In many places, water can overwhelm storm drains and wastewater systems during extreme rains, instead of soaking into the soil. That can cause floods in communities that have more impervious surfaces (think parking lots, sidewalks and driveways) than open space, says Yiwei Huang, an assistant professor of landscape architecture at Purdue University.

“A rain garden works like a catch pond and even small ones, if there are enough of them, can help with flood mitigation,” Huang says. “These types of gardens are a way for all of us to critically think about how we treat water and make use of it versus making it the enemy. They can solve an urban problem in a sustainable way.”

Even in places that don’t typically flood, rainwater often carries pollutants from roofs, streets and the like, dumping them in rivers and streams. The plants and soil in a rain garden can serve as a filter, helping to improve the water quality.

And of course, at a time when water is a precious commodity, an established rain garden can be self-sustaining, decreasing water use. “Think of it as greater-good gardening,” says Jeffrey Longhenry, a landscape architect in New York City.

Here’s what a rain garden is not: It is not designed to retain standing water and become a breeding ground for mosquitoes. It is not a lily pond. It is not a water feature. Water is retained for a day or two at most, then absorbed into the ground.

Building a rain garden of your own is fairly simple and inexpensive. The only costs are any new plants you need and the initial sweat equity of creating the plant …