Stormwater retention is a hot-button issue among landscape architects. It’s something that all designers need to consider and can pose challenges on specific sites as well as in larger ecological systems. As landscape architects, we strive to implement creative practices to mitigate stormwater issues.
In the past, designers have tended to select wet site-tolerant plants for these installations; however, while bioswale soils may be wet for brief periods, they are more often very dry between rainfall events. The authors tested several plants for their wet and dry tolerance and developed a bulletin describing many woody plants that are well-adapted to these conditions of alternately wet and dry soils.
In Structural Soil—Part 1, the many problems confronting urban trees were outlined and the need for a specially designed growing medium for such trees was amply demonstrated. In Part 2, the authors describe their development of CU-Structural Soil™ and share lessons learned after using this load-bearing soil for 15 years.
Why was CU-Structural Soil™ developed?
Soils under pavement need to be compacted to meet load-bearing requirements so that sidewalks and other pavements won’t subside and fail. Soils are often compacted to 95% peak (Proctor or modified Proctor) density before pavements are laid. When trees are planted into these soils, root growth is severely reduced or eliminated beyond the tree-planting hole. When root growth is restricted, tree growth suffers as water, nutrients and oxygen are limited. The need for a load-bearing soil under pavement gave rise to the development of CU-Structural Soil™, a blended soil that can be compacted to 100% peak density to bear the load of a pavement while allowing tree roots to grow through it.
A New Medium to Allow Urban Trees to Grow in Pavement
The fact that trees have difficulties surviving amid the conditions of urban and suburban environments is not a surprise. Urban areas for the most part are not designed with trees in mind. Trees are often treated as if they were afterthoughts to an environment built for cars, pedestrians, buildings, roadways, sidewalks and utilities. Studies point out that trees surrounded by pavement in the most urban downtown centers live for an average of 7 years (Moll, 1989; Craul, 1992), while those in tree lawns, those narrow strips of green running between the curb and sidewalk, live for up to 32 years. These same species might be expected to live anywhere from 60 to 200 years in a more hospitable setting.