Gardens for the protection of the SF Bay
By Lauren Lewis
The winter rainy season is a happy time for our gardens, and a less happy time for the San Francisco Bay. That’s because heavy rains, which typically happen a handful of times each winter, bring more water than Bay Area cities’ sewer systems can handle, and the result is more minimally-treated wastewater making its way into the Bay.
Most of San Francisco has a combined sewer system, which means that sewage and stormwater travel through the same pipes. Under normal circumstances, all that combined wastewater goes through primary treatment (basins for removing settled heavy junk and floating light junk) and secondary treatment (use of microbes to consume organic matter) before being discharged into the bay. When there’s a storm, there’s more rain in the system than can be treated in the secondary treatment plants, so the discharged water retains all its organic matter, like bacteria, and people who come in contact with the water are more likely to get sick from it. In the 90% of the city with a combined sewer system, the key mitigating factor is how much water we can prevent from entering the storm drains.
In the other 10% of the city, stormwater and sewage travel through different pipes, and stormwater flows directly to the bay, untreated. In these areas, the key to protecting the bay is improving the cleanliness of the water going down storm drains.
In both cases, there’s a variety of practices that the city and its residents can use to keep some stormwater from getting into the bay. The most basic way is plant-covered ground. Even bare soil allows for more water infiltration than cement or asphalt, but a well vegetated piece of land can do so much more than soil alone. The plants hold the soil in 
place, so the water doesn’t wash it away. Plant roots, dead plant matter, and all the soil organisms plants support make soil more porous, so more rainwater can soak in.
“Rain gardens” situated where rainwater gathers and flows are an inexpensive way for the city to prevent some stormwater from reaching drains, and if you look carefully you’ll notice more and more of these popping up around the city. Bits of sidewalk torn out and replaced with gardens can have a similar effect (in addition to the obvious side-benefit of beautification!). The city of San Francisco partners with Friends of the Urban Forest to do this, and you can spearhead a sidewalk garden with FUF’s help.
If you’ve read any of our previous blog posts, you’ve probably noticed some repetitive themes for how we garden and how we want others to garden, and this post is no different. Our advocacy is for dense and diverse plantings, hidden organic matter, and location-appropriate plant choices- practices that improve the water-infiltration capacity of soil, and also apply to all the various topics of our previous posts. What a wonderful thing. When you copy nature’s patterns in the controlled environment of a garden, your garden and the much bigger world it’s connected to all benefit.
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