A Steep Slope in Seattle: A Silva Cell Case Study

– At Fifth Avenue and Madison Street, five blocks from the Seattle waterfront, three trees are thriving on a steep urban incline.
– The trio was given proper soil volume to maximize their canopy potential and create a more durable street and sidewalk for a heavily trafficked downtown area, across the street from the Seattle Public Library.
– In four years, the tree canopy width of the Scarlet Oaks on Madison has doubled. In contrast, the trees across the street that were planted without Silva Cells have seen little to no growth.

The Weisman Design Group, a sustainable solution landscape architecture firm in Seattle, was faced with challenges of a substantial incline on a busy sidewalk and street that would make construction and planting complex.  The firm acts as a steward for the planet and aims to connect its designs with the natural environment.  Peter Nelson, a principal with WDG, kept all of this in mind, as the team looked for a solution that would achieve maximum tree canopy and a safe and workable space for locals and tourists. Nelson said, “The project aimed to provide a vibrant and durable streetscape for pedestrians in downtown Seattle. This included selecting trees and shrubs that would be able to grow and thrive in a harsh environment. This project is part of a larger effort to increase the overall tree canopy across the city.”

Heavy pedestrian and vehicular traffic combined with an unusually steep slope on the street meant the trees needed the right amount of support, technically and physically. DeepRoot was able to provide a solution that would both support the trees’ growth over the long term and address the challenging 12% slope that existed on the sidewalk.

“Silva Cells allowed for much needed uncompacted topsoil under the sidewalk. They had to be installed in a stairstep fashion down the slope because of its steepness. We used DeepRoot’s excellent technical support to assist in the design and detailing of this complex project.”

-Peter Nelson, WDG

42 2X Silva Cells were installed at the corner of Fifth Avenue and Madison Street across from the Seattle Public Library in May 2017. This Silva Cell-to-tree ratio gave each tree 344 ft3 of soil volume. The results just three years later reveal the power of Silva Cell technology.

Figure 1: One of Three Scarlet Oaks. Photo facing west toward the waterfront. May 2018

“The trees have doubled in canopy width and are thriving.”

-Peter Nelson. WDG

Figure 2: August 2015. Google Street View. The three trees on the left were planted without Silva Cells. The three trees on the right were planted with Silva Cells.

Figure 3: May 2019. Google Street View. The three trees on the left were planted without Silva Cells. The three trees on the right were planted with Silva Cells.

The significant growth of the three trees can best be appreciated when comparing them with the trees on the other side of the street. The trees on the north side of Madison Avenue, along the sidewalk in front of Seattle Public Library, were planted without Silva Cells. Dated images courtesy of Google Street View reveal that the trees on the left have been there even longer than the three trees planted with Silva Cells, but the older trees have hardly grown.

With climate change presenting more frequent and extreme weather events such as intense storms and intense extreme heat events, a large urban canopy is a vital part of all landscape architecture and urban planning.

Installation Summary:
Number of Silva Cells: 42 2X
Total soil volume:
1,033 ft3 (29 m3)
Soil volume per tree:
344 ft3 (10 m3)
Number of trees and type:
Three Scarlet Oaks
Type of project:
Integrated trees stormwater
Project designer:
Weisman Design Group
Project contractor:
Gary Merlino Construction Co. Inc.
Installation date of Silva Cells:
May 2017
Project time line:
2017