Rain Gardens are sunken planted areas that slow, capture, clean, and often infiltrate stormwater runoff from impervious surfaces. Benefits of rain gardens include recharging groundwater, increasing in-stream flows, reducing downstream flood risks, and providing habitat for birds, insects, and small animals. Rain Gardens are critical living infrastructure for healthy water systems.
Rain gardens work best in gently sloped areas. Sites with steep slopes, near fault lines are not well suited for rain gardens. Though sites with heavy clay soil do not absorb rainwater quickly, rain gardens on these sites can greatly reduce irrigation need and improve drought and fire resilience. Well-drained soils, especially those located in groundwater recharge zones are perfect places to harvest stormwater in rain gardens.
Costs for rain gardens are generally very low in comparison to other methods of water harvesting, like rain tanks. For residential sites, costs can be as low as the price of a shovel, a simple water level, climate appropriate plants and some mulch. For larger projects involving high capacity detention ponds, constructed wetlands, and street retrofits with bioswales, costs increase. Rain gardens, however, are one of the most economical ways to harvest the rain.
WHY CAPTURE STORMWATER?
Stormwater is free, and in urban and suburban areas in particular, rooftops, parking lots and roads generate a huge amount of it. When stormwater rolls into creeks and streams it is often hot, and almost always carries pollutants like motor oil. Rain gardens can help clean, cool and slow down stormwater runoff. This improves water quality and fish habitat in our rivers, streams, and ultimately in the ocean.
DESIGN NOTES & CONSIDERATIONS:
- Learn about your soil and the slope of your site. Observe where water moves and where it sits during a rainstorm.
- Test how quickly your soil drains. Dig a 1 square foot hole and fill it. Time how long it takes to drain. If it does not drain within a day, your site may be better suited for a pond or constructed wetland! Rain gardens must drain within 24-48 hours to avoid mosquito breeding.
- Do a bit of math to estimate how much water you can capture in your rain garden. If you are routing roof runoff or overflow from a rain tank to your rain garden, remember that a 1 inch rainfall on 1000/square feet generates roughly 600 gallons of runoff.
- Plan your overflow route, and be sure that water flows through it, not back towards your home or another structure.
- Keep rain gardens between 1-2 feet deep. Trees and other plants around your rain garden can only benefit if their roots can reach soil moisture. Most plant roots only reach 1-2 feet in depth.
- Mulch your rain garden. Use organic material at least 6 inches deep to help absorb stormwater and hold it in the soil. Mulch will also help reduce weeds and support plants through hot and dry times by shading the soil and keeping their roots cool.
- Plant a variety of native plants appropriate to your area inside and around the rain garden. Most plants and trees will prefer to live around the edge of a rain garden, while riparian water-loving plants like rushes and sedges will do best inside the rain garden.