The Field

Trees are important to the composition of urban design proposals. Drawings and sections show healthy, mature trees lining streets and punctuating plazas. There is an unspoken conclusion that a street without trees is not a complete street. Yet there is a critical component missing from most of these renderings.

Drawings almost always show the tree magically rising out of the ground plane with no means of support. Typically the sidewalk paving is shown right up to the trunk of the tree, the critical swelling of the trunk flare at the base of each tree above ground is not drawn. Also unspoken is the assumption that the trees will somehow find rooting space. The messy details of how the tree grows are left to the next phase of the design process. To be fair, urban design drawings, particularly the ubiquitous “typical” sections, also omit the building and light pole foundations. These omissions in the beginning of the planning process are to be added as the project moved forward. It is reasonable to assume the engineers and architects will put foundations under buildings and light poles, unseen structures typically built into the very first cost estimates. But sadly and all too often, the tree’s requirements and cost are ignored throughout the entire process.

There are two basic elements of the tree that urban designers must incorporate into their drawings, reports, and cost estimates. These are (1) sufficient soil volume to support the size tree expected to grow and (2) acknowledging the structural requirements of the tree where it meets the ground.

Sufficient soil volume

Creating soil volume in cities can be expensive and is not optional, it cannot be value engineered, as there is no substitute. Trees require about 1.5 to 2 cubic feet of loam soil per square foot of tree canopy area, defined as the area within the MATURE diameter of the trees canopy spread. Mature tree canopy spread is easily referenced by species in books such as Michael Dirr’s Manual of Woody Landscape Plants. The rest is a simple equation to calculate the volume and an effort to find the space under the sidewalk to fit.

An industry rough rule of thumb is that 1,000 cubic feet of loam soil is the minimum required, but this is still not sufficient to support a large mature tree such as an Oak or Plane tree. While this may feel intimidating to designers who have never done it before, calculating the needed soil volume is significantly easier to do than calculating the number of lanes of traffic required to make traffic flow or the number of cars and space needed for parking within the design, both common calculations in the urban design phase of a project. An urban designer does not have to be a tree expert to make these calculations and rough out the required space, but they do need to take on this task.

1,000 cubic feet sounds like a lot, but it is not always required to purchase all this soil. Many times the existing soil conditions may be suitable to provide some of the trees soil needs. Occasionally there may be adequate existing soil quality and/or some of the volume to grow the tree. In these conditions, adding volumes less than the minimum to the existing soil may support satisfactory results. Evaluating the existing soil quality factor may require an expert in the field of urban tree and soil assessment.

There are many optional approaches to adding loam soil under pavement, each is different in its effectiveness. Stone based structural soil such as CU Soil only contains 20% loam soil and thus requires 5,000 cubic feet of material to get to the 1,000 cubic foot minimum. Sand-based structural soils have limited independent testing and have not performed nearly as well as loam soil. A recent study at Bartlett Research Labs showed that its performance was well below loam soil. Simply adding more of sand based structural soil may not get better tree results due to the nature of the material. By far the most efficient system to increase loam soil under pavement is using a suspended pavement structure such as Silva Cells filled with unscreened loam soil. In all approaches, the amount of loam soil is the design goal and none of the approaches should be considered as equals.

Accommodating the trunk flare

The second problem with trees and urban design concepts is when they ignore the basic structural requirements where the tree transitions into the ground. Trees require that the base of the trunk (trunk flare) and the emerging roots near the trunk swells to a size that supports the trees against failure. Intrusion by the growing trunk flare and emerging roots is a frequent cause of damage to sidewalks near trees.

For example, a tree growing in the 1,000 cubic foot minimum soil will grow a trunk that is larger than 20 inches in diameter when measured at 4.5 feet above the ground (diameter at breast height, or DBH). For a tree of this size, the tree will swell, called the trunk flare, at the soil line larger than 40 inches in diameter. The roots that emerge from the trunk flare will expand with a force that easily lifts pavements for a distance of another 6-8 feet away from the tree. These roots can be directed downward by including root guides along the edge of the pavement, but these roots have to find additional soil under the pavement.

The net effect of these enlarging structures is that the area around the tree is a dynamic space that will modify the shape and elevation of any structure placed on top. In order to accommodate these structures, large spaces around the tree must be left open or covered with surfaces that allow slow gradual change in the shape and elevation. A good rule of thumb is that the size of the opening should be at least 5 feet minimum width. This size assumes that a root guide is in place and that there is sufficient soil under the pavement to support the maturing tree. This space immediately around the tree should never be considered a part of the permanent walking zone.

Yet all too often, urban designers think it is good design to place paving as close to the trunk of the tree as possible either with hard pavers or metal tree grates. These concepts become embedded in urban design documents that are difficult to change even when the folly of this approach is illuminated. Metal tree grates cause the greatest damage to trees and should not be used in any urban design recommendations as they may damage the tree and/or become a tripping hazard for pedestrians. Flexible materials such as mulch, gravel, resin coated rubber aggregate, or loose laid pavers may be acceptable provided the designer keeps them at least 6 inches away from the newly planted tree trunk and accepts that this paved surface will move and lift over time.

Urban designers are obligated to design spaces that respect principles of biology and physics. They are also increasingly in need to prioritize resilience and environmentally sustainable design. When designers bend the science that supports smart, long-term urban tree design, they do so at great risk to their clients and the future image of the spaces on which they work. Dead and declining trees or failed paving systems in the future do not make a great urban landscape.

by James Urban, FASLA. James is well known for his skills in the areas of urban arboriculture and soils, including the preservation and installation of trees in the urban environment and the specification and installation of specialized planting soils for roof gardens, urban landscape plantings, and rain water management. 

Reference List

1. Dirr, Michael. Manual of Woody Landscape Plants (2009). Stipes Publishing, Champaign, IL.
2. Loh, F.C.W., Grabosky, J.C., and Bassuk, N.L. Growth Response of Ficus benjamina to limited soil volumes and soil dilution in skeletal soil container study (2003). Urban & Fischer Verlag, Urban Forest, Urban Greening, 2.
3. Urban, James. Up By Roots: Health Soil and Trees in the Built Environment (2008). International Society of Arboriculture Books, Champaign, IL.
4. Study – Soils Under Pavement, Dr. Tom Smiley, Bartlett Research Labs, Charlotte, NC, in progress, personal communication, 2015.
5. Study – Sand based Structural Soil vs Loam Soil in Boston, Dr. Tom Smiley, Barteltt Research Labs, Charlotte, NC, in progress, personal communication, 2014.

[1] The root ball of a tree is the soil that is around the tree when it is dug in the nursery. It is usually sized as 10 inches in diameter for each inch of trunk caliper. The caliper of a newly planted tree is the diameter of the tree measured at about 12 inches above the ground for larger trees and 6 inches above the ground for smaller trees.