Rain garden

If you live in a typical tract-style home in a place like Santa Rosa, some 25,000 gallons of water will drain off your roof this winter.

That water will race across your property, perhaps via a pipe, winding up in the street and down a storm drain. Laden with debris, silt, pet and bird waste, motor oil from the driveway and any herbicides and pesticides you might have put on your lawn or planting beds, the runoff will make its way to a creek and eventually the ocean.

That is 25,000 gallons of water wasted, water that could have been used to replenish the water table and feed the plants in your garden.

That figure boggles the mind of Barb Triol. A retired Agilent engineer, Triol contemplated that figure with consternation. Multiply 25,000 gallons by however many residential roofs there are in Sonoma County and you have an astonishing volume of dirty water that could be put to far better use. And that is just roof runoff. Other hard and impermeable surfaces, like driveways and patios, also repel rain water that otherwise would seep into the ground.

Triol took up the cause. She teaches free classes through the Sonoma County Master Gardeners on how to capture some of that precious liquid by making a rain garden.

“It’s one thing people don’t realize,” she said. “When water goes into the ground and is seeped into the ground, it’s filtered. The roots and the microbes in the earth cleanse and filter the water and clean all the pollutants out. It’s a natural thing that has been happening forever.”

Rain gardens are shallow, man-made depressions in the ground that capture rainwater, and slowly drain and filter it back into the earth.

Talk “drainage” and many people’s eyes glaze over with visions of engineering plans, backhoes, pipes and big dollar signs. But for the average homeowner, a rain garden is a fairly easy, do-it-yourself weekend project that doesn’t require specialized skills. And it can be made into an attractive landscape feature when planted with native plants that are drought tolerant in the summer but can take a heavy inundation of water in the winter.

Triol, whose mantra has become “Slow it, spread it, sink it,” will share tips on how to install a rain garden — along with other ways to capture rainfall — during a two-hour drop-in workshop at 10:30 a.m. today at the Cloverdale Library.

More common in wetter climates in the East, Midwest and Northwest, rain gardens are just starting to catch on in California, parched by three years of drought. They are seen not just as a way to capture rainfall to feed the garden, but as a way to reduce runoff, erosion, street flooding and pollution of waterways.

“Roofs have nitrates in the dust and potassium and phosphorus coming from bird excrement. When those are taken through the riparian system it’s detrimental,” said Rick Taylor, who teaches in the Sustainable Landscape Professional Certificate program at Sonoma State. “Nitrogen can cause algae blooms in water, which can suck the available oxygen out. It alters the ecosystem in a profound way.”

A rain garden is one of a number of tools to capture rainwater. Many homeowners are starting to install rain barrels under their downspouts to capture rainwater to feed their gardens. But that requires multiple barrels or a very large tank, and the water can be used up quickly.

Several years ago, Taylor, owner of Elder Creek Landscapes, found himself, almost at the last minute, installing his first rain garden in Sebastopol. He said he was going forward with a standard front yard installation, figuring that it just wasn’t big enough to have any impact if he were to take any water retention measures.

“Then I realized I wasn’t being nearly as creative as I could be, and I looked at what we really could do in that yard.”

He ran a pipe from the downspout to a swale or trench dug into the ground that would guide the runoff to a rain garden, which he planted with carex pansa, a native California grass. For good measure, he added a very small gravel bed beyond that to further capture and filter any water than might spill over the rain garden. All of that was strung out within a space of only about 25 feet.

The whole thing didn’t really cost any more than a conventional drainage plan.

“The budget had built in (an allowance) for grading and drainage. It just took a little more thought and a bit more effort than the standard pipe and trench,” he said.

But creating your own rain garden isn’t too hard. First, you will need to find a location. The best place is at least 10 feet away from the house, in full or partial sun. You also want to make sure the depression is large enough to handle the amount of water you’re going to divert off your roof. Factors that determine the proper dimensions of your rain garden include the size of your roof or surfaces you will be capturing water from, the distance from your collection point and the type of soil you have.

For instance, clay retains more water than loamy or sandy soil, so you’ll need to amend it, typically with 50 percent sand, 25 percent topsoil and 25 percent compost, according to Triol. You’ll also want to add 3-4 inches of mulch. Determine your soil type by digging a hole 12 inches deep and six inches wide and filling it with water three times. Let it drain completely each time. Fill it again. If it drains in an hour it’s sandy soil. If it takes up to 8 hours, it’s loam. If it takes longer, it’s clay.

The Master Gardeners have an 8-page handbook detailing how to build a rain garden, which includes tables and formulas to help you calculate how big you need to make your garden. If it is more than 30 feet from the house, it doesn’t need to be as big because the rain will have had more time and space to be absorbed into the soil.

Sandy Metzger, who has installed her own rain garden in her rural Santa Rosa garden and has written about the topic for the Master Gardeners, said most rain gardens are about 6-8 inches deep and flat at the bottom, with a deeper depression in the middle. You’ll want to divert water from your roof’s downspout with perforated piping or by creating an attractive rock-lined swale or dry creek bed leading from the downspout to the rain garden.

Now comes the fun part — adding plants to gussy it up. Remember, it isn’t a pond, but a garden. Not all plants will thrive in this environment. Triol said you will need to pick plants that can tolerate a lot of water and “wet feet” in winter and need little or no summer irrigation.

Metzger and other experts recommend using natives and Mediterranean plants. Things like California poppies and other native wildflowers, goldenrod, buckwheat, butterfly bushes, milkweed, blanket flowers, verbena, catmint salvia, native ornamental grasses and purple coneflowers, to name a few.

You can reach Staff Writer Meg McConahey at meg.mcconahey@pressdemocrat.com or 521-5204.