CORVALLIS, Ore. — As water runs off roofs, driveways and patios, it can pick up pollutants and flow straight into streams, wetlands, lakes and groundwater.
“Homeowners interested in treating stormwater runoff with rain gardens should first familiarize themselves with how runoff from their roof, driveway, sidewalk and roads are currently being routed and treated.”
For years, stormwater pipes and ditches moved runoff off sites quickly, with little infiltration or treatment, said Derek Godwin, water management and conservation specialist with the Oregon State University Extension Service. That approach reduced ponding but increased pollution and flooding.
Municipalities now use rain gardens — a type of green infrastructure — to slow and treat runoff before it reaches waterways, and they encourage homeowners to build them, too.
Designed and installed correctly, rain gardens act like tiny treatment facilities. Water gathers, soaks into soil and is taken up by plants. Remaining water percolates to deeper soils and groundwater or moves more slowly to nearby streams. Plants and soils filter nutrients, sediment and toxic materials before they reach waterways.
Check your runoff routes first
“Homeowners interested in treating stormwater runoff with rain gardens should first familiarize themselves with how runoff from their roof, driveway, sidewalk and roads are currently being routed and treated,” Godwin said.
Surface flow can be directed into a rain garden if there’s enough area and infiltration. Place the garden downslope from foundations. If excess water remains, route it to a safe location at a slower rate than from impervious surfaces.
If downspouts are piped underground to a curb drain, disconnect and redirect to a rain garden. Check with your local jurisdiction first. If approved, they will outline requirements, including where to send overflow during large storms to prevent erosion and protect nearby homes and roads.
For step-by-step guidance, see The Oregon Rain Garden Guide, which Godwin co-authored.
Design and plant by zones
Rain gardens can be beautiful and provide bird and pollinator habitat. Because a rain garden is designed to drain, you don’t need plants that sit in water for months, such as broadleaf cattail (Typha latifolia).
Areas where water enters are frequently flooded. Use plants that tolerate short periods of standing water and extended saturated soils there. On the rim, choose plants suited to drier conditions.
The guide includes designs and plant lists for sunny and shady sites across Oregon. It also shows which plants fit the garden’s three zones: top (dry), slope (moderate) and base (wet). The U.S. Department of Agriculture’s wetland plants database is another good source for native plant choices. Nurseries that specialize in ponds can help as well.
Establishment and maintenance
Select plants that can handle dry summers with little to no extra water or fertilizer after two to three years of establishment, Godwin said. Keep stormwater out of a new rain garden for about a year so plants and soils can settle.
Aim for low maintenance, but expect routine care: weeding, pruning, mulching and sometimes moving plants that don’t tolerate wet-and-dry cycles.
What not to do
It may seem logical to route runoff to a spot that already has standing water in winter. Don’t. Stormwater rain gardens need well-drained soil to treat water. Avoid adding more water to wet areas; instead, plant wetland-adapted species there to enhance habitat.
Rain gardens slow water and let it soak in, reducing erosion and flooding. Fine sediment carried by fast-moving water harms fish habitat and other aquatic life that rely on clean gravel to feed and spawn. Many pollutants attach to sediment; for example, excess phosphorus from fertilizers can fuel algae blooms and lower dissolved oxygen.
“A rain garden would be like other gardens around your yard — it’s meant to drain; it isn’t supposed to be a wetland,” Godwin said. “The difference is during the rainy season it gets more water than other gardens. You’ll have to choose plants that can withstand higher amounts of water during the wet season. On the other hand, they also have to survive our dry summers.”
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