General Considerations for Planting Street Trees -
by John T. Whaley, RLA and Certified Arborist

        Planting trees along streets can be visually pleasing in most urban and suburban
locations. They also provide welcoming shade, provide natural character including color
and form to an area and help to enclose or define a streetscape space by defining the edge
of the street and adding a defined scale to the area. However, there are many things that
need to be considered before planting trees along a street, especially in the narrow tree
lawns associated with cities and suburban residential development.

       This article will try to highlight some basic information that will help designers,
community planners, shade tree commissions, and home owners in deciding the best
course of action for planting trees along their streets.

General Planting Criteria:
         Space to plant trees along a street is usually very limited in width. When a tree is
to be planted between a street curb and a sidewalk (tree lawn), these physical barriers can
constrain the normal growth of the root system. The width of the tree lawn limits the size
of the tree that can be supported by the soil and water available in the tree lawn space.
The tree lawn width and type can also limit the permissible growth of branches before
they need to be pruned for pedestrian clearance (8 ft. height) or street vehicles (14 ft.

        Trees should be located as far behind the curb line as possible. If the tree lawn
width is 3 ft. or less, than it is not suitable for planting street trees. A tree lawn width of
+3-1/2 ft. to 6 ft. can accommodate a tree with a future branch spread of 25-30 ft. Larger
growing trees can eventually grow to have a trunk greater than 2 ft. in diameter. Mature
trees can cause serious and costly damage to adjacent curbs, sidewalks, and overhead or
underground utilities. The width of the tree lawn indicated below can help determine the
size of mature trees that should be used.

      Width of Tree Lawn                  Size of Mature Tree
          3 – 4 ft.                       small - usually under 25 ft. height
          4 – 6 ft.                       medium - 25 to 40 ft. height
           + 6 ft.                        large - taller than 40 ft.

          Trees should be planted only where it is possible to provide a proper soil volume
to support the trees potential growth. Small trees require at least 100 c.f. (5’x 5’x 4’
deep) of soil volume per tree while medium and larger growing trees require at least 400
c.f. of soil volume. Most feeder roots are located within the top 2 ft. of the root zone.

        Grassed tree lawns require periodic maintenance such as weed control, mowing,
edging, and tree debris removal. These maintenance operations can damage the trees in
the tree lawns when poor maintenance practices are performed by persons operating
mowers, string-line trimmers or other lawn maintenance equipment. Keeping the grass
away from the tree trunk by maintaining a large and properly mulched tree pit can help
reduce potential damage.

        Tree wells are often created in high pedestrian traffic areas. These wells are often
covered with a tree grate, gravel or bark mulch, or a hard surfaced material that will allow
water to percolate into the well area. Tree wells should be as large as possible but should
not be less than 5’x 5’ or a 25 s.f. area. If a square-shaped tree well cannot be provided,
then construct a rectangular shaped tree well approximating the desired size, i.e. 4’x 6.’
A tree well created for a young tree can also provide a place for other vegetation such as
annual flowers to be planted in order to bring additional seasonal color to the streetscape.

      Tree well placement in paved city sidewalks must provide sufficient widths to
accommodate street furniture use and provide clear pedestrian passage that includes any
Americans with Disabilities Act (ADA) provisions.

            Gingko in large tree well with        Paving bricks can allow future trunk expansion
            granite block surface cover           but rarely remain as a level surface cover

                Mulched tree pits will require periodic weeding and mulch replacement

        Tree grates (metal or plastic) are often used at tree wells for placing trees in areas
surrounded by pavement or in paved tree lawns where pedestrian traffic is expected.
They allow for water and air to enter the root zone at the tree well (pit). The grates help
to keep the tree pit surface more level to reduce the potential for tripping on uneven tree
pit surfacing material such as wood bark, stone, block, or brick. The grates also allow for
the street trees to be placed further away from the curb line and help to dress-up the tree
pit appearance and reduce soil compaction.

        Metal tree guards are often used in high pedestrian traffic areas to provide young
trees with a greater degree of trunk protection as the tree develops. The guards can be

very decorative and add interest to the streetscape but must be removed in later years as
the tree matures.

                Tree grates provide a level walking surface                   Tree guards protect
                                                                              young trees

        Street trees should be kept as far back behind the edge of curb as possible for
safety purposes and for the eventual tree trunk development. For traffic speeds over 25
mph, the street curb may be of insufficient height to prevent vehicles from jumping the
curb and colliding with the trees. The trees may need some other type of barrier
protection or be placed further from the roadway surface to prevent the trees from
becoming roadside obstacles.

        In many cases, it may be possible to plant street trees in a group within a
designated planting area instead of using individual planting pits or tree wells along the
street. These planting islands usually provide better tree survival opportunities and can
create greater visual appeal than individually dispersed trees since other plants can be
added to the streetscape beds.

