They're both! Rain gardens are typically depressed garden beds where rain accumulates and slowly soaks into the ground. By directing downspouts to rain gardens, we can reduce the amount of runoff that reaches the street, then the storm sewer, and ultimately erodes the banks of our local creeks and lakes. More water soaks into the ground, recharging shallow aquifers and capturing sediment and excess nutrients in the soil; the cooler, cleaner water can then recharge the creeks as baseflow.
Many rain gardens are planted with native plants with long root systems that can withstand longer periods of inundation than varieties we typically find at lawn and garden shops. Just like any flower bed or vegetable garden, some maintenance and possibly thinning will be needed throughout the year. A good mix of plants is best; some that like wetter conditions, and some that like to dry out a bit more in between rain events. Blue vervain, cardinal flower, various sedges, black eyed-susans, and more will provide color and texture at different times of the year, creating a landscape feature that is beautiful and functional.
Check out the Chicago Botanic Garden and the University of Wisconsin-Extension for great tips on designing, building, planting, and maintaining rain gardens.
