What Exactly Is a Bioswale?

Updated: Jul. 11, 2023

You've likely already seen these planted ditches on the side of the road. Here's how they work.

With the impact of climate change felt nationwide, many Americans are searching for tangible changes they can make at home to help the environment. From investing in an EV to replacing water-guzzling grass with eco-friendly lawn alternatives, homeowners can make several sustainable choices to nurture their local ecosystems.

One high-impact solution city planners and homeowners alike are turning to is the installation of bioswales. Chances are, you’ve seen a bioswale on the side of the road, but may not have recognized it as such. For the curious and environmentally conscious homeowner, consider this your guide to bioswales: what they are, how to spot them, and how you can build one for your own home.

What Is a Bioswale?

Much like rain gardens, bioswales are meant to absorb water runoff. However, while rain gardens focus on catching runoff from a house’s gutters and yard, bioswales “are designed to capture much more runoff coming from larger areas of impervious surfaces like streets and parking lots,” according to the Milwaukee Metropolitan Sewerage District (MMSD).

Additionally, bioswales typically involve more complex infrastructure, utilizing pipe underdrains and overflow structures, as well as the typical layers of soil and gravel. This allows the bioswale to filter pollution out of stormwater. Both rain gardens and bioswales use native plants to absorb water and prevent erosion.

How Do I Spot a Bioswale?

A Public Bioswale Made Of Drought Resistant Plants, Grasses, Rocks And Mulch.Billy Hustace/Getty Images

To find a bioswale, look alongside the streets the next time you’re out running errands. Bioswales are often placed in medians or running parallel to roads. Then, check the shape of the garden strip. Is it concave, like a drainage ditch? Is the garden filled with landscaping plants and rocks? Are there any drainage grates near it? If so, chances are, you’re looking at a bioswale.

How Can I Build a Bioswale?

The process for building a bioswale will depend greatly on whether you intend to install one on private or public property. Check out our suggestions for each below.

On Your Own Property

If you would like to build a bioswale on your own property, follow these steps:

  1. Choose a location. Your bioswale should run alongside a flat and poorly draining area, like a driveway. Do not build over a septic tank or utility lines, and be sure to call 811 to have your utilities flagged before digging.
  2. Dig the right ditch. The depth of the ditch should not exceed the limit of how much water your soil can absorb in 24 hours. Test this by digging a coffee-can-sized hole in your garden, filling it with water, and measuring how much drains off in 24 hours. For example, if the water in a 12-inch hole completely drains off in 24 hours, your bioswale should be 12 inches deep. The ideal bioswale slope ratio is 4:1, according to the National Association of City Transportation Officials (NACTO), so once you’ve determined the depth of the bioswale, multiply it by four to determine the width of the bioswale slope for each side of the deepest point. In the previous example, the bioswale would be 1-ft. deep and 8-ft. wide. This optimizes both collection and filtration.
  3. Consider an underdrain. If you want the most highly filtered water, install an underdrain, or a perforated PVC pipe. This will encourage water to disperse throughout the soil, but isn’t necessary for smaller installs.
  4. Decorate with rocks. Fill part of your bioswale with splash rocks and decorative rocks, placing the largest rocks in a bottom layer and smaller ones on top. As with creating a bocce ball court, this allows for the best drainage. Have fun with your design, making rock “rivers” or patches in a visually pleasing pattern.
  5. Finish with native plants. Finally, top the bioswale off with native plants. Find which plants are native to your area here and use this guide to score great deals on native plants. Make sure to place high-moisture plants at the deepest point of the bioswale and moderate-moisture plants on the slopes. Whenever possible, use more mature plants to ensure they can endure the rain and use landscape fabric to secure them.

 On Public Property

A Bioswale In A Public Park Constructed Of Dirt, Tree Bark, Rocks And Drought Resistant PlantsBilly Hustace/Getty Images

If you are interested in helping to build a bioswale in your neighborhood, you will first have to consult with zoning professionals, the local government, an engineer and a landscape architect. Permits will certainly be required!

Fortunately, many metropolitan areas are constantly seeking volunteers to build bioswales and have numerous nonprofit organizations dedicated to this task. Check out the below programs for the most populated US cities, or search for volunteer opportunities in your area.

What Else Can I Do to Help the Environment?

The possibilities are endless! For a quick DIY project, try building a rain barrel to collect water for your yard. Or, make a pollinator-friendly garden filled with flowers that support bees. Look into inexpensive eco-friendly upgrades for your home and try to reduce your use of plastic. Every little bit helps, so no matter what appeals to you, know that you’re making a difference.