure from unavoidable clearing - Trees in a forested area typically have small canopies (low live crown ratio) and few lower limbs (i.e., pines). Once surrounding vegetation is removed, they may become structurally unsound. Such trees are also not aesthetically appealing.

Assessing Individual Trees for Preservation

Where necessary a certified arborist should be consulted during the design phase and as necessary during construction.

Accurate site information is essential in determining which plants, particularly trees, can be preserved and which will require removal. Project planners and designers should take measures to anticipate impacts from both construction activities and the project itself by visiting the site during the early stages of planning. Survey information is helpful in providing plant locations, but it does not replace a site visit. For instance, survey information often does not show the diameter of the tree trunk and therefore does not necessarily show the extent of impact the tree will suffer from changes in sidewalk or street re-alignment. Survey information also does not relay information about the health, age or structure of the tree.

During the project planning process, designers should identify on plans trees that might be affected and determine whether to protect or remove and replace existing vegetation. Trees and other vegetation should be evaluated for preservation in terms of the following considerations:

• Species characteristics - All species have characteristics that allow them to adjust to change or tolerate damage to different degrees. Some species are much more tolerant of disturbance and tolerate a much wider range of conditions. Some species are short lived and are not worth putting extra effort into preserving. Consideration should be taken as to:
  • Life span
  • How well the species adapts to disturbance
  • Resistance to decay and compaction
  • Tendency to fail when exposed to wind
  • Habitat value
  • Invasiveness
  • Historic or cultural value
• Size - The larger the plant size, the larger the area of root zone requiring protection.
• Age - Older plants may be less tolerant of construction impacts and will require larger protection zones.
• Overall health of tree or community of plants - Healthy plants will withstand removal of more roots, provided there are no stabilization problems. Plants that are diseased, damaged, or have pests should be considered for removal and be replaced.
• Structure of plant - Plants that show signs of decay or potential for decay, have been improperly pruned, or street trees that are poor specimens should be considered for removal.
• Live crown ratio - This is the ratio of the length of the live crown to the height of the tree. Trees with live crown ratio greater than 50 percent are the best candidates for preservation. Tall trees with small crowns are structurally unstable. Many forest trees have low live crown ratios due to closer proximity of other trees. Forest pine trees in particularly, have low live crown ratios.
• Crown integrity - Crown should have healthy foliage, which typically signifies a healthy tree.
• Pruning history (cut leaders, topped) – Improperly pruned trees are susceptible to decay.
• Construction impacts – Construction activities may impact the trees’ ability to survive.

Determining Tree Protection Zone

If it is desirable to retain a tree, there are several guidelines to determine the tree protection zone that will be required. These limits will indicate boundaries for permanent protection measures, such as walls, as well as temporary construction measures, such as fencing. Trees should be protected to the edge of tree canopy, or 1.5 times the drip line for narrow-canopied trees. Using trunk diameter is another method that may be more appropriate: for every inch of trunk diameter at 4.5 feet above grade, allow 1 to 1.5 feet of space from the trunk. This method is not dependent on the tree canopy, which may be small or irregular and not reflect the root zone area. In general, a minimum of 6 feet should be protected around a tree regardless of its diameter.

Other factors (determined as part of the plant evaluation) should also be considered. For instance, older trees may be more likely to die due to root loss and should have a larger root zone protection area. Some species are more likely to fail due to root loss and these should also have larger root zone protection area. In general, to determine tree protection zone:

• Evaluate species tolerance of the tree: good, moderate, or poor.
• Identify tree age: young, mature, or over-mature.
• Estimate distance from trunk that should be protected based on canopy size or trunk diameter.

Design Modification Measures to Preserve & Protect Trees

If a tree is to be preserved, design measures should focus on preventing injury to trees, particularly to the root zones, during construction and after construction. Design strategies include:

• Shifting sidewalk or roadway geometry or reducing cross section to avoid the tree.
• Narrowing sidewalk to minimum accessibility thresholds in immediate vicinity of existing trees.
• Adjusting sidewalk profile (or bridging portion of sidewalk within root zone) if it can be done without compromising accessibility, to avoid excavation.
• Using retaining walls and/or steepened slopes to minimize cuts or fills over existing root systems. For example, one can steepen the grade as it gets closer to the tree root zone area. Not only does this reduce the impact to the tree, but will also reduce the amount of cutting or filling in that area and reduce associated costs.

Measures to Protect Trees from Construction Activities

The following measures can be specified to protect plants during construction and staging, as illustrated in Figure 13‑2:

• Fencing should protect as much of the root zone area as possible. No activities should be allowed within this zone including no vehicular or foot traffic and no storing of machinery or supplies.
• Include items for arborist services, pre-emptive air excavation and root pruning, irrigation of disturbed roots, trunk armoring, and mulching for root zones as necessary.
• Include wood chips, plywood, or other soil protection method where storage or foot traffic cannot be avoided.
• Provide items to bridge the root zone area when it cannot be fenced off. Plywood or steel plates supported by railroad ties or on top of wood chip mulch to cover the root zone area can be used so that the weight of a vehicle or equipment is distributed over the sheeting, not the ground.

Figure 13-2: Tree Protection

Source: MassDOT

Restoration and Enhancement

Restoration and Enhancement in Naturalized Areas

The principal considerations governing plant design in natural sites are clear zones and sight lines, maintenance considerations, frequently dry and windy microclimate conditions, and relatively poor soil conditions along the roadside. Typical landscape conditions at natural sites are illustrated in Figure 13‑4.

Planting in Clear Zones

MassDOT does not recommend woody planting in clear zones. Native grasses and native seeding may be options for clear zone areas that are infrequently mowed. However, areas that are mowed more than once/year or more should be planted with turfgrass, or a blend of turfgrass with native warm-season grasses.

