By: Glen Martin, LLA and DTC Landscape Architect — Rain gardens are a simple way to help improve the quality of stormwater runoff from a development. Rain gardens are shallow depressions designed to hold stormwater runoff from impervious surfaces of roofs and pavements and allow the stormwater to infiltrate into the soil. Rain gardens can be planted with native grasses, trees and shrubs to create an aesthetically pleasing landscape feature.
By holding stormwater, the rain garden allows runoff to be absorbed into the soil and prevents pollution from entering the storm drainage system and nearby water bodies. Rain gardens also help recharge groundwater. Native plants within the rain gardens help absorb stormwater runoff and filter pollutants. By allowing infiltration, rain gardens can reduce the impacts of impervious surfaces from proposed developments. Rain gardens are one technique to help reduce the impacts of stormwater runoff while simultaneously beautifying the landscape with native plants and providing habitat for wildlife.
Step 1. Choose your location. The rain garden needs to be constructed on a level or gently sloping open area at least 10′ from a building foundation. It should be located in a low part of your yard or project area which drains well. A sunny or partially sunny location is preferred.
Step 2. Size your rain garden. If you are capturing stormwater from paved or roof areas you will need to size your rain garden based on those areas. The type of soil in the proposed location will determine the size of the rain garden. In general, rain gardens with sandy loam soils should be 20% of the drainage area size, 30-35% for loam soils, and 45-60% for clay soils.
Step 3. Prepare the rain garden. The rain garden should be 4-8″ deep with a level bottom. After excavating the depression, create a planting bed 8-12″ deep within the rain garden by tilling and amending the existing soil with compost and washed concrete sand. These amendments will improve the soil for the rain garden plants and increase soil permeability.
In areas where existing soils are poorly drained, the addition of perforated pipe below the rain garden can ensure that ponding does not extend for too long of a period creating a nuisance. Also, it is important to ensure that provisions for drainage overflow are incorporated into the layout and design of the rain garden. This way during heavy rains an overflowing rain garden will direct water away from buildings and other sensitive features. Overflows can take the form of small surface swales or piped outlets.
Step 4. Plant the rain garden. Plant the rain garden with a variety of grasses, wildflowers and shrubs native to your area and that can tolerate wet and dry conditions. Mulch the garden with a 2-3″ layer of shredded bark mulch.
Step 5. Water the rain garden. The plants will need to be watered for the first several weeks if there is no rain. This will allow them to get established.
Now sit back, relax, and enjoy your beautiful new rain garden.