               Planting island along street             Downtown street median planting

         The placement of street trees should not impair visibility at driveways and
intersections or of any traffic signs, signals or street lighting. As a general spacing
criteria locate trees at least 10 ft. from driveway entrances and 25 ft. from an intersection
if the intersection is controlled by traffic signals, or 45 ft. from an intersection if the
intersection is controlled by STOP signs (measured from the projected point of curb line
intersection). Trees should also be kept at least 20 ft. away from street lights, utility poles
and alley entrances to avoid blocking light, allow operational work around poles, and to
provide unobstructed sight lines.

        Street trees are also very prone to direct or indirect damage or vandalism from car
doors, and pedestrians especially if the trees are located along streets that are used for
parking. Trees limbs that will eventually extend over the curb line and intrude into
vehicular traffic lanes can be greatly damaged by high-sided vehicles such as trucks and

         Street trees can be planted on one side of a roadway or on both sides. The spacing
of the trees can be in a uniform formal arrangement or placed in an informal arrangement
as space allows. Trees planted along both sides of a street can be spaced so that the trees
are directly opposite each other or the trees can be arranged alternately. Alternating the
trees will allow more space for tree crown development. The future crown development
potential of the tree will help determine how close the tree should be planted to buildings
and other upright structures such as lighting and utility poles. City buildings that use
awnings or canopies over the sidewalk will restrict the type and size of street tree that can
be used.

         A street that is wide enough to accommodate parking along one or both sides of
the street can often allow the use of medium or larger sized trees since truck and bus
traffic generally will not come in contact with spreading tree branches. The limbs of the
trees can be pruned-up to accommodate vehicles that will park or pass along the curb.

                        Trees limbed up for               Lower limbs removed
                       parking and pedestrians             for taller vehicles

    If possible, use larger growing trees at greater spacing for wide streets, avenues, and
parkways to create a proper scale proportion to the openness of the area. Smaller growing
trees can be spaced closer together. In most street tree applications, it is desirable to have
the mature tree crowns touch or overlap the crown of the adjacent tree. Use the following
general criteria for tree spacing:

      Large sized trees (+40 ft. height) – space 40-70 ft. or greater for very large trees
      Medium sized trees (25-40 ft. height) – space 30-50 ft. apart
      Small sized trees (under 25 ft. height) – space 20-35 ft. apart

                                 Wide streets with wide tree lawns
                                 are best for larger growing trees

    Future pruning may be necessary to keep a tree growing within a designated tree lawn
space. As young, small caliper trees begin to grow, periodic trimming of the lower and
side branches over both the curb line and the adjacent sidewalk will normally be required
to reduce interference with pedestrian or car traffic. Spot removal of dead or damaged
limbs will also keep the young tree healthier and keep the tree looking nice.

General Criteria for Utilities and Street Furniture
        Underground and overhead utility lines present a challenge to the installation,
survival, and overall growth potential of street trees.
        Underground utility criteria – The proposed tree pits should be kept away from
all underground utilities such as gas, water, sewer, communication, and electric cables.
Future maintenance of these lines can adversely affect the health of the street trees by
cutting developing root systems. Tree roots can also intrude on the underground lines and
may create disruption to the utility service. It is recommended to keep trees at least 5 ft.
away from any underground utility service leads and 10 ft. away from primary or
secondary service lines and manholes.

         Overhead utility criteria – In most cases, do not plant under or near overhead
utility lines. If trees are still to be considered, do not select trees that have the potential to
grow higher than 30 ft. height. Trees with a potential height of 30-45 ft. are questionable
for use near overhead utility lines unless wires are high enough or far enough away to
prevent future contact. Trees with a potential height of over 45 ft. should be kept at least
40 ft. away from overhead power lines, buildings, or other trees.

        Other Placement Considerations: Potential street tree locations require
consideration of the placement of various traffic and business signs, utility and light
poles, parking meters, fire hydrants, mail boxes, benches, and other street furniture
amenities in urban areas. Accessibility and usage space must be provided for all
streetscape amenities. The opening of car doors at delineated vehicle parking spaces
along the curb can damage the street tree or the vehicle for trees located using poor
spacing criteria. Young trees planted too close to signs can eventually grow large enough
to obscure the sign and thereby require future pruning that may be detrimental to the
future shape of the tree. Relocating the sign can be a future option but is rarely
accomplished in a timely manner due to other community priorities.

                              Avoid planting trees too close to
                              signs or other street furniture

Planter and Garden Wall Use:
        Street planters can be of various sizes, shapes, and materials and are used to grow
plants in an outdoor environment. They can provide a means of introducing shrubs and
small trees into a limited street space. Some planters may have closed bottoms and act as
large planting pots. Some planters have drainage holes that will allow excess water to
seep out of the planter. Periodic movement of planters containing smaller growing plants
to different locations along the street can also change the appearance and function of the
outdoor space.

                      Small planters can be used to add plants along a street and can
                         often be moved to change the appearance of the area

        The size of the planter limits the maximum size of root development of the plant
and thus the mature size of the plant that can be used. The planters also expose the roots
to various extremes of drought and temperature. Insulating the inside of the planter may
help prevent the roots from freezing during winter. The recommended minimum planter

size for street trees is 6 ft. diameter with a 2 ft. root zone depth. Periodic watering,
fertilization, and allowing for proper drainage from the planter are important growth and
maintenance considerations.