Maintaining Sight Lines

Sight distance requirements on curves, and at interchanges and intersections limit the placement, height, and spread of plant material used. Sight distance requirements will depend on design speed, road geometry, and traffic controls. As with clear zones, low shrubs are technically feasible, but over time larger trees will have to be removed from shrub beds. Shrubs and perennials must be low growing. Larger trees should be set back to prevent branch encroachment and prevent blocking of traffic signs and signals, particularly for interstates and heavily traveled arterials.

Figure 13-3: Typical Landscape Areas at Natural Sites

Source: MassDOT

Design to Accommodate Roadway Maintenance

Successful integration of the road into the natural landscape requires anticipating typical maintenance practices. General planting guidance for maintenance of natural sites includes:

• Clear zones are typically mowed 1-2 times per year, while 8- to 10 foot strips adjacent to the pavement may be cut more frequently. Similarly, roadside swales may be mowed 1-2 times per year. Grasses provide the typical groundcover.  Native seeding that includes flowering species are ideal where mowing is limited to once per year or less.
• In general, in natural areas, avoid planting woody material within 10-15 feet of a pavement edge. This area is typically mowed more frequently and may be used for snow storage.
• Embankments protected by guardrail typically do not require regular mowing except as necessary to keep woody vegetation from encroaching on the road.
• Plantings along the roadside must be able to tolerate salt runoff and salt spray from de-icing operations.
• Locate large trees away from the road and use appropriate shrub and understory edge species in areas that are closer to the roadway.
• Avoid locating evergreen trees on the south side of east-west tangents to reduce potential roadway icing due to shade.
• Plant in groups to facilitate mowing and use perimeter mulching to help suppress weed growth during establishment.
• Reduce mowing areas by planting or allowing naturally occurring woody species to grow in expansive open areas.
• Recycled pavement (“pavement milling mulch") is often used to suppress vegetation under and adjacent to guardrail. Extending depth of this material (i.e. 6 inches or more) can substantially improve effectiveness of weed suppression.

Buffers and Screening

Landscaping can provide a buffer between the road and adjacent land. Frequently this buffer is intended as a visual and aesthetic screen, either of the highway itself, or of undesirable views from the highway. Earthwork landforms and berms can provide sound attenuation and add variety and shape to the landscape. Technically, trees and shrubs offer very little sound attenuation; however, because they can provide a visual buffer they can mitigate the overall perception of traffic.

When designing vegetative screening, use a diverse palette of plant material rather than a continuous wall of one species and size. Where space is adequate, provide a mix of evergreens and deciduous trees and shrubs, creating a mixed woodland strip.

Plantings can provide several types of buffering benefits, including the following:

• Habitat Edge/Buffer - In natural sites, a vegetated roadside edge provides a buffer for forests and other habitat areas. Plantings provide edge cover, screen wind and snow, and filter air and trash from roadway traffic. The vegetated edge also provides habitat for edge species and food and cover for wildlife movement along the roadway corridor, as further discussed in Chapter 14.
• Water Resource Protection - Plants are a critical part of water protection. They function as a physical barrier, protecting water from litter and other pollutants as well as slowing the flow of water to allow it to gradually filter through the soil, preventing erosion and flooding. Plants immediately along the water edge shade and cool waterways, protecting the habitat of fish and other wildlife.
• Shading and Screening - Trees on a rural roadway provide shade and reduce glare, lessening driver fatigue. Shrubs in the medians help screen headlights of opposing traffic. Trees can also be used to screen undesirable roadside views such as landfills, scrap yards, and other industrial uses. Evergreens provide winter screening.
• Wind and Snow Breaks - Trees can prevent snowdrift and serve as windbreaks. Evergreens and dense deciduous shrubs are best for this use.
• Aesthetic Interest - Seeding native grasses and wildflowers along with a limited mow program provides pollinator habitat and allows for more interesting and attractive views of clear zones and open areas.

Medians in Natural Sites

In addition to the safety requirements of preserving clear zones and sight distances, medians in natural sites have many of the same environmental constraints as those in urban areas. However, they offer some additional design considerations.

• Shrub plantings in the medians provide visual interest and screen headlight glare from opposing traffic. They also help reduce snow drifting.
• Rural medians provide an opportunity for storm water storage, conveyance and filtration. Plant selection and location should anticipate these functions. The MassHighway Stormwater Handbook provides more information on these topics.

Steep Slope Conditions

Steeper slopes tend to be dryer due to increased run-off and (if south facing) increased solar exposure. Planting recommendations, including maintenance considerations, include:

• Native grasses provide excellent long-term stabilization once established.
• Biodegradable erosion control such as compost blankets and jute mesh are recommended to stabilize soil during grass establishment.
• Specify slope plant material in small sizes to facilitate transplanting.
• Plant in close groups to decrease wind damage to plants and to enhance root shading.
• South facing slopes require heat and drought tolerant species. North facing slopes require species that tolerate some shade and a cooler microclimate.

Reinforced Slopes

Biotechnical stabilization integrates plant material with layered geogrid reinforced systems, riprap, and/or gabion systems. This method provides the benefits of bioengineered vegetative systems with the more predictable benefits of non-living structures and measures. The addition of deeply rooting plant material to the stabilization system can lead to longer term stability than is provided by strictly structural solutions. Further, the addition of vegetation provides ecological/habitat value, improves aesthetics, and reduces heat, glare, slope runoff amounts and rates, ice buildup and potential movement of structural elements over time.

