Each time it rains, water runs off hard surfaces, such as streets, parking lots, roofs or driveways, and is carried by storm sewer pipes to nearby lakes, creeks or wetlands. The U.S. Environmental Protection Agency estimates that pollutants carried by rain water runoff account for 70% of all water pollution.
These storm sewers were designed to prevent flooding, but not designed to prevent pollution from entering our waters. A solution to this pollution is to mimic natural hydrology by planting rain gardens to capture and filter storm water runoff before it gets to streets. Allowing water to soak into the soil also recharges ground water aquifers, aka drinking water for most of Minnesota.
What makes a rain garden different from a traditional garden? Rain gardens are strategically placed to intercept runoff and designed to temporarily hold and soak in rain water runoff. Rain gardens are effective in removing up to 90% of nutrients and chemicals and up to 80% of sediment (dirt) from the rain water runoff. Although called “rain” gardens, they are actually dry most of the time and are built to drain within a few days to prevent them from breeding mosquitoes. To ensure proper drainage, soils are typically amended with compost and sand.
What types of plants thrive in rain gardens? As a rule, native shrubs, grasses and perennials are best suited for rain gardens, because they are more tolerant of unfavorable growing conditions, such as being flooded one day and dry for weeks until the next rain. Plants that can handle having “wet feet,” such as blue-flag iris, cardinal flower and dogwood, are generally happy in the bottom of rain gardens, whereas most other native plants that prefer a more consistent soil moisture can be planted on the sides of the rain garden.
What’s the best place for a rain garden? Although an existing low-lying area might seem like a natural spot for a rain garden, it is better to avoid these areas to ensure proper drainage. Also, it is generally recommended to stay at least 10 feet from the house and at least 50 feet from a septic system or slopes greater than 15 percent. Water can be routed to the rain garden above ground or below ground using drain tile or corrugated pipes. A typical rain garden for a residential home or small building is between 100 and 400 square feet.
Regardless of the size, each rain garden can make an impact. The rain garden pictured takes runoff from the street saving about a half pound of phosphorus and 240 pounds of sediment each year from reaching nearby waters. To learn more about rain gardens, visit bluethumb.org.