SDOT is Giving Trees Room to Grow
Last Tuesday, April 22, when people around the world celebrated Earth Day, the Seattle Department of Transportation (SDOT) began work on its first substantial public works project installing Silva Cells. The cell system provides the space for large trees to grow with healthy roots in a manner compatible with other infrastructure in the right of way.
Since Tuesday, crews have been excavating the ground and beginning to install a total of 738 cells, which will support more than 40 trees along Mercer Street, Aurora Avenue/SR99 and Dexter Avenue as part of the Mercer Corridor construction project. Roughly 10 to 16 four-foot by eight-foot cells are being installed for every London Plane and Scarlet Oak tree that is to be planted.
Trees along unique urban corridors like Mercer Street are an iconic symbol for Seattle, The Emerald City. Trees are identified as a hallmark of Seattle by those who go to the Seattle Center or routinely travel along this street and enjoy the green forest canopy that provides a park-like atmosphere.
Trees cleanse the air, filter out pollution from stormwater, reduce run-off that can lead to flooding, provide shade to cool paved surfaces and reduce the “heat island effect” common in urban environments. The unique design of the Mercer Corridor project allows the use of large scale tree species in limited right of way space, ensuring maximum benefits from each and every tree.
Silva Cells look much like a series of industrial strength coffee tables that support the pavement above ground surrounding the tree while eliminating the need to compact the soil. Compacted soil is very harmful to a tree’s root system. The space under the structural system allows roots to grow in organic rich soil below the pavement throughout the cells.
While the cells are good for trees, they also have real benefits for protecting water quality. Stormwater can be filtered by soil surrounding a tree and also by soil underneath the pavement which retains the water for longer periods of time after storm events.
A study of trees throughout North America showed the life span of a typical urban tree can be as short as seven years. With these cells, SDOT can help the corridor’s trees live well beyond that timeframe. Making sure trees grow to maturity benefits the environment. Trees increase their growth rates, sequester more carbon and reduce stormwater runoff at an ever increasing level as they age.