A rain garden is a specially designed, shallow depression that collects and filters stormwater runoff from impervious surfaces, such as rooftops, driveways, and patios. It functions as a temporary water-holding basin, allowing rainwater to slowly soak into the ground instead of flowing directly into storm drains. The garden’s soil and deep-rooted native plants naturally filter out pollutants like fertilizers, herbicides, and sediment before the water recharges the local groundwater supply. Installing a rain garden improves local water quality and reduces the volume of stormwater runoff, lessening the risk of localized flooding.
Selecting the Best Location and Drainage Assessment
The location of a rain garden requires careful planning for safety and effectiveness. The garden must be situated downslope from the runoff source, such as a downspout or paved area, to naturally capture the flow of water. Place the rain garden at least 10 feet away from any building foundation to prevent moisture from compromising the structure. Before digging, call for utility locations to avoid damaging underground gas, water, or electric lines.
A percolation or “perc” test is needed to determine the soil’s ability to absorb water. This test involves digging a hole 6 to 12 inches deep, saturating the soil, and timing how long the water takes to drain. For correct function, the water should drain completely within 24 hours, indicating a minimum percolation rate of about 0.25 inches per hour. If drainage takes longer than 36 hours, the location may be unsuitable or the soil may require significant amendment.
Determining the Correct Size and Depth
The size and depth of the rain garden are calculated based on the impervious drainage area—the hard surface that drains into it. Measure the square footage of the roof section, driveway, or other hard surface directing water toward the garden. The rain garden’s surface area should typically be between 5% and 10% of the impervious area for a standard 6-inch ponding depth. For instance, 500 square feet of roof drainage requires a target garden size of 50 square feet.
The intended ponding depth, the water held temporarily before overflow, typically ranges from 4 to 8 inches. This depth is measured from the bottom of the basin to the overflow point. If the percolation test indicated slower drainage, use a shallower garden (closer to 3 inches) or increase the surface area to compensate for the reduced infiltration rate. Ensuring the captured water soaks into the soil within 24 hours prevents mosquito breeding.
Step-by-Step Excavation and Shaping
After determining the size and shape, mark the garden’s outline on the ground with spray paint, ensuring the basin bottom is level. Excavation involves removing soil to a depth of 4 to 8 inches, plus any depth needed for mulch or soil amendments. The basin bottom must be flat to promote uniform infiltration across the entire footprint.
If the garden is on a slope, use the excavated soil to construct a raised edge, known as a berm, along the downhill side. This berm holds the water and should be firmly compacted for stability, typically built 6 to 8 inches high and at least two feet wide at the base. The overflow mechanism is a small spillway cut into the berm at the lowest point, designed to direct excess water toward a safe discharge area. Line the inlet where runoff enters the garden with rocks or gravel to manage incoming water and prevent erosion.
Plant Selection for Rain Garden Zones
Plant selection is fundamental to a rain garden’s long-term success. Plants must tolerate both brief periods of standing water and extended periods of drought once the water has drained. Native species are highly recommended because they are adapted to local climate conditions and require minimal maintenance after establishment.
Rain gardens are typically divided into planting zones based on moisture level. The lowest area, the inundation or ponding zone, requires plants that thrive in saturated soil and tolerate temporary standing water, such as Blue Flag Iris or Swamp Milkweed. The side slopes and perimeter, known as the transition or high zone, dry out more quickly and are suited for plants tolerant of drier conditions, like sedges, rushes, or Little Bluestem grass. Selecting a variety of plants for each zone ensures the garden remains functional and visually appealing.
Ongoing Care and Maintenance
Maintenance is relatively simple once the rain garden is established, but requires focused attention during the first one to two years. Newly planted gardens need supplemental watering, about an inch per week, during dry spells until the root systems mature. Initial weeding is necessary to prevent aggressive species from crowding out the desired native plants.
Regular inspection of the inlet and overflow areas is necessary to ensure they remain free of debris, such as leaves or sediment, that could block water flow. If mulch washes away or erosion occurs around the inlet or berm, use rocks or additional soil to stabilize the area and slow the water’s entry. Fertilization is unnecessary, as the garden is designed to absorb excess nutrients from the runoff. If the garden holds standing water for too long, it indicates a drainage problem that may require loosening the soil or adding amendments.