To incorporate plants with geogrid systems, the facing of these slopes will require wrapping the slope face with rolled erosion control materials to ensure surface stability during plant establishment. Plants must be sufficiently drought and heat tolerant to withstand the steep slope conditions. In a similar fashion, gabion terraces may be inter-planted with seedlings or dormant plant material. A diverse palette of rapidly growing cool season grass species mixed with durable native (warm season) grasses and wildflowers species, and rooted and dormant tree, shrub and vine species should be considered for any slope reinforcement site. The solar orientation (slope aspect) should guide the selection of appropriate plant species.

Where placed stone or riprap is used for steep slopes or bridge abutments, loam pockets and tube plantings can provide vegetative cover and wildlife habitat. When planting in pockets, potential dislodgement of plants and soil can be avoided through various preventative measures. Techniques can include tying woody stems to embedded rocks or other structural members and wrapping soil surface with sturdy, biodegradable erosion fabric.

Bioengineering and Stream Bank Stabilization

Bioengineering is the combined use of plant material and various natural and manufactured structural materials to stabilize, restore and/or reconstruct slopes, stream banks and shorelines. Because bioengineering seeks to mimic natural slope stabilization processes, rocks, boulders, logs, and dead brush may be used in combination with plantings. Existing soils may be enhanced with placed topsoil, marsh base and/or compost or leaf mulch. Biodegradable fabric is frequently used to hold soil in place prior to installation of plants. The term bioengineering covers a broad range of stabilization techniques, and therefore, allows for more site- specific treatment than strictly structural/conventional solutions. In some instances, bioengineering can provide more long-term stability than conventional solutions alone.

As plants establish, the slopes become increasingly stabilized. However, there are some factors which must be considered:

• Bioengineering depends on the performance of living material, which is difficult to predict, particularly during the initial establishment stage.
• Availability of water for irrigation, particularly in upland sites, may be limited. 
• Proper stream bank repair requires an understanding of the specific stream type and stage of morphology. Additional information is available from the MassDOT Rivers and Roads Program
• Bioengineering for stream stabilization is generally limited to bank slopes of 2:1 or flatter and has varying effectiveness depending on the morphology of the stream.
• Because they are constructed of living material, bioengineered systems must be monitored and maintained for at least a year or more following installation to ensure proper establishment of desired plant material and the control of invasive plant species.

Specifications on stream bank stabilization in the vicinity of bridge abutments and other critical slopes is provided by FHWA Highway Engineering Circular 23 (HEC 23 Bridge Scour and Stream Instability Countermeasures).

Stormwater Management Facilities

Consistent with the goal of preservation, roadside design should pursue stormwater management strategies that minimize the disturbance of existing vegetation and encourage healthy vegetation to manage water naturally. Standards for water quality may still require the construction of basins and other structures specific to managing treating stormwater. See Chapter 8 for more information.

Constructed basins and channels designed to improve infiltration of runoff substantially re-shape the roadside landscape, changing how it is maintained, and therefore how it looks and functions. Basins should be designed to minimize disturbance to existing vegetation and topography.  The basin grading should be shaped to ensure access for maintenance, ease of mowing as required, and should blend into the surrounding topography. Configure basins to allow for appropriate screening and buffering of adjacent residential areas. Avoid locating woody plants where sediment will accumulate, or on constructed retaining berms where planting may compromise the structure. Woody plants are not recommended in the basin area or on constructed embankments holding water.

Wetland Mitigation

The MassDOT Wetland Design Guidance should be followed along with relevant regulatory literature regarding the performance objectives of wetland mitigation work.

Integrating Planting with Highway Structures

Designers should consider the following general guidance when integrating planting with highway structures such as bridges:

• Plants should be selected for restoration purpose or to visually blend the bridge into its environment in urban areas.
• Plant locations should allow access for periodic bridge inspection.
• Locate shade trees to avoid branch encroachment over bridge deck and locate plant material away from walls to allow for snow storage.
• For bridges over waterways, landscaping may also be used to restore areas used for temporary bridges and/or staging areas.
• Slope stabilization, including protection against scour, should be installed to look as natural as possible. This can be achieved by tapering armored stone aprons from bridge abutments out toward the bank so that they blend into the existing stream bank, rather than constructing abrupt edges that are perpendicular to the contour of the bank. Bio-stabilization and compost blanket surfacing can further blend the armored surface with the natural riverbank form and habitat.
• Where existing patterns dictate, wildlife passages may be warranted, and surface treatment and landscape materials should integrate these passages with existing pathways, as described in Chapter 14.

Planting for Interchanges

Designers should consider the following general guidance for plantings near interchanges:

• These areas are frequently used as construction staging areas. Check with the Districts to evaluate whether interchange should remain open.
• Native seeding is ideal for revegetation of interchanges. Seed mix should be selected with respect to expected or planned vegetation management, which should be coordinated with the Districts.
• Planting in interchange open space (outside of sight line and clear zone limits) can provide screening of headlights and reduce snow drifting.
• Locate plant material so that branches and stems do not obscure sight distances to and from ramps.

Noise Walls

Noise walls are installed where traffic noise exceeds or is expected to exceed established threshold levels. The introduction of these structures along the roadway can substantially depreciate the visual quality of the roadway corridor and have mixed impacts on abutting land uses. Noise walls can particularly exacerbate the climatic stresses on roadside plants since the flat surfaces can substantially increase reflected solar gain, desiccating winds, and snow throw and salt spray accumulation.

Sound attenuation specifications control the materials, proportion and geometry of sound walls. Materials and installation costs further limit the alternatives available for the design. Aesthetic treatment lies in mitigating the effects of the large surface area, as described below.

