April 1, 2012

CU-Structural Soil

— Thinking outside the planting box

Patented, recipe soil improves the odds for urban trees.


Assailed by pollution in the air, soil and water, street trees typically must grow in the cramped quarters of a conventional four ft.-square planting box, their roots vainly stretching for life support in the compacted soils required to provide the structural integrity of the surrounding sidewalks and streets. “It’s like lowering the tree into a coffin,” says Dr. Nina Bassuk speaking at the Spreading Roots conference at the Toronto Botanical Garden last fall. A professor and program leader of the Urban Horticulture Institute at Cornell University, Dr. Bassuk has developed a growing medium that offers these trees a lifeline.

About 20 years ago, Dr. Bassuk sought a solution for the opposing requirements of trees and the built elements around them. The problem was, how to supply the volume of soil trees need to sustain healthy growth, while meeting installation specifications that require soil to be highly compacted to bear the weight of concrete, asphalt or pavers. Using air excavation tools and ground penetrating radar, Dr. Bassuk was able to trace the way tree roots actually grow in the compacted soils of paved areas. What she discovered was that roots sought the route of least penetration, spreading shallowly between the pavement and the surface of the compacted substrate, rather than growing through it or rooting more deeply to reach beneath it. This shallow rooting causes the heaving of pavements and sidewalks and limits the longevity of the tree itself.

The medium Dr. Bassuk developed to solve the problem is CU-Structural Soil. A mixture of 80 per cent crushed gravel and 20 per cent soil bound together with a hydrogel tackifier, CU-Structural Soil creates a rigid framework able to support the weight of pavements without compacting the soil. This is achieved as the stones form a stable, lattice-like framework that creates pore spaces large enough to accommodate the soil. There is also enough room to allow for the free movement of roots, air and water into these soil pockets.

“It’s important how it’s made,” says Gino Turchiaro, in charge of sales and product development at Hermanns Contracting, one of two companies licensed to produce CU-Structural Soil in the Greater Toronto Area. “The components must meet certain specifications,” he says. The stone must be of a uniform size of between ¾ and 1½ inches in diameter. It is also critical that it be washed to remove the fines which may otherwise clog the pores, he adds. The soil itself must be a minimum of 20 per cent clay to allow adequate cation exchange capacity. Also, it must incorporate enough organic matter—between two and five per cent—to ensure adequate nutrient and water-holding capacities and encourage microbial activity. Only when mixed according to this strict recipe is the soil successfully suspended in the pore spaces between the framework of stones.

Specifically designed for trees planted on paved sites such as pedestrian malls, sidewalks and parking lots, CU-Structural Soil is used directly below these areas, allowing two cubic feet of soil for every square foot of crown projection for each mature tree. It can be used in trenches, under permeable and non-porous paving, and in island beds in parking lots and plazas. It can also be used to create break-out zones under existing sidewalks for example, to provide a way for roots confined to narrow curb-side strips to grow under the sidewalk and into adjacent yards and green spaces. The one application it is not meant to replace is the conventional tree pit. Because the success of CU-Structural Soil is based on providing large enough volumes of media for roots to grow into, the meagre dimensions of tree pits offer inadequate space for CU-Structural Soil. In these cases, a good, well-draining soil is recommended.

When designing for tree plantings, Robert Beaudin, senior landscape architect at Janet Rosenberg and Associates Landscape Architects of Toronto points out that many municipalities now require minimum soil volumes of about 30 cubic metres for a single large tree. “When we use structural soil,” he says, “municipalities often consider only the soil portion of CU-Structural Soil, which is about 20 per cent of its total volume.” The implications of including only the soil portion of CU-Structural Soil in the soil volume calculations leads to a plan with a larger area of structural soil than regular soil. “Despite these difficulties,” he says, “structural soil provides some benefit when planting in potentially compacted spaces.”

When installing CU-Structural Soil, “timing is important,” says Turchiaro. “Order the right amount for the trench you’re filling,” he advises. If delivered before the site is ready, or in greater quantities than can be used immediately, the soil must be covered to prevent contamination with sand or other elements on the job site and to maintain moisture levels. At Hermanns, the soil is stored under optimal conditions to preserve its integrity. Ordering “as needed” ensures a more successful installation than storage at the job site. “Once a contractor knows [about these requirements],” says Turchiaro, “there are usually no problems in the co-ordination of a CU-Structural Soil installation. It just takes a bit of forethought,” he says.

Over the last 25 years, there have been many CU-Structural Soil installations in Canada, from Dartmouth, N.S., to Edmonton, Alta. In the Toronto area, Marco Marrone of Four Seasons Site Development Limited, has been involved in several CU-Structural Soil installations, including the Bloor Street project. Here, CU-Structural Soil was used above utilities in areas too shallow to accommodate the modular Silva Cells, another urban tree planting system employed on the project (see Landscape Trades, January 2012). Where the two systems are being used, says Marrone, it is critical to use a barrier at the interface of the two systems, to mitigate soil infiltration between the two soil types, which would jeopardize the performance of the CU-Structural Soil.

“Compared to other systems, one of the benefits of using CU-Structural Soil,” says Turchiaro, “is its simplicity of installation. It’s easier and more cost effective to access adjacent underground utilities to perform maintenance and repairs, especially in municipal streetscapes.” When it comes to selecting and installing CU-Structural Soil, Turchiaro adds, “It all comes down to awareness, confidence, and showing the research and history that trees thrive.
Lorraine Flanigan is a Toronto-based garden and horticulture industry writer.