A bioswale is a low-lying, linear depression that directs the flow of water while letting it percolate into the soil. This process is known as bioretention: using biology to retain, or slow, water.
How do bioswales, streetscapes and rain gardens work?
Bioswales, streetscapes, and rain gardens apply the same strategy in different formations: they use soil and plants to manage stormwater and reduce erosion in cost effective, environmentally-friendly manners. The plants and soil – or bioretention media – serve as natural filters that remove silt and pollution from runoff water.
How do bioswales remove pollutants?
Bioswales behave like mini constructed wetlands. After a rain, water flowing off of surfaces – typically roofs or roadway – gets diverted into beds of hardy grasses and other plants. Then, the following occurs as illustrated in the photo:
- Soil in the bioswale catches contaminants in the runoff.
- Oil and metal contaminants are broken down by soil microbes. This changes their chemical structure so they are no longer toxic.
- Contaminant-eating microbes need oxygen to work. Wetland plants bring oxygen into the soil.
Water leaving the bioswale is cleaner than when it came in.
What are other ways bioretention soils help reduce pollution?
- They slow the flow. Water that falls to the earth during rain events has a chance to get absorbed by the earth and return to the groundwater table, as opposed to getting whisked away by sewers and drains.
- They filter out pollutants. Ideally water that reaches surrounding lakes, streams and bodies of water does not have debris or pollutants.
What elements of rain gardens should I look for in a rain garden, streetscape, or bioswale?
A cross section reveals four different layers:
- Dry – The top where floodwaters never reach. Drainage here is good because it’s at the top of a slope.
- Mesic – This level, just below the dry zone, experiences occasional, brief winter flooding and summer drought.
- Moist – The zone approaching the bottom that experiences frequent winter flooding. The number of plants that can grow here without summer water is limited.
- Wet – The bottom of the swale will be saturated for a large portion of the year; water plants can be grown here if supplemental water is given in the winter. Without summer irrigation, fewer plants can grow here.
How do I build a rain garden or bioswale in on my property?
A rain garden is a planted depression where run-off from roofs, driveways and other surfaces is directed so that it can soak back into the soil naturally rather than run into storm drains. The soil and plants in these areas filter out some impurities before the water drains into sewers, groundwater, rivers and streams.
- Choose a location: Choose a spot where water can be easily directed through the landscape or from downspouts. Make sure to leave at least 6 feet from your house, and allow for overflow away from foundations and other structures.
- Prepare the site and soil: Amend the soil so the mix is roughly 50% native soil, 30% soil amendment or compost-based product, and 20% pumice.
- Mulch: Two kinds of mulch are important in a rain garden. A mulch of pea gravel or river rocks at the point where water enters will help prevent erosion; this mulch should be thick enough that no soil shows through. The rest of the rain garden should have a high-quality soil amendment or compost-based product 1-3” deep added once a year as spring rains taper off. This will help suppress weeds and maintain moisture levels during dry periods.
- Water during the first couple years: All plants (even drought-tolerant ones) will need supplemental watering the first 1-2 years until they are well established.
- Avoid fertilizers and pesticides: These should both be avoided in your rain garden whenever possible; part of the goal is to help keep these synthetic chemicals out of local waterways. If necessary, use granular, low phosphorous, organic fertilizer, and the least toxic pesticide available. Plant selection is critical; consult with the nursery specialist for plants that will thrive in this environment.
How does compost fit in to bioretention?
Specially formulated soils rich in organic matter help provide both short-term and long-term positive impacts on soil structure. With compost, the soils resist compaction in finer soils and provide greater drought resistance and water holding capacity in coarse, sandy soils. Soil porosity is key in soil structure and the coarse organic texture of compost creates an environment for better root development. Compost increases Cation Exchange Capacity (CEC) which is the ability for soils to retain micro nutrients for the plant to utilize and lowers nutrient leaching. Compost supplies many beneficial micro-organisms and nutrients to soils and growing media as well as bind and degrade specific pollutants – a strong characteristic in bioretention soil use.
The benefits of using compost for plants, the environment and completing the recycling loop in our communities are tremendous. As a plant organic and nutrient source compost works with soil biology naturally to increase soil organism activity. This relationship between planting soils and compost derived from green waste can support a wide variety of soil amendment needs for growing plants, stormwater management and soil erosion.
Harvest offers custom blends in almost all of its markets. Our soil specialists can help meet your needs. Minimum quantities apply.