Roughly 80 percent of urban tree health problems originate from conditions below ground. A tree is supported both structurally and nutritionally by its roots -- any limitations placed on its roots will result, directly or indirectly, in future health problems. These resources explore the fundamental relationship between soils and roots, and how to grow trees to create the best growing conditions below ground.
Providing uncompacted soil volume for tree growth
Soil volume and tree growth are directly related. Several techniques exist to protect soil volume under pavement from compaction so the soil can support root and tree growth. These methods can also be used in biorentention practices that treat stormwater runoff.
Tree Space Design is a publication that explores the relationship between soil volume and tree growth and provides strategies to grow trees 'out of the box'.
Engineered Rock Based Soil
Structural (or engineered) soil is a medium that has been used to grow trees in areas where the soil must be compacted to support pavement. The soil consists of a particular mix of crushed gravel and clay loam soil. When properly composed and installed, the gravel in the mix provides a locked weight-bearing matrix that can support pavement. The voids between the gravel are mostly filled with the clay loam, which holds moisture and nutrients needed for tree growth. A hydrogel is added when the components are mixed to prevent the separation of soil and gravel components.
Cornell Urban Horticulture Institute offers a variety of resources on Cornell's (CU) Structural Soil. This is a patented formula available only through licensed producers. Other types include Stalite, Swedish and University of California (UC) Davis' structural soil.
Structural Cells or Suspended Pavement
Suspended pavement is a general term for any technology that supports the weight of paving and creates a subsurface void space that is filled with soil for root growth. Using new technology, suspended pavement can now be made more cheaply and simply through the use of soil cells. Frames (cells) can be stacked up to three high, with a deck installed above the top frame. The modular design allows the structure to support the pavement, provides space for soil, and the flexibility to size the soil volume as needed for each site.
The costs to install these systems are much greater than simply putting a tree in a 4 x 4 foot cut out, but the outcomes are much better. The trees have access to uncompacted soil volume, allowing them to thrive and grow to a size yielding maximum ecosystem benefits. If we consider trees as green infrastructure and as sources of necessary services like clean air and water, the trees will pay the costs of these systems back quickly.