• Where other factors do not prevail, the wall should be sited such that it blends into the terrain and with existing vegetation.
• Architectural design that emphasizes and articulates posts can break up the surface of the wall and provide the appearance of a more decorative wall.
• Modular construction that breaks the surface into smaller units can relieve the monotony of continuous wall surfaces.
• Using natural materials or simulated natural materials provides visual relief.
• Composite masonry units can be molded into a variety of forms that mimic stone masonry walls. However, where the reveal of the “grout” lines is shallow, shadow lines can disappear and the visual effect is lost to views from the road.
• As setback from the highway increases, the opportunity for providing a sustainable planted roadside buffer also increases. Walls may also be constructed so as to weave through existing vegetation. Walls with more vegetation on the highway side are less susceptible to graffiti, however, the large scale of sound walls presents an imposing structure near residential areas.

Planting and plant species appropriate for walls within 20 feet of the traveled way will be limited on the roadway side. Unless plant care beyond the contract warranty is available, plantings are not recommended.

Restoration and Enhancement in Urban/Suburban Corridors

Many of the roadway projects in Massachusetts pass through densely settled areas. These “streetscapes” are primarily pedestrian environments. This section focuses on the pedestrian landscape bound by the limits of the right-of-way and comprised of sidewalks and crosswalks. It may include decorative pavement, street trees, decorative lighting, and, where appropriate, other furnishings such as bicycle racks, benches, and trash receptacles.

Locating Street Trees

Street trees provide substantial community and environmental benefits. The desire for and feasibility of planting trees as part of a transportation project should be identified in the early stages of project development and should be in accordance with any municipal masterplans or restrictions.

Street tree plantings are frequently provided as part of state highway projects, but maintenance often becomes the responsibility of local municipalities. For this reason, street tree planting design must consider not only conventional traffic issues of sight lines and clear zones, but also the maintenance requirements of plant material. For plantings on municipal properties, the municipality is expected to sign a letter of agreement that accompanies the MassDOT Plant Care & Management Recommendations.

Historically, trees have often been planted in strips too narrow and pits too small to adequately support long-term growth. The designer should not propose to replant in these existing sites without considering whether the soil volume, soil quality, microclimate, moisture levels and other site considerations are appropriate for the species selected, as described below.

Back-of-Curb Planting

Locating trees along the traffic side of sidewalks provides a buffer between the sidewalk and the street and may provide a visual traffic-calming cue to drivers. This location is referred to as back-of-curb. The planning and development of streetscapes should aim to provide sufficient area for tree planting, while accommodating pedestrian and utility demands.

Historically, trees have often been planted in strips too narrow and pits too small to adequately support long- term growth. In general, planting strips less than 3.5 feet wide are too narrow for typical street trees species. These strips do not contain sufficient soil volume for most species, do not provide sufficient area to protect tree trunks and roots from pedestrians, bicycles, parked cars, and snow storage, and, due to roots seeking soil outside the planting area, do not provide optimum long-term conditions for sidewalks.

In addition, there is often insufficient area left for tree planting behind the curb due to roadway expansion on one side, sidewalks/path expansion on the other side, and constraints from overhead and underground utilities.

Design recommendations include:

• Different species have different soil volume and moisture requirements. Choose species most likely to survive in the microclimate, site and soil conditions of the planting area.
• Shared root space provides a benefit to trees. Link tree pits or tree lawns to create spaces with greater soil area and volume.
• As recommended by urban forestry practices, aim for a buffer of at least 6 feet, preferably more.
• Ensure that municipalities will water trees where there is insufficient soil volume to provide the moisture that the trees will require.
• While narrow tree strips may be viable, municipalities should expect diminished tree growth and health, shorter plant life spans, and increased maintenance.

Back-of-Sidewalk Planting

Where sidewalk conditions (including accessible pathway requirements) limit the feasibility of back-of-curb planting, and where abutting land use allows, current urban forestry practices recommend back-of-sidewalk planting, opposite the roadway. Under these circumstances, the design process should include agreements with adjacent owners, especially where installation will encroach on private property.

Where right-of-way limits fall close to or on the back of the sidewalk, a portion of the sidewalk and sub-grade may be cut out to accommodate the root ball, provided there is adequate passage between the back of the curb and the back of the sidewalk, as shown in Figure 13‑4.

Figure 13-4: Back of Sidewalk Tree Planting

Source: MassDOT

Where possible, these locations are likely to offer a larger planting area for trees, and, while planting in back of the sidewalk does not provide a physical buffer between the sidewalk and the street and may change the conventional image of the tree-lined street, there are several significant benefits:

• Trees planted in areas with larger volumes of soil where possible, these locations are likely to offer a larger planting area for trees, and are more likely to survive and grow faster than trees in individual tree pits or strips because the roots have greater access to moisture and nutrients.
• Trees on back of sidewalk are less likely to lift sidewalk pavement, as the roots will grow toward open soil and sources of water.
• Conflicts with overhead utilities along the sidewalk are avoided.
• Trees are less likely to be injured by vehicles and car doors.
• Trees will be less likely to obscure traffic signs and lights.
• Setback plantings are out of the way of future street repair such as curb resetting or installation.

Other Street Planting Strategies

Where sidewalk space is constrained and back-of-sidewalk planting is not possible, other considerations include:

• Locate trees where conditions are more favorable.
• Design subsurface planting pits that extend beneath and provide structural support to sidewalks, allowing sufficient room for root growth.
• Expand sidewalk width at tree planting pits using porous flexible pavement (rather than using tree grates).

