How to Build a Rain Garden That Actually Works
A rain garden is not a pond and not a swamp. It is a shallow, planted depression that catches runoff from a roof, driveway, or lawn, holds it briefly, and lets it soak into the ground instead of rushing to a storm drain. In the Pacific Northwest, where winter brings long, soaking rains and summer brings months of drought, a rain garden has to do two opposite things well. The plants you choose and the soil you build determine whether it succeeds at both.
Start with the soil, not the plants. Most Northwest yards sit on compacted glacial till or heavy clay, and water will not infiltrate either one at a useful rate. The standard fix is to excavate six to twelve inches and backfill with a mix of roughly sixty percent sand, twenty percent topsoil, and twenty percent compost. This blend drains quickly in winter but still holds enough moisture for roots in summer. Test infiltration before you plant: fill the basin with water, let it drain, and fill it again. If the second fill drains within a day or two, you are in business.
Siting and Sizing Without Guesswork
Place the garden where water naturally wants to go, downhill from your downspouts or at the low edge of a lawn. Keep it at least ten feet from your foundation and away from septic fields and utility lines. As for size, a rain garden typically wants to be somewhere between ten and twenty percent of the area draining to it. A thousand-square-foot roof does not need a thousand-square-foot garden; a hundred to two hundred square feet of well-built basin will handle a surprising amount of water if the soil drains.
You do not need a permit for a small residential rain garden in most jurisdictions, but check before you dig, especially if you are in a floodplain or near a wetland. And always call the utility locator before your shovel goes in.
Planting for Two Seasons at Once
The trick of a good rain garden plant list is that every plant must survive winter saturation and summer drought. That rules out most conventional perennials and points straight at our native wet-meadow species. In the center, the lowest and wettest zone, use slough sedge, tufted hairgrass, and soft rush. These tolerate standing water for days and then tough out August. Around them, in the middle zone, plant red-twig dogwood, Pacific ninebark, and clustered wild rose, all of which handle fluctuating moisture and provide structure and wildlife cover.
On the upper rim, where it is driest, use Oregon grape, nodding onion, and yarrow. These bridge the gap between the rain garden and the surrounding landscape. Mulch the whole basin with coarse wood chips, but keep mulch away from plant crowns so they do not rot. In the first summer, water deeply every week or two. After that, the garden should run on rain alone. Expect it to look ragged in February and glorious in June, which is exactly how a Northwest rain garden should behave.