This article was originally published on May 26, 2022.
As urban areas expand throughout the planet, the more challenges develop with water treatment. Sewage is one thing, but storm runoff is also a problem. The rain that falls on cities and towns picks up pollutants or nutrients before it washes into waterways.
Rain gardens are one way to alleviate some of the pressure from water runoff. They act as sinks, absorbing the precipitation back into the soil where plants or trees can use it.
“It’s a low-cost way to treat water, to take pollutants out of the water and improve the water quality,” says David Deegan, a spokesperson for the U.S. Environmental Protection Agency’s Boston regional office.
Why a Rain Garden is Important
Before we invented cities and concrete towns, forests and grasslands had a natural way of dealing with the runoff from rainstorms. The soil would absorb this water and the roots of plants and trees would suck much of it up before it had a chance to seep into nearby rivers and lakes.
But as cities grew larger and larger, relying more on concrete sidewalks, roads and glass building fronts, these new structures diverted the rain in large areas into sewers. The runoff then drained into rivers, where sometimes water treatment facilities would filter.
“Rainwater is hitting roofs, parking lots and streets, and running off to waterways next to them,” says Deegan. “You’re concentrating water in fewer spaces.”
The issue is rain picks up pollution as it travels through cities, whether it’s garbage, fertilizer or chemicals like road salt in the winter in northern regions. In smaller towns, or cities without water treatment facilities, these nutrients can change the ecology of waterways and other bodies of water. They can result in an increase in blue-green algae (cyanobacteria) blooms, which can cause problems for the health of humans and wildlife.
“We have concerns in a lot of water bodies in the Northeast for elevated nutrients and the impacts that causes,” Deegan says.
It can also create a bad aesthetic for towns that could affect tourism and general peace of mind. “If the water looks kind of green or scummy, you’re probably not getting in for a dip,” Deegan says.
Cities with water treatment facilities can also find runoff costly. Some sources estimate cities need billions of extra dollars to manage stormwater across the U.S. The EPA has even created a public outreach program to help people reduce stormwater runoff called Soak Up the Rain.
But there are other reasons to build a rain garden other than pollution and the regional water supply. These landscape features can also help properties control water run-off problems. There might be swampy parts of your yard near the roof drain. Or water may gather in the low parts of your yard during wetter seasons. Rain gardens can help drain and absorb some of this excess water runoff.
What is a Rain Garden?
If residents and businesses implemented rain gardens, they could fill in some of the funding gaps. Essentially, they are nothing more than fancy holes. You can dig ditches into the side of the road, or intersperse them between the stalls in parking lots or create a garden with specific infrastructure underneath. The idea is to give rain a place to gather locally before it hits the storm drain, or just washes off a parking lot into a nearby creek.
Aside from retaining the water locally, many plants in rain gardens also help filter excess nutrients that might run off from fertilizers in lawns and gardens. They can filter out some chemicals, stopping them from reaching waterways, and ultimately our oceans, through absorption, decomposition and other processes.
The region can vary the type of rain garden you might need as it depends on the local ecology and the amount of rain. But the general rain garden needs a base area of small rocks or sand. The EPA has a database that can help determine what plants might work best in a rain garden depending on your area.
How To Build One
On a simple level, it all starts with digging. When creating a rain garden, you should dig down anywhere from one to five feet deep — though even as much as half a foot might suffice, depending on the area. Next, fill the hole with larger stones, then cover that with a layer of smaller stones roughly the size of a large marble, or a walnut.
“You basically want to dig a pit and put in materials to increase the drainage, then put in a layer of sand,” Deegan says.
Depending on your aesthetic taste, you can either leave rocks exposed on the surface, or you can cover them in sand. You can also add some landscape fabric that keeps sand and other sediment in place. Next, you can lay down a layer of topsoil or mulch.
Taste and region may vary your plant options, depending on what’s suitable locally. Deegan says that ornamental grasses work well, but you could also plant flowers and shrubs. This would make it look more like a garden and less like a culvert or construction feature. Here’s one example of how to build a rain garden, focusing on New Hampshire. Other examples can be found on the Soak Up the Rain website. You can highly personalize these techniques in yards, but they are also useful in larger properties or businesses.
“They can be really pretty decorative,” Deegan says.
Small towns or municipalities can find rain gardens especially useful since their budgets are smaller and they wouldn’t need to invest in storm water drainage or treatment, Deegan says.