Build A Rain Garden (Hardwear Sessions)

By Cynthia Houng

In November, after 8 months or so of dry weather, California's storm season begins. These winter rains provide the bulk of California's water supply. Though the state tries to capture and retain as much of the winter rainfall for the coming year, we lose a good percentage of our rainwater as runoff.

There are 2 reasons why California's winter rains, and especially the first heavy rains, have trouble making it into our reservoirs and water tables: (1) California's prolonged dry season, which creates dry, compacted soils that struggle to absorb rainfall; (2) the state's increasing urbanization, and the hard surfaces that accompany urban sprawl, do a poor job of absorbing and holding onto rainwater. As a result, the rainfall does not recharge the water table. Instead, the rainwater heads straight for the ocean.

With California's increasing urbanization, runoff has become a major environmental problem. The Environmental Protection Agency ranks runoff among the top 3 threats to American watersheds. Runoff collects chemicals, heavy metals, and other pollutants, and delivers it to vulnerable aquatic ecosystems. It also enters the watershed with incredible speed and force, literally scouring away sandbars, wetlands, and estuaries.

Hard surfaces, such as roofs, roads, driveways, parking lots (and even lawns) encourage rainwater to run quickly, and in large volumes, into storm drains and out to sea. Unlike meadows, marshes, or woodlands, whose soft surfaces soak up the rainwater and slow it down, these hard, man-made surfaces amplify the water's speed and force. Runoff also causes erosion, washing away valuable topsoil, and in some cases, runoff can trigger mudslides or landslides.

Rain gardens allow gardeners to recreate the natural hydrological cycle on a small scale. These gardens, which are designed to help slow down runoff and sponge up rainwater, can also help improve local water quality, filter out pollutants, decrease erosion, and even help replenish the local water table.

A rain garden should improve the soil's ability to filter and absorb rainwater, so locate your rain garden away from areas with poor drainage. If your property includes an area where water consistently pools during a rainstorm, consider channeling water away from these 'problem' areas, using pipes or dry wash, towards your rain garden. A "dry wash" (akin to a seasonal creek that only runs during the rainy season) can also be used to direct rain water away from the house foundation.

The garden can also include a water feature, like a permanent pond or a vernal pool. Water features attract, and sustain, wildlife, and provide valuable habitat for frogs, salamanders, and other aquatic creatures.

Be sure to locate your rain garden at least 10 feet away from the foundation. (Gardeners with space limitations should consider installing a rain barrel or cistern to catch rainwater for landscape use.)

As you design your rain garden, consider using native plants, which will provide food and shelter for local wildlife, and will also require less maintenance than exotics.

Here in California, gardeners can mimic a variety of natural landscapes, from seasonal wetlands and vernal pools to dry desert washes or fan-palm oases. Consider your site, as well as your local microclimate.

In the East Bay Hills, gardeners are blessed -- or cursed, depending on your perspective -- with a variety of microclimates. Gardeners living in the North Berkeley hills experience a wetter, cooler climate, where ferns, redwoods, and dogwoods thrive. Gardeners in my area -- just a few miles north of Berkeley -- live in a drier, sunnier environment, where the native plant community is best described as "coastal prairie." My backyard rain garden would resemble a wet spring meadow or vernal pool, and include sedges (Carex spp.), fescues, blue-eyed grass, Douglas iris, California checker mallows, mariposa lilies, and other meadow plants.