Betting on Soil Cells

Soil cell root structured tree planted as a test case for Atlanta’s 8,000 Trees for Downtown Initiative

Trees grow better with more soil and in better soil than found in most urban sites. In urban areas, trees compete for space in the built environment with roads, sidewalks, buildings, underground infrastructure and utilities, and even the air space above them. We need to intentionally plan for trees and plant them with enough open ground area and soil volume to allow a tree to thrive and reach its optimal size faces leave less open ground for trees to grow. “Soil cells” can be used to create more soil volume in places that may have limited space. The structures increase accessible soil volume and reduce soil compaction while allowing paved surfaces, such as a sidewalk, to be constructed above the tree root zone.

Testing Our Luck

In April 2024, Trees Atlanta installed a soil cell system at our headquarters. A street tree was planted near the Atlanta Beltline-side entrance of our campus using the Stratavault soil cell system. Experts from CityGreen, the manufacturers of Stratavault soil cells, traveled from Canada to coach us through this initial installation. Various brands are in the market and several were reviewed. A deciding factor in our choice was that Stratavault is made of 100% recycled plastic.

The tree planted is a Georgia oak (Quercus georgiana) and is optimistically named “Lucky.” Lucky is planted in the same planting strip where eight Georgia oak trees were planted in the previous year. Trees Atlanta will be monitoring Lucky and the eight trees (that were planted without soil cells) to evaluate soil conditions and tree health over many years. More immediately, a big benefit of the experiment has been getting hands-on experience installing the soil cells and test tree.

A very important canopy project that is already underway is the planting of 8,000 trees in Downtown Atlanta by Trees Atlanta, in partnership with the City of Atlanta, Central Atlanta Progress (CAP), Atlanta Downtown Improvement District (ADID), and other city stakeholders, plus additional funding support from USDA Forest Service. The goal is to increase the canopy coverage in Downtown Atlanta from its baseline of 3% and increase it to 18%. The baseline urban tree canopy (UTC) cover percentage of Downtown was determined using 2008 satellite data (study published 2014). The UTC of 18% is the projected canopy growth by 2040 of 8,000 new trees planted along with the existing trees in Downtown according to the Downtown Atlanta Urban Tree Planting Plan Report, 2021.

Achieving this planting goal requires new tree wells to be cut and created in existing paved areas in Downtown Atlanta. Tree wells are spaces reserved in paved areas that are opened to the ground. Unfortunately, retroactively-created tree wells are typically not large enough to provide good soil volume for healthy tree growth while also preserving sufficient paved area for pedestrians. One solution is to create soil volume under the sidewalks using soil cells. Trees Atlanta staff wanted to get a closer look at and test the Stratavault system before installing the soil root structures throughout Downtown Atlanta.

Soil Cell Root Structures

Soil cell root structures are modular systems designed to meet site specifications and offer high-quality, uncompacted soil volumes for tree roots. These structures support infrastructure, increase soil volume, reduce soil compaction, and increase water and oxygen exchange to tree roots which contribute to better tree canopy growth. Soil cell root structures can also prevent future damage to sidewalks. The cells provide space underground for root expansion through each cell versus being forced upward toward the surface. The modular quality of these structures allows them to be installed around existing utilities buried under sidewalks, a major challenge for planting in city spaces, and allows trees to be planted in previously “unplantable” locations.

It’s Not Luck, It’s Hard Work

Madison Williams, Trees Atlanta Downtown Planting Manager at the time, coordinated the project involving heavy earth moving equipment, removal and replacement of existing sidewalk pavement, hand digging, and problem-solving through a new product installation. Staff from other Trees Atlanta work teams couldn’t resist helping to install the soil cells.

The test location was on site at the headquarters campus. The test site mimics typical planting locations throughout the city: a narrow planting strip bound by the roadside curb and a paved sidewalk.

Williams explains the installation process in her own words:

The first step of installation is to determine the dimensions for the concrete cut. The proper dimensions can vary depending on location.
Ideally, we want to reach 250 cubic feet of soil volume for an understory tree, 500 cubic feet for a midstory, and 800 cubic feet for an overstory tree. Space is limited Downtown, so we strive for the largest concrete cut possible.
Lucky is a 2.5’’ caliper tree. For a tree of this size, the average depth for a tree pit usually ranges from 3 to 4 feet. However, we needed to dig further due to a large utility running through the middle of the planting location. After three days of hand digging, our tree pit was approximately 12’ x 11’ x 5’. The staff-dubbed “pit” was ready for Stratavault installation.
After removing the soil, it’s important to make the base of the pit level. To do this, we poured gravel into the pit and used a compactor to push it down. Once the base was level, we laid a filter guard to prevent the roots from impeding and began to assemble the Stratavault. Assembly is efficient and somewhat similar to Legos, where each piece can easily be removed and attached elsewhere.
With the Stratavault assembled, we were ready to backfill the pit with curated soil. The soil Downtown isn’t of the highest quality, so we worked with a local company to curate a mixture of sand, topsoil, and fertilizer to backfill our tree wells.
With soil in place, it’s almost time to plant our tree. We selected a Quercus georgiana, or Georgia Oak, for this spot due to its ability to grow in granite outcrops. It’s our goal to plant more of these in Downtown Atlanta.
Once Lucky was placed in her forever home, we laid a filter guard over where the sidewalk would be. A filter guard is lined with a recycled plastic grid that helps prevent the roots from impeding the concrete. Finally, it was time to repour the sidewalk concrete and seal the Stratavault into place.
Lucky’s installation was made even more special by the camaraderie of planting as a team. What began as a Downtown Planting project quickly blossomed into a Trees Atlanta project, with each department coming out to help in different ways. Uniting behind a shared mission is Trees Atlanta’s specialty, and that sentiment was felt throughout the process.