Even with a rain barrel in your garden, if you have more downspouts than barrels, or a low section of the yard, you will still need to deal with excess water running off your roof, sidewalks, patios and other impervious surfaces. Planting a rain garden is a perfect way to treat storm water runoff, and maintain a beautiful landscape at the same time.
Plant selection for rain gardens varies from region to region, but the overall goal remains the same: to slow runoff from hard surfaces, filter and return the water to the underground aquifer, while reducing erosion and pollution from runoff.
Characteristics of a Rain Garden
Rain gardens are designed to fully drain by four hours after receiving 1 inch of rain. There are three areas of the garden: the outer ring, which dries fastest; the middle ring which generally dries out within the 4 hour time span; and the center, or lowest part of the garden, which may take longer to dry. All plants in the rain garden need to withstand the following conditions:
- Extreme water fluctuations from flooding to drought
- Concentrated amounts of nutrients
- Heavy metals and other pollutants that run off impervious surface like roofs and driveways
- Lightweight soil that helps the garden drain quickly
Because the gardens are designed to quickly treat water and allow it to soak into the ground, soil and soil amendments are usually added to the native soil where the rain garden will be planted. Lightweight, fast-draining soil is a must to ensure that the rain garden functions properly. As an alternative to major soil amendments, rain gardens planted in heavier clay soil can be larger than those planted in heavier soils.
Siting and Planning a Rain Garden
A rain garden fits well in the lowest area of a garden. Water from impervious surfaces will naturally flow down to that area, so it makes sense to situate the garden in a depression. A few areas where you should not site a rain garden:
- Right next to the house
- On high ground
- Near utility lines or pipes
The best spot for a rain garden is in a naturally occurring depression in the ground where water from the downspout, driveway or sidewalk will naturally flow, or can be easily directed. The garden should be situated perpendicular to the flow of water into the garden, in order to catch and hold the most runoff.
For a rain garden to be most effective, it needs to be large enough to treat the amount of water that will flow into it after a 1 inch rain. An overflow area is also a good idea, in case more rain than the garden can handle falls within a short period of time. A grassy area or other garden is enough of an over-wash area. Anything planted, and not impervious, will allow overflow to still be treated. Here are some statistics for size of garden vs. volume of water treated:
- Sandy soil drains twice as fast as soil with a high clay content
- Plant a garden that is between 20% and 80% as large as the area contributing runoff to the garden. Sandy soils need a smaller garden, heavy clay soils need a larger area.
If more than 300 square feet is needed for rain gardens to treat all of the rainwater runoff, split the garden into two. It will be easier to care for.
Plants for Rain Gardens
Plant selection varies significantly by climate region. Here are some suitable plants for rain gardens around the United States. (Check with your local cooperative extension or regional horticultural publication to see which plants are native to your area.)
Shrubs: American Beautyberry, Bottlebrush Buckeye, Buttonbush, Winterberry Holly, Inkberry, Oakleaf Hydrangea, Rose of Sharon, Summersweet Clethra, Virginia Sweetspire, Wax Myrtle
Perennials: Asters, Blackeyed Susan, Blue Lobelia, Cardinal Flower, Cinnamon Fern, Goldenrod, Ironweed, Joe-Pye Weed, Liatris, New England Aster, Red Columbine, St. John’s Wort, Swamp Hibiscus, Switchgrass
Handling Excess Rain Barrel Runoff
With heavy rains, rain barrels will need an over-flow runoff area. To comprehensively treat your stormwater runoff, make sure to route your rain barrel runoff to your rain garden, as well. You will then rest assured that you’re doing everything you can to treat and return runoff to the natural water table.