Site selection is the first, important step to make when planting trees in an urban environment. Quality sites have a higher probability of supporting long-lived and healthy trees, whereas poorly chosen sites produce poorer-quality trees. Urban environments are challenging for tree survival. They often contain soil contaminants, air pollution, high velocity wind, compacted and poorly aerated soil, higher temperatures, altered soil drainage, and small planting areas. Considering these challenges, choosing the best site for planting is critical for optimal growth and survival, followed by careful preparation of the site, tree selection, and planting.
Site Characteristics
Knowing the characteristics of a planting site will help you determine whether the site will support healthy tree growth and development. The Site Factors Checklist below may be useful when evaluating site conditions. Some of the important site conditions to consider include:
- Soil characteristics
- Environmental conditions
- Site location
- Soil volume
- Social influences
- Maintenance requirements
Site Location
The site location offers clues on potential stresses that may impact tree health and maintenance. For example, a tree located within a downtown sidewalk will probably require more maintenance than one located in a park. Sites with pedestrian and vehicular traffic require special attention.
Streets, Sidewalks, and Other Paved Areas If the site is located near a street, sidewalk, bike path, or other paved area, several site factors must be considered.
- Pedestrian and vehicular areas — For any site near where pedestrians or vehicles travel, tree species selection is critical. Species with thorns or prickly foliage or soft, messy fruit should be avoided. Trees with drooping branches will require frequent pruning. For public safety, it is always important that traffic lights, signs, and intersections not be obstructed by trees. Select a species tolerant to high salt levels in the soil if the site is located near a road where de-icing salts are used.
- Conflicts with roots and pavement — Tree roots may grow under asphalt or cement pavement, which can cause the pavement to crack and buckle. Some communities have tried using root barriers and root training to avoid root-pavement conflict. There are different types of root barriers, from cylinders to herbicide strips, that are placed in the planting site. They are designed to physically deflect the roots away from the pavement. In some cases, they do prevent root growth near sidewalks, but they may also limit tree growth. Root training is an option that uses chemical and physical barriers, deep fertilization, and irrigation or aeration structures to improve the soil conditions in the deeper soil horizons. If the barriers are successful, the roots will grow deeper, avoiding surface problems such as cracked sidewalks.
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| Photo Credit: Raina Sheridan |
Structures Trees need to be far enough away from buildings to allow for proper crown and root development (Gilman 1997). Trees that grow large, such as oaks, should be planted at least 15 feet from a building. Small and medium-sized trees may be planted closer to the building, but regular pruning may be required (Gilman 1997).
Utility Lines Utility lines for water, sewer, phone, electric, or cable may cause problems for trees. When selecting a site, check for underground or aboveground lines that might interfere with the future growth of the tree.
- Aboveground utility lines — If the site has aboveground utility lines, select a small-growing species that will top out at least 5 feet below the wire, or select a species with a narrow crown and place it so it will not grow into the utility line (Gilman 1994).
- Belowground utility lines — The planting site should be located at least 12 feet from a major underground utility line for large trees (Gilman 1994). A common misconception about tree roots is that they actively grow into sewer and water lines. Roots will follow a path of least resistance and only grow into sewer and water lines that are broken.
Site Activities The type of activities — past, current, and future — on the site can help in evaluating the planting site. Has construction occurred on the site that may have changed soil conditions? How many people or vehicles use the area around the site? Are there safety concerns, such as personal welfare or property damage? Will the trees need to be protected from compaction, vandalism, or potential injuries?
This type of information can usually be determined by visiting the site and talking with people who are familiar with it. The property owner or local planning departments are good resources for finding out about future plans for the site.
Urban Planting Sites
Several types of planting sites are unique to urban areas, including street lawn, tree pit, roadway, planter, and cluster planting. These sites may require special considerations when selecting a species and choosing a proper planting technique.
Street lawn The street lawn, also known as the tree lawn, is the space between the curb and the sidewalk. Depending upon on the mature size of the planted tree, the street lawn should be at least 3 feet wide. If there is a choice, a street lawn is preferred to a tree pit because the street lawn has a continuous strip of soil. Do some checking before planting in a street lawn because of the potential conflicts with pavement, utilities, and local highway department guidelines.
Tree or planting pit Tree or planting pits are small areas of soil within a sidewalk, parking lot, or other paved area. They are common in urban areas because often this is the only space available for planting trees. They also offer the advantage of softening the hardscape in urban areas. Trees planted in tree pits usually require special attention because of the unique growing conditions at the site.
Roadway Tree plantings in the median and on the sides of the roads provide many benefits, such as intercepting dust and particulate matter; reducing glare, noise, wind, and erosion; visually separating opposite lanes of traffic; and reducing mowing costs. However, trees near roadways can be damaged by vehicles, lawnmowers, string trimmers, herbicides, and de-icing salts. Drainage problems are common, because the sites usually have disturbed soil that has been placed on top of compacted soil. It is also common to find construction rubble from road projects in the soil. Each state’s department of transportation usually has specific guidelines for plantings near roadways, such as species selection, planting distance from pavement, and distance between trees. It is important to work with them, especially during the planning phase.
Planters/ Containers Planters or containers are an option for sites where it is impossible to plant because of poor soil, lack of soil, underground utilities, or other factors. Planters can also be used for architectural design purposes. Tree planters come in various shapes and sizes and are made of plastic, wood, cement, or other composite materials. They should have thick walls, be a light color, and measure at least 18 inches deep with an adequate drainage hole.
Cluster planting Cluster plantings, clumping, open-space planting, and urban tree islands are different names for planting groups of trees in a large space. Cluster plantings provide many benefits, such as reduced maintenance costs, shelter from weather extremes, and increased life spans for the trees. However, since the trees are located close together, insects and disease can move quickly from tree to tree. Overcrowding may also become a problem. Proper species selection and spacing are critical to avoid these problems.
See other parts of this 3 part Tree Planting Series:
To learn this content and more for volunteer credit and a certificate of completion, go to campus.extension.org and enroll in eLearn Urban Forestry: eLearn Urban Forestry: Online Training in Urban Forestry – eXtension
Also see:
- Trees in Urban Environments
- Planting Sites Unique to Urban Areas
- Guidelines for Planting an Urban Tree – eXtension
- Urban Forests and Pollution – eXtension
- Designing Urban Forests – eXtension
- Urban Heat Island – eXtension
- Sources of Assistance in Urban Tree Care for Energy Conservation
- Uses of Urban Vegetation to Moderate Climate
By: Ed Macie, Regional Urban Forester, USFS Southern Region; Sarah Workman, UGA Warnell School of Forestry and Natural Resources; Holly Campbell, Southern Regional Extension Forestry