Bioswales are patches of vegetation made up of grass, flowers, trees or other plants that absorb storm water runoff, helping to degrade or remove pollutants before it flows untreated into any nearby bodies of water, or into sewer systems. Bioswales can be used to form channels that direct the flow of and filter the water, or they can be placed in strips (sometimes called biofiltration strips or filter strips) to catch water that flows over in thin sheets from paved areas. Some bioswales also include other mechanisms to further direct and filter runoff, such as under-drains and infiltration trenches.
Bioswales remove contaminants like heavy metals, oil, grease and sediment from runoff. They also cool water that has heated up while traveling across pavement before it reaches natural bodies of water, where warmer water could harm wildlife. They can be used in parking lots in place of storm drains, and, in urban areas that don't have a lot of plant cover, they can help prevent sewers from overflowing due to too much rainfall going directly down the drain.
The vegetation will vary by region, and unfortunately, bioswales are not ideal for arid climates. But in places that can support them, bioswales can do a lot of good. They also look like little landscaped parks in some cases, which are more aesthetically pleasing than concrete drainage structures. Bioswales can even end up sheltering small forms of wildlife like butterflies and birds. They're a win-win for nature.