General Location and Design Guidelines

• The outside face of the tree trunk at its estimated mature size should be at least 1.5 feet behind face of curb.
• Locate trees and select species such that the canopy will not block signs, signals, or street lights.
• Locate trees so that trunks do not obscure sight lines at intersections and curb cuts.
• Where there is on-street parking, coordinate tree locations with parking stalls to minimize conflicts with car and truck doors.
• In commercial areas, allocate space for bicycle parking to deter people from locking bikes to trees.
• Avoid planting under utility wires. Otherwise, choose appropriate species/cultivars: low growing, columnar, or finely branched trees that can be pruned without causing major damage to the tree.
• Locate trees away from hydrants, electric boxes, lights, and other utility structures that may need to be accessed.
• Identify subsurface utilities. In general, most tree roots are found in the top 18 to 24 inches of soil. However, excavation for sidewalks, root balls, backfill and drainage must be considered. Tree roots do not generally migrate toward underground lines, however, where soil surface area and volume is limited, roots may follow moisture and air along drainage and sewer lines underneath streets and sidewalks.

Vertical Clearance

Vertical clearance requirements may dictate species selection and/or selective pruning of trees upon or after installation.

• Sidewalks typically require a 6.5-foot vertical clearance
• Sidepaths require a minimum of 8-foot vertical clearance (refer to Chapter 11 for detail)
• Vertical considerations for roadways include 8-foot clearances for bicycle traffic and as much as 14-foot of clearance to avoid interference with buses. Pruning after installation may accommodate these conditions.

Median and Island Plantings in Urban Areas

Occasionally communities desire median or island plantings to enhance the visual quality of the roadway corridor. These locations are extremely challenging, particularly in high volume locations. The environmental stresses and maintenance demands are, along with safety considerations, greater than on the roadside. Clear commitment from the local community to maintain plantings is essential. Median areas are highly visible and highly susceptible to trash build up and thus particularly challenging for maintenance. Weeds must be regularly removed, plants watered, grass mowed, branches pruned, and dead plants replaced. Planting should be selected to avoid the emergence of weeds, minimizing the need for frequent manual weeding or herbicide application.

General guidelines include:

• Median plantings in urban areas should be limited to areas where design speeds are less than 45 mph and where plants will not present a hazard or obscure sight distances or traffic signs and signals.
• From a horticultural standpoint, soil cross section for planted medians should be at least 10 feet for trees, 6 feet for shrubs.
• Selected trees should be upright, narrow, and high branching, in order to minimize interference with vehicles and pedestrians. Smaller tree species, including multi-stem trees tend to perform better than shade trees, but must not obstruct sight lines.
• Typically, the frequency of curb cuts and intersections will require maintaining sight lines across most, if not all, of the median, limiting shrubs and perennials to plants less than 3 feet high, including curb and earthwork height.
• Plants should be located as far from the curb as possible to prevent injury from salt, sand and snow.
• Where snow storage or traffic conditions may result in back-of-curb erosion, provide an 18 to 24-inch wide paved apron behind the curb.

For landscaping guidance specific to roundabouts, refer to Section 5.7 of the MassDOT Guidelines for the Planning and Design of Roundabouts.

Alternative Pavement Surfaces for Streetscapes and Stormwater

Decorative pavements are often desired to reflect architectural materials and details in historic districts, to beautify significant downtown locations, or to highlight pedestrian zones. In general, the designer should consider textured pavement and color admixtures.

Modular pavements, used for enhancement or for stormwater capture, should be limited to non-traversable areas, such as the buffer zone, or “furniture zone” between the pathway portion of sidewalk and the street, or for non-traversed portions of medians.  Apply unit paving only where pavement line is continuous. Minimize breaks requiring cutting and hand fitting of units.  Anticipate utility fixtures and develop templates to fit whole units around fixtures without gaps.

Modular pavement can be expensive to construct and presents challenges to maintain. Its application should be limited to projects that have explicitly identified the enhanced treatment as a priority and where there is local municipal commitment to maintenance, including proper repairs when pavements are disturbed or damaged.

In addition to modular pavements, there are flexible pavement materials that can offer stormwater benefits as well as an alternative surface color.  Cost is a consideration, and regular maintenance to remove sediments is necessary for stormwater capture.

Note that any consideration of pavements for stormwater capture must anticipate how the water will be drained from beneath the pavement.

Lighting

Constructability and maintenance requirements for roadways, as well as cost, may limit the types of lighting fictures used. Communities are typically responsible for additional maintenance and installation costs of non-standard fixtures and lights. Aesthetic considerations include the detail and finishes of the posts and lamp housings, the post height, brackets, and the color quality of the light. The following general guidelines apply to the installation of street lighting, which is also discussed in Chapter 16:

• Top-mounted decorative lights can be obscured by adjacent street trees.
• It is advisable for light fixtures to cast the light downward to avoid obscuring the night sky and contributing to light pollution.
• Shorter lights are more to human scale, but provide a reduced spread of light. To provide adequate lighting, they must be installed at closer intervals. This increases installation, operation, and maintenance costs.
• In determining compatibility of illumination and planting, consider pole heights, light cones, the shape of the tree, and the horizontal location of poles relative to tree trunks.

Bicycle Racks

Bicycle racks are an integral part of accommodating bicycle transportation. Bicycle racks should be located where they are convenient to the users and where they will not interfere with pedestrian and vehicular traffic. Providing bicycle racks helps discourage users from locking bikes to railings, street trees, and which other furnishings. General guidelines are presented below and additional references are provided in Section 13.7:

• The number of spaces should anticipate volume trail use but also be sensitive to site constraints.
• Placement of racks, and associated surface area must also take into account the anticipated use, as well as the variety of bicycle types anticipated.
• Typical inverted “U” or post and ring systems are preferred, but any rack that meets the following criteria may be used:
  • Rack must enable the frame and one or both wheels to be secured, preventing the bike from tipping over.
  • The rack should be anchored so that it cannot be stolen with bikes attached.
  • For parallel storage, arrange rack elements 30 inches on center to allow space for two bicycles to be secured to each rack element.

