A Smooth-Sidewalked Future Free from Tree Root Tumbles

Tree-root taming: Silva Cell installation at 4th and Virginia (Photo: City of Seattle)

So we were excited to see that the city is trying out a solution for sidewalk-ruining roots, at least. They're using something called Silva Cells as they plant some red oak trees in front of the Escala downtown. We asked Shane DeWald, landscape architect for SDOT how it works.

As you can see in the photo, the installation requires excavation of the area designed for installation of the blocks which are then back-filled with amended native soil--a mix specifically suited to encourage root growth. The blocks provide the support for sidewalk above them, providing benefits including both the prevention of sidewalk damage and the appropriate growing conditions for the large scale streets that Seattle is noted for.

So, to recap, the Silva Cells create a large, underground root-space for the tree (the "cells" are stacked three deep) and leave room between the sidewalk, which the cells support, and the soil the tree roots grow in. They also help capture a large amount of stormwater runoff, which is especially needed in impermeably surfaced downtown. What they don't do is work with existing tree root problems--you have to install them before the trees are planted.