Raingardens
Transistions garden at Ellerslie Show. Image - Judy Grindell
Raingardens are shallow depressions, typically planted with plants (preferably native), strategically located to collect, infiltrate and filter rain that falls on hard surfaces like roofs, driveways, alleys, or streets to minimise negative impacts of excessive runoff from these surfaces on lakes and streams.
Raingardens are designed to take the place of your stormwater system and soak up and filter the water that comes off your roof and paved areas.
Other terms include mini-wetland, stormwater garden, water quality garden, stormwater marsh, backyard wetland, low swale, wetland biofilter, or bioretention pond. The variables include dimensions, design, engineering components, and plant selection.
Building a raingarden
The design of a raingarden can be varied to accommodate soils, watershed hydrology, existing drainage patterns, aesthetics, microclimate, and purpose.
Basic design components include:
- Grass filter strip
- A shallow surface-water ponding area
- A bioretention planting area
- A planting soil zone
- An underdrain system
- An overflow outlet structure.
There is no standard size for a raingarden. One formula provides that the bioretention area should be 5% to 7% of the drainage area that the raingarden is intended to accommodate. A raingarden should be placed near impervious surfaces so that rainwater will drain into the dip or depression. Locate the garden strategically near impervious surfaces, such as alleys, sidewalks, driveways, and under downspouts or gutters, to capture the rain as close as possible to the point where it falls.