Benches

Benches not only enhance the livability of public spaces, but because they also provide respite, they potentially expand the range of people who will make use of paths and sidewalks. When appropriately sited and properly constructed, benches create more attractive and comfortable human environments. Their design and construction should be appropriate to the character of the site. Backless benches allow two viewing perspectives. At the same time, bench backs and arms improve accessibility.  Additional arm rests discourage sleeping on benches, which may not be desirable. Benches should be located where people are likely to sit to wait, people-watch, rest, or look out at scenic views. They should not block pedestrian movement or impede wheelchair access. Bench placement should provide companion spaces for wheelchairs. Avoid placing benches where they will not be used.

Trash Receptacles

Municipalities desiring receptacles must carefully consider resources for maintenance, as well as ease of access, to determine if receptacles should be provided, how many, and where they should be located.  A period of monitoring may be necessary to determine the appropriate number of receptacles needed. In addition, receptacle design should consider capacity needs, security, weight, and vandalism, as well as lids to limit litter and rodent problems. Avoid locating trash receptacles adjacent to benches and other areas where people sit.

Information Signs

Information signs are necessary to guide people unfamiliar with the local area. Local zoning regulations are valuable in controlling the appearance, size and location of signs. Signs should be visible and conveniently located but should not interfere with vehicular or pedestrian circulation. Less signage is better than excessive signage, which creates clutter and tends to be ignored. Signage should be consistent with municipal formats where relevant.

13.5 Planting Material Selection

Selecting the appropriate species is an essential part of creating a successful roadside landscape. Plants should be selected based not only on how they will thrive in that particular site, but also on how they may interact with and impact surrounding ecological systems. Good plant selection will create sustainable plant communities and transition areas, benefiting human and ecological systems. Poor selection can lead to plant failure which can result in soil loss, compromised highway structures, and the encroachment of invasive species, causing long-term negative impacts to surrounding as well as distant ecological systems.

Roadside conditions are typically harsh, limiting the palette of plants for landscape restoration and streetscapes. The following conditions should be analyzed to guide location and selection of species:

• Climate and microclimate of the roadside vary from site to site and within sites. Slope angle and orientation will dramatically affect ground temperatures and water demands of plant material. Desiccating winds increase with closer proximity to the roadway. Keep in mind changes that new construction will bring.
• Existing vegetation should be evaluated with regard to plant communities, invasive plant species, native species, and the function that that vegetation serves (wetland buffer, screening, habitat area, erosion control, etc.). Indirect construction impacts on existing vegetation, such as changes in light or exposure, should also be addressed.
• Existing soils and proposed fills must be assessed as to drainage characteristics, compaction potential, organic content and fertility, as well as potential exposure to salt run-off and other soil contaminates.
• Existing hydrology will determine erosion considerations and species choices. Wetland areas will require additional protective measures, as described in Chapter 8.
• Existing habitat should be assessed prior to choosing species for restoration. Habitat preservation and restoration should avoid inadvertently attracting species closer to roadway. Habitat considerations along roadways are further discussed in Chapter 14.

General Plant Material Selection Guidelines

The following aspects should be considered when selecting species for planting:

• Hardiness - Plants should be hardy to the region in which they will be planted. Roadsides have extremes of microclimates: south facing slopes get full sun and heat, north facing slopes are cool and shaded, areas close to the road have high wind conditions and salt spray. In urban environments, asphalt, building masses, automobiles, and reflective surfaces generate radiant heat, creating a microclimate hotter and drier than the general climate.
• Existing Plant Species and Communities - Surrounding vegetation should be assessed to determine what species are doing well in the site’s general soil and climate conditions. Determine whether there are invasive species in the surrounding area that may out-compete new plantings. Surrounding vegetation can also be used as a clue to pH and moisture levels when selecting new species.
• Soil Quality - Soil should be evaluated for texture, pH, density, salinity (conductivity), and organic content. Most soils close to the road are infertile, dry, compacted, and contain salt and other pollutants. New soils brought in after construction have better soil quality in terms of texture and density, but they tend to lack necessary soil organisms. The designer should select plant species appropriate to anticipated conditions.
• Diversity of Species – A diverse selection of plants, including native species that are already existing in the landscape should be used. Mixing different sizes and types of plants (i.e., shrubs, evergreens, deciduous trees, and groundcovers) helps create an interactive plant community. While the diversity opportunities are much more limited on streetscapes than in naturalized areas, diversity of species remains important. There are many benefits to choosing a variety of species, including pest or disease resistance within the population. Choosing cultivars that are more tolerant of drought and more resistant to pests and diseases is also important.
• Plant Sizes and Densities – Plant sizes (in age) should be relatively small. Quantities and spacing (particularly for reforestation type plantings) should be close due to the stresses and mortality during establishment. Varying ages prevents all trees from maturing and having to be replaced at the same time, thus maintaining general canopy cover at all times. Where conditions permit, smaller species can be planted between or in front/back of larger shade trees. The following general species selection considerations apply:
  • Do not use invasive, likely invasive, or potentially invasive species.
  • Use native species, particularly in ecologically sensitive areas, when possible.
  • Choose trees that are low maintenance (no spraying, pruning, fruit litter).
  • Choose trees that are tolerant of specific site conditions (narrow spaces, overhead wires, salt, compacted soil, low soil volume, pollution).

Native Species

Native species are species that occur in a region as result of natural forces rather than as having been brought in by humans, either intentionally or accidentally. A species may be native to a large area, such as North America, or may be native only to a small area, such as Cape Cod. The plant’s native range, or region, is typically associated with environmental factors such as climate, soils, and topography. When choosing native species, it is recommended to choose plants as locally “native” as possible.

