We profiled rain gardens in the July 2007 issue of our Gardener's Notebook. You can read it here and also read the Chronicle's rain garden article, below.
Rain gardens capture storm water, clean it up
SF CHRONICLE
Ron Sullivan, Joe Eaton
Wednesday, June 18, 2008
It's a bit odd that "green" roofs have become popular here sooner than rain gardens. A green roof certainly does a lot to compensate for the footprint of a building in terms of microclimate heat, maybe habitat loss and certainly rain runoff - the last of which carries with it the prospects of overtaxed sewage systems, flooding, erosion and the pollution of streams and the bay.
Look at those admonishments stenciled over storm drains for clues: Runoff from paving and other hard surfaces carries particulates and pesticides, petroleum and fertilizers, and other nasties into the water in small bits that accumulate to toxic levels.
Roof gardens require attention to engineering, weight, leakage potential and access for maintenance. Hauling the lawn mower up the chimney is the stuff of sitcoms, but even the toughest plants need attention unless you decide to cultivate a garden of weeds and whatever else chance might deliver. It's fun until chance delivers an acacia and its roots creep up on you with ill intent.
Rain gardens are easier. A rain garden is a planting designed to soften the destructive force of rainstorm water and reduce runoff by slowing it enough to allow more to soak into the ground. It might use plants, swales (shallow channels), rocks of various sizes, and/or temporary ponds to do this. Depending on the plants used, it might or might not need irrigation in the dry season.
You can use rain gardens to compensate for the runoff from your house and garage roofs, and driveway, sidewalk and patio - all impermeable surfaces - and they can be pretty and provide habitat for other pretty creatures.
There are formulas to calculate how much runoff your roof is responsible for in a year. Multiply your roof's square footage by 623, divide the result by 1,000 and multiply that by the number of annual inches of rainfall wherever you live. Compensating precisely is not the point. Anything you can do to reduce immediate rainwater runoff is a good thing.
Rain gardens also conserve water where we're going to need it - in our gardens. Pete Veilleux mentioned one very basic system in the June 13 Chronicle's quarterly Green section: Unhook some downspouts from the storm drains or aim them away from the walk or driveway; connect the spouts to perforated pipe wrapped in landscape cloth; lay the pipe in swales or depressions of your own design to carry water into your garden; conceal, if desired, with mulch.
Cheryl Sullivan, a senior landscape architect with Cunningham Engineering in Sacramento, spoke about rain gardens, also called bioretention cells, during a recent climate-change conference at UC Davis. She made several suggestions about creating such gardens, one of them urgent: "If you disconnect downspouts from underground drainage," she said, "create a swale or slope to take it directly to the rain garden; don't have it pooling around the building."
A rain garden is an anti-Army Corps of Engineers, old-style approach. When creeks got channelized and concrete dikes raised along riverbeds, the idea was to move water away as fast as possible. The problem is that there's always somebody downstream who objects to being on the receiving end.
Rain gardens make moving water slow down, absorb, nurture. Vegetation does this via stems and roots, and it's also a natural filter for pollutants, both because it slows the flow, letting particles and substances drop out onto the streambed, and because lots of those pollutants are actually plant nutrients (animal fecal particles, for example) or get harmlessly absorbed and chemically broken down by the plants and soil microorganisms. Biological sewage treatment plants like Arcata's work this way.
Because these "non-point source" pollutants generally reach hazard levels only when accumulated from a big area and concentrated into a small one, they've never yet been caught threatening a bioretention site or the groundwater there, Sullivan told us.
If you get a sheet of water flooding your yard, sidewalk or street after a hard rain, you can help solve the problem. A rain garden isn't a pond (though it can include one), so there doesn't have to be a mosquito or stagnant-water hazard; it should drain in about 48 hours. It can be a swale - a fairly shallow depression in the ground, lined with plants or drainage rock - and/or a sink of sorts that lets the rush of a rainstorm stand still long enough to percolate away into the ground.
"Rain gardens cost only three or four dollars per square foot or less to install," Sullivan told us. "You do have to know something about your soil conditions and plants. The range of plants is huge for California - in general the plants need to tolerate wet to dry conditions, and be appropriate for the local soils. Bunchgrasses can line the bottom of a swale or a slope; plants like manzanitas that resent bad drainage should be at the top."
Above all, she said, "Experiment with plants! Landscapes are always changing, in reality, the garden is a piece of performance art."
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