Bioswales are shallow channels dug into a gentle slope to direct rainwater into a specifically designed area where it can be absorbed naturally and filter off toxins in runoff before they contaminate natural water supplies. Plants are the perfect way to utilize these toxins and excess nutrients, converting them to vegetative matter that can be removed by pruning.
The most common plant for a bioswale is grass. Usually the bottom channel at the very least is planted with a thick covering. The grass can cover the sides and the edges as well. Some bioswales are planted with grass only. The grass used needs to be drought resistant, but also tolerant of extended wet and potentially saturated periods. Typically you want the grass to be able to live on rain water alone, even during the summer.
Check for a list of native or suitable species that are recommended for bioswales in your specific climate. Often the types of grasses you can use are limited to those mandated by local environmental or building codes.
When planting seed, try to plant when there is little threat of storms so the seeds have a chance to germinate and root firmly before any water floods the bioswale, potentially washing them away. If rains can't be avoided, it may be necessary to build temporary dams to prevent water from draining into the bioswale until the grass is established.
Sod is an alternative to seeding, and it establishes quicker. Any holes or parts of the sod that do not take root need to be filled in immediately to prevent erosion. Keep the grass height at least six inches above the level of the water surface during maximum expected capacity.
Woody Plants and Trees
Along the sides and edges of the bioswale is a good place for shorter shrubs and woody stemmed plants. These help hold the soil in place during periods of high water flow, and the stems and branches help slow the progress of water and debris as it flows through the bioswale. They also can add color or hide a large bioswale, turning it into a rain garden. Use native shrubs or shrubs that can withstand dry periods when there is little rainfall to fill the bioswale.
Trees along the upper edges of the bioswale also help hold the soil in place and prevent erosion. They can help slow the progress of the water into the bioswale, increasing its contact with the ground. Caution should be taken when planting trees near bioswales because their leaves have the potential to fill it with sediment that will require more-frequent maintenance.
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