        Raised garden wall planters can be constructed in various shapes using material
such as wood timbers, precast block, stone, or brick and can be constructed with mortar
or laid dry. These planters have an open bottom and allow for better root growth and
plant development. Sometimes these planters can be used along streets within a tree lawn
for walls having a low height coursing of one, two, or three rows.

        Garden walls can add color and texture to an area and help protect the tree from damage

General Tree Selection Criteria:
        Trees to be used for street tree planting must exhibit the ability to withstand the
many stresses associated with urban planting. These stresses include poor soil, drought,
air pollution, restricted root and top growth area, vandalism, compacted soils, and
reflected heat from adjacent paved surfaces. Ensure that the selected trees are hardy to
regional climate changes.

       Select narrow, columnar or fastigiate formed trees or trees with upright (vase
shaped) branch development since they are best suited for restricted space areas. Larger
spreading trees can be used if there is more area available for tree crown development.

                                    Fastigiate shaped trees fit well
                                    along narrow sidewalks bounded
                                    by cars and buildings

         The branching structure and potential foliage density of the trees must also be
carefully considered. Many small-sized trees are very dense and instead of framing
space, they fill the space and obscure views. Densely branched trees are also likely to
attract birds to nest or roost. The birds can cause their own future maintenance noise and
waste-dropping problems.

       Avoid using trees that have the potential to drop large quantities of or large-sized
vegetative substances such as leaves, fruit, flowers, and branches onto pedestrian,
parking, or vehicular traffic areas. Avoid trees with thorns.

       Avoid selecting trees that develop abundant surface growing roots and trees that
are prone to storm damage.

                              Abundant surface roots can cause
                              several future grass, sidewalk, and
                              curb problems

        Large growing trees such as Sycamore, Elm, Oak, and Maple when planted too
close to sidewalks and curbs can cause future sidewalk deterioration, sidewalk heaving,
and the potential for future curb displacement. The young trees that look good when
planted in fairly large pavement tree wells will eventually out grow the planting pit and
lead to future maintenance and safety problems as well as extensive pavement and curb
rehabilitation work.

                                 Mature Pin Oak causing extensive
                                  sidewalk and curb damage

       Avoid planting large quantities of a single tree specie or variety in a certain area.
This will limit the potential large scale loss of the trees due to pests or disease as was the

case with large numbers of American Elms lost to Dutch Elm disease nationwide. Ash
trees are currently under attack by the Emerald Ash Borer in several states.

         Select trees that have been nursery trained for use as street trees. These trees have
been pruned to develop adequate clearances for pedestrians and vehicles and are grown
with a single trunk. These trees will generally require limited future pruning to shape the
tree. Some tree species have male and female trees (i.e. Gingko). If the tree has both
male and female forms, plant only the male or sterile female trees to prevent unwanted
fruit from littering the ground around the trees.

                                The upright shape of Zelkova lends
                                itself too many street applications

Street Tree Terminology:          (See Figure 1)

Tree Lawn – Type 1: An area of varying width generally located between the curb line
and the sidewalk that can be used to plant trees.

Tree Lawn - Type 2 & 3: An area of varying width generally located between a
sidewalk and a building line/building lot that can be used to plant trees.

Building Line – A line of buildings, vertical structures, building lots, or right-of-way
generally in urban or commercial areas, that defines the width available behind the curb
line to install sidewalks and tree lawns.

Curb Line – The edge of a road bordered by a raised concrete, stone, or asphalt curb. A
curb constructed at an 8-inch height is usually high enough to act as a barrier in order to
help keep the movement of vehicles on the desired road surface. Lower height curbs may
not be capable of acting as a barrier to vehicular movement.

Tree Well – An opening provided in paved areas of a sidewalk for planting trees.

Tree Grate – A grate usually constructed of metal that covers a tree well (pit) in a paved
sidewalk and has openings that will allow water and air to get into the tree pit.

The grate opening around the trunk of the tree should be capable of being enlarged to
allow for the tree trunk to expand with normal growth. Select grates with openings that
will not allow shoe heels to penetrate the grate.

Streetscape – The viewed space or perceived street character that is defined by the
distance that buildings are set back from the curb line on either side of a street. This
character is comprised of the mix of items that make up the street locale including
sidewalk and street pavement types, street trees and other landscaping, street signs, store
fronts, advertising, awnings, street lights, benches, and other outdoor street furniture.

     Street Trees by Roland Daniels, The Pennsylvania State University, College of Agriculture, 1975
     Street Tree Factsheets, edited by Henry D. Gerhold, Norman L. Lacasse, and Willet N. Wandell,
        The Pennsylvania State University, College of Agricultural Sciences, 1993
     City of L.A. Bureau of Street Services, Street Tree Division website - Tree Spacing Guidelines
     Ames, Iowa City Government, Tree Planting and Maintenance website


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