This story was originally published in May 2019.
As rain pours down rooftops, through gutters, across lawns and into culverts, it collects pollutants and carries these toxins and bits of trash into local rivers and streams. Along the way, rain collects in depressions in the landscape, which often results in soggy patches of lawn and shallow pools that serve as prime breeding areas for mosquitoes.
This can all be fixed with a rain garden.
Rain gardens are beds of plants that are located where rain naturally gathers or where rain is diverted through the use of drain pipes or swales. Designed on a slope or at a low-point on a property, these gardens are filled with plants that can withstand short periods of saturation as well as dry conditions.
“A rain garden allows for water to slow down and soak in,” said Amanda Rockler, watershed restoration specialist with the Maryland Sea Grant Extension Program. “It captures runoff between the runoff source and its destination.”
Each time it rains, the plants in a rain garden absorb the runoff and help filter pollutants from the rest. According to The Groundwater Foundation, properly designed rain gardens can remove up to 90 percent of nutrients and chemicals and up to 80 percent of sediments from rainwater runoff. And compared to a conventional lawn, rain gardens allow for 30 percent more water to soak into the ground.
“More and more, rain gardens are being adopted as a practice to deal with stormwater, specifically to slow down and treat pollutants running off hard surfaces,” Rockler said.
In addition to cleaning the environment, rain gardens are much more aesthetically pleasing than soupy lawns and washed out ditches. And they provide food and shelter for pollinators, songbirds and other local wildlife.
Planning a rain garden
Rain gardens vary greatly in design and size. They can be small and simple, such as a strategically located flower bed that’s watered by a home’s downspout or a sloped driveway. Or they can be more complex, with water directed by rock swales or underground pipes, and overflow structures or outlets for heavy rain events.
“The first one we made was pretty large,” said Cathy Lotzer, who has helped plan and install two community rain gardens in Marshfield, Wisconsin. “We put it in a community park that has soccer fields and a lot of green space and parking lots. So the rain garden was built to filter off some of that salt from the parking lots and runoff from the fields.”
The construction of the two gardens was the work of the Marshfield Area Groundwater Guardian program, which is sponsored by The Groundwater Foundation. But Lotzer has also constructed a personal rain garden right in her yard, where the rain pools after flowing off the roof of her house.
“All I did, instead of building a raised bed, I dug down,” Lotzer said. “It can be just that simple.”
“At your home, it’s a place for your water to go that isn’t a ditch or storm sewer,” she added. “It’s immediately putting [the rain] in your garden, feeding your plants and then recharging the aquifer.”
When planning out your garden, there are several steps you can take to ensure that it effectively filters run-off and the plants in it thrive. These steps include: carefully selecting a location where water flows to or can be diverted to; conducting a soil infiltration test to see how permeable the soil is; calculating a specific garden size based on the size of the drainage area; and deciding how to channel the water to the garden (if needed).
Then it’s time to dig.
Creating a rain garden
The typical rain garden design requires you to dig down 18 to 30 inches to loosen and amend the soil, Rockler said. Depending on what soil you’re working with, you may then need to mix in compost or even add a gravel reservoir to allow for proper drainage. Water in rain gardens should soak into the soil within 24 to 36 hours, according to the University of Maryland Extension.
“It depends on what you find,” Lotzer said. “If you dig down and find a lot of clay, sometimes it helps to put sand or peat or gravel in there so the water has a fighting chance to filter down.”
The rain garden shape will vary depending on your location and preferences, but in general, the garden should be constructed so that its surface gently slopes to a low point that is about 6 inches below the garden’s edge, according to the University of Maine Cooperative Extension. This creates what’s known as a pooling area.
Also, if the rain garden is built on a slope, you’ll need to build a berm or hump on the lower end of it to keep the water in the garden.
Lastly, you’ll need to select your plants. Many factors go into what species of plants you’ll choose for your rain garden, including sunlight, zone hardiness and moisture needs, but for this particular type of garden, selecting native plants may be best, according to the Environmental Protection Agency and several other sources on rain gardens. Adapted to local soils and climatic conditions, native plants typically require less maintenance than non-native species, they attract and feed pollinators, and they are more resistant to local pests and diseases.
“We had nearly 500 plants that we used to cover the 500-square-foot garden we did,” Lotzer said. “Wild petunia, cone flowers, loosestrife, columbine, New England aster, wild geranium — we had 23 different kinds of plants.”
Choose water-loving plants for the lowest part of your garden and dry-tolerant plants for the edges, suggests the University of Maine Cooperative Extension in an online resource. Then cover your garden with mulch to discourage weeds and trap in moisture.
“Don’t be afraid to just experiment,” Lotzer said. “There are so many resources online to help.”
The Groundwater Foundation and several other environmental organizations and universities provide online guides on how to build rain gardens. There’s also a mobile app called Rain Garden for Apple and Android devices, created by the University of Connecticut. The app helps you design and size your garden, choose plants and provides information on how to install it.