As water runs down roofs, over driveways and patios and off other impervious surfaces, it may pick up pollutants as it flows directly into streams, wetlands, lakes and groundwater aquifers.
Stormwater runoff is often routed directly to streams through stormwater pipes and ditches with little infiltration or treatment, according to Derek Godwin, Oregon State University Extension Service water management and conservation specialist. That’s because the focus has been on getting the water off the property quickly and efficiently.
However, stormwater runoff has become the No. 1 source of residential pollution for waterways. It also increases potential flooding.
Municipalities have responded, installing rain gardens—a type of green infrastructure—in public and commercial spaces to slow down and treat the water before it enters streams, wetlands, lakes and groundwater aquifers. They encourage homeowners to build them, too.
Designed and installed properly and planted with the correct plants, rain gardens are like tiny water treatment facilities. Water gathers in the rain garden, soaks into the soil and is taken up by plants. The water not used by plants moves into deeper soils and groundwater or flows more slowly to nearby streams. The plants and soils filter nutrients, sediments and toxic materials from the runoff before it gets to waterways.
“Homeowners interested in treating stormwater runoff with rain gardens should first familiarize themselves with how runoff from their roof, driveway, sidewalk and roads is currently being routed and treated,” Derek says. “Runoff water moving on the surface could be incorporated into a rain garden design with sufficient area and infiltration rates. Runoff drains into the rain garden downslope from foundations. If it can’t all be absorbed, it flows out to a safe, designated location at a slower rate than from impervious surfaces.”
Derek recommends checking with your local jurisdiction to see if building and routing runoff through a rain garden would benefit water quality, reduce flooding and be permitted for individual property conditions. If approved, the municipality will provide any requirements for safely routing water during large storms to prevent erosion and protect nearby homes, roads and other infrastructure.
Rain gardens can be designed with a variety of plant types and sizes that offer beauty, and bird and pollinator habitat. Because the garden is designed to drain, you don’t need plants that withstand months sitting in water. However, areas at or near where water enters the rain garden are frequently flooded and require plants that survive short periods of standing water and extended periods of saturated soils, while plants on the rim should be suited for drier conditions.
Derek recommends selecting plants that can withstand dry summers with little to no extra water or fertilizer after they’re established in two or three years. He also suggests keeping stormwater runoff out of the rain garden for at least one year to allow plants and soils to settle in. The goal is low maintenance, but, like any areas of the garden, it will need some upkeep.
For more information about rain gardens and how to build one, refer to The Oregon Rain Garden Guide at http://tinyurl.com/5bzy9rd2.
Information courtesy of Oregon State University Extension Service.