In addition to environmental benefits, native species reflect the region’s ecological characteristics. For instance, sugar maples or New England Asters along a New England roadside give a sense of place and identity to the roadway or streetscape. In general, native species are better adapted to the native climate and soils of a region. However, roadsides and other developed or exposed areas may not have typically “native” conditions. Therefore, species should be chosen with regard to the specific site conditions.

Special attention should be paid to sensitive areas and efforts should be made to plant native species in ecologically sensitive areas along waterways, wetlands, and areas noted for rare and endangered species.

Invasive Plant Species

The Massachusetts Invasive Plant Advisory Group (MIPAG) defines invasive plants as "non-native species that have spread into native or minimally managed plant systems in Massachusetts, causing economic or environmental harm by developing self-sustaining populations and becoming dominant and/or disruptive to those systems." Many of these plants came from European settlers who brought plants with them for human and animal consumption, for medicine, to make products, and for ornamental use. Some non-native species were accidental introductions, arriving with ships, in packaging, or with cattle. Many new weed species continue to enter the United States as seed contaminants.

Executive Order 13112 on Invasive Species requires Federal agencies to the extent practical to prevent the introduction of invasive species, respond to and control populations in a cost-effective and environmentally sound manner, to monitor invasive species populations, and to restore native species and habitat conditions in ecosystems that have been invaded.

Invasive species are common along roadsides because they survive and thrive in harsh climates and poor soils. Traffic, construction, and roadside vegetation management, including mowing, also contribute to the spread of these species along the roadway corridor. Invasive species have not only spread along roadways, but have also spread into minimally managed environments and are displacing native species. Managing invasive plant species for roadside visibility, safety and general maintenance has proven expensive and difficult. While no longer planting these species will not eliminate them from the environment, it may help to contain the population. For current listing of invasive plants in Massachusetts, refer to the MIPAG website. Plants listed by MIPAG as “Invasive”, “Likely Invasive”, or “Potentially Invasive” shall all be considered invasive.

Existing vegetation should be identified and measures taken to prevent the spread of invasive plants during and after construction. 

Plant Selection for Natural Sites

Generally, natural sites involve larger expanses of land, more extensive vegetation and associated wildlife, and significant water bodies. There is a wider range of species that can be selected for these areas, however, plants located close to the roadway must be able to tolerate climate conditions of exposure, high winds, salt spray, and full sun. Unless new soil has been brought in, soils close to the road are typically infertile, very dry or fluctuating between wet and dry, and often contain salt and other contaminants. Typical of naturalized roadside environments, there is little maintenance either to get plants established or to ensure their survival once established. The following functions and restoration goals should be considered when choosing plant species:

• Erosion Prevention - Deep-rooted species, such as warm season grasses and perennials are recommended for erosion control. They hold soil in place, can absorb run-off, and help water infiltrate the soil. Quick-growing species are good for immediate erosion control.
• Water Protection - Using plants to benefit hydrologic systems is important to protect water bodies, wetlands, and aquifers. Deep rooted species in particular, including native grasses and perennials, help water infiltrate through the soil and absorb excess nutrients.
• Habitat Restoration - All plants provide habitat. However, different species offer different kinds of habitat. Shrubs offer different forms of cover and food than trees, and evergreens offer different habitat environments than deciduous plants as well as providing winter cover. Consider a variety of types of plants (trees, shrubs, groundcovers) to provide a range of food and cover throughout the year. Planting native species helps support habitat for native wildlife. Edge conditions, such as where forest meets open areas, provide cover and a protective buffer from the road.
• Pioneer Plants - The roadside conditions after construction may be significantly disturbed and altered such that native species or species established in the surrounding environment will not thrive in the newly constructed roadside environment. In such cases, imitating typical succession patterns by using a mix of the surrounding species with fast growing, pioneer species that thrive in infertile soils - such as cherries, poplars, sumac, birches, and silver maples - helps create a forest type microclimate. These pioneer species grow fast, providing shade and cover for those species that are more difficult to get established.

Grassland Restoration

Natural sites along the road, particularly those that have suffered damage due to construction projects, offer an ideal opportunity for grassland restoration, including cultivation of native species and provision of pollinator habitat. Clear zones within the roadside right-of-way may be converted to grassland, ensuring the long-term establishment of grasses and perennials. Reducing mowing and incorporating more native grasses and wildflowers provides more food and shelter along these corridors than continually mowed areas.

Native grassland plants not only benefit insects and wildlife, but they create a more visually interesting and attractive roadside throughout the seasons, helping to alleviate driver fatigue. Native plant cultivation along the roadside may be accomplished by reducing areas that are mowed and seeding. Mowing once a year allows wildflowers and grasses to set seed for the following year.

Benefits of grassland restoration include:

• Reduction in air pollution and mowing expenses due to reduced mowing (once a year).
• Flowers and seeds provide a greater source of food attracting insects and birds.
• The diversity of herbaceous species will likely increase due to a reduction in mowing. This in turn may encourage greater diversity of wildlife.
• The reduced mowing cycle will encourage the growth and dispersal of New England natives such as goldenrods, asters and milkweeds, all of which provide an important food source for native wildlife.
• Taller herbaceous material provides better cover for birds and small mammals that nest and move along the highway corridors. Mowing later in the season allows many species of grassland birds and mammals to nest and to successfully raise their young.

Seeding Native Grasses and Perennials

Roadsides are an excellent place to begin re-vegetating by seeding perennials and native warm-season grasses. Highly disturbed areas or areas with new soil provide an ideal site for establishment since there is less competition from weeds and cool season grasses.

