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Conventional versus expanded tree basin, San Francisco style

Image: Dolores Street, San Francisco

We're back from our California research trip where I conducted field work in Sacramento and San Francisco. In yesterday's post, I featured a photograph of an assessment of Sacramento's urban heat island. Today's post features two types of street tree planting areas in San Francisco: a conventional tree pit and an expanded tree basin. The former is typically 3 feet by 3 feet. The expanded basin in the photograph is 54 feet by 6 feet!

Image: Valley Street looking east towards Dolores Street, San Francisco

Providing expanded tree basins in San Francisco's neighborhoods is a project of the Friends of the Urban Forest (FUF). Using the city's Permeable Sidewalk Landscaping Permit, FUF staff and neighborhood residents remove sidewalk concrete blocks to provide greater planting and growing space and other environmental benefits.

The replacement of concrete by plants allows for the absorption heat instead of reflection of it. This absorption of solar energy reduces the Urban Heat Island effect, a phenomenon which can significantly alter surrounding ecosystems and increase the amount of energy used in the city. These open areas also absorb more rainfall, reducing the strain on our combined sewer system, a system that mixes building waste with street water runoff before processing. In periods of high rain fall our sewer system can't handle the volume of water that passes through it and the combined sewage is released directly into the bay without being cleaned. Removing a 3' square of concrete will reduce the amount of rainfall run-off by five gallons for every inch of rain that falls. (