Native warm season grasses tend to be more aesthetically attractive than cool season grasses because, unlike the cool season species whose flower stalks topple in summer, the stems and seed heads of these later-flowering grasses persist well into winter, maintaining their ornamental appeal.

Perennials and native grasses have several benefits over annuals and cool season grasses but are more difficult to get established. Unlike annuals and cool season grasses, which usually spread by rhizomes, native perennials and warm season grasses tend to grow very deep root systems, sometimes down to 12 feet deep. However, to establish the initial root system, they partition more energy to the roots, resulting in slow aboveground growth of perhaps only 2-3 inches after the first season. It is because of this deep root system that once established, perennials and warm season grasses are longer lasting and more drought tolerant. The deep roots also provide additional benefits such as:

• Erosion Prevention—the deep fibrous root systems hold the soil.
• Improved water quality—deep roots absorb more water than sod and help to prevent runoff, which might result in less fertilizers and pollutants ending up in the water supply.
• Improve soils—the greater root bio-mass provides increased organic matter in soils and more rapid infiltration rates.

Due to establishment of extensive root systems, perennials and warm season grasses can take three to five years before they flower. Native and perennial species should not be chosen if the goal of the seeding project is quick cover and/or quick color.

Coordination for Reduced Mowing

Restoration seeding will need to be on a limited mow schedule. Coordination with Districts will be required.

Plant Selection for Urban Areas

Street trees enhance the corridor visually and ecologically. They provide shade and cooling, improve air quality, manage stormwater, and provide habitat. Street trees should also be considered as part of a larger system in that they function as links for animals migrating or seeking food within a larger habitat corridor.

As discussed in the “Urban/Suburban Corridors” section, streetscapes are difficult environments to establish plants.   Multi-modal pavement demands limit the planting area, and overhead wires, utility poles, hydrants, mailboxes, and light fixtures further restrict locations for trees and the variety of trees that can be used. The streetscape climate is characterized by heat radiation from buildings and pavement, exacerbated wind tunnels, heavy shade from buildings, along with additional stresses of de-icing salts.  Additionally, urban soils, which are typically compacted and low in organic content, discourage growth of desirable plants. Soil volume in tree planting pits or trenches may be inadequate for healthy growth of most tree species.

Plant selection considerations include:

• Use of a variety of species.
• Columnar cultivars should be considered for narrow areas.
• Trees that reach mature heights of 25 to 30 feet are best for areas where there are overhead utilities. The small upper branches of these trees may be pruned, but the heavy limbs, capable of breaking wires, remain well below the wires.
• Where soil conditions and overhead clearances allow, larger shade trees that can be limbed up may be preferable to smaller trees.
• Avoid species and cultivars that drop fruit or have excessive leaf litter, species that require frequent pruning or irrigation, and species with thorns.
• Pathways for bicycles typically require higher clearance than sidewalks.

Tree planting for urban/suburban areas should be coordinated with the Town Tree Warden and with any Master Plans the municipality may have.

13.6 Plant Care

Plantings are a visual enhancement, provide natural stormwater management, climate mitigation, and support much needed pollinator and wildlife habitat. However, they require care. Prior to proposing a planting design, designers shall ask municipalities to review the MassDOT Plant Care & Management Recommendations and agree to perform the future maintenance that will be required.

13.7 Plans and Documentation

Documentation of projects is essential and additional information on project planning and design is provided in Chapters 2 and 18. Not all of the documentation recommended below is required for all projects. The type of project, its complexity, and the extent of change to the landscape will determine the extent of documentation.

From the standpoint of landscape and context analysis, photographic documentation is the most useful, simplest, and least expensive form of documentation. In addition to publicly available orthographic aerials, digital photography has made the cost of collecting and distributing images negligible.

A further benefit of digital photography is the ability to readily edit and illustrate, in either elaborately rendered or simple diagrammatic form, the type and extent of changes for a given proposal.

Preliminary Development

The following are recommended design considerations during project planning and at the preliminary design stage:

• Aerial orthophotos.
• A comprehensive site photo documentation set.
• Large or complex projects should be superimposed on corresponding aerial orthophoto.
• Additional diagrammatic information may be included on aerial photograph to annotate graphic data for better understanding of context.

25 Percent Submittals

The following activities should be completed in preparation for the 25 Percent submittal:

• Early environmental coordination.
• Overlay of project on environmental layers.
• Digital site photo documentation.
• Narrative of site changes and estimated impacts.
• Photo documentation of existing structures (i.e., bridges).
• For Design Public Hearings, plan superimposed on aerial orthophoto, including approximate limit of impacts, such as grading.
• For complex or controversial projects, visual simulation showing changes.
• For projects where landscape is integral to the purpose or is substantially affected, a narrative of impacts and proposals.
• For streetscape/enhancement projects, approximate locations and type of landscape materials, including preliminary species list.

75 and 100 Percent Submittals

The following activities should be completed in preparation for the 75 and 100 Percent submittals:

• Construction drawings showing location and specific species of plants and other features.
• Tree protection shown on construction plans. Landscape plans shall show locations with symbol tags of proposed plants.
• Include a summary plant list with botanical and common names, plant sizes and item numbers. Include a comments column indicating, (as required) height to lowest branch, cultivation (container size, b&b, bare root, cutting, etc.), or other information.
• All planting sheets shall include key showing symbols and names of plants on that sheet.
• Where different types of seeding are used for different areas, call out non-typical locations on construction plans, landscape plans, or area detail, as appropriate.
• Estimate of landscape materials including quantities and sizes.
• Special provisions as required.
• Plans and CAD files as described in Chapter 18.

13.8 For Further Information

• Landscape Design and Roadside Maintenance