A rain garden is a shallow, landscaped depression designed to capture stormwater runoff from impervious surfaces like roofs, driveways, and sidewalks. This specialized basin allows rainwater to slowly soak into the ground (infiltration). The primary function of a rain garden is to improve local water quality by filtering pollutants and sediment before they enter waterways. This process also significantly reduces the volume of stormwater runoff, which helps mitigate localized flooding and recharges groundwater supplies.
Selecting and Sizing the Site
The success of a rain garden begins with careful site selection, which involves identifying the source of runoff and assessing the soil’s ability to drain. The garden must be positioned to receive water flow, often downslope from a gutter downspout or paved area. It should be located at least 10 feet away from any building foundation to prevent water from seeping into the structure. The site must also be situated a minimum of 10 feet from septic system drain fields and should not be placed over utility lines or in areas with a consistently high water table.
The soil’s drainage capacity is determined with a simple percolation test. This involves digging a hole to the proposed garden depth, saturating the soil with water, and then measuring the rate at which the water level drops. A suitable site must drain at a rate between 0.5 and 2 inches per hour to prevent standing water from creating a mosquito breeding habitat. If the test shows an inadequate drainage rate, the soil within the basin will need to be amended.
Sizing the garden involves calculating the total area of the impervious surface that will drain into it, such as a section of a roof or a driveway. For simple residential applications, the rain garden’s surface area should ideally be 10% to 20% of the calculated drainage area. The exact percentage depends on the soil type; slower-draining clay soils require a larger, shallower garden to accommodate the same volume of runoff compared to faster-draining sandy soils.
Constructing the Basin
After determining the appropriate size and shape, the area must be marked and excavated to the planned depth, typically between 6 and 12 inches. The basin floor must be perfectly level to ensure water ponds evenly across the entire surface for uniform infiltration. Any soil removed during excavation can be reserved for use in the berm construction.
If the percolation test indicated poor drainage, the native soil on the basin floor must be amended by mixing in coarse sand and compost to create a more porous medium. The recommended mix is 50–60% sand, 20–30% compost, and 20–30% topsoil to promote rapid infiltration. This new soil mix acts as a filter, allowing water to pass through while trapping pollutants.
The defining characteristic of a rain garden is the constructed earthen berm, a low, raised edge built on the downhill side of the basin using compacted soil. This berm must be stable and watertight to temporarily hold the incoming stormwater. The slope should be gradual, ideally following a 5:1 ratio to prevent erosion and blend naturally into the landscape. A stable overflow or spillway must also be incorporated, often a rock-lined channel or vegetated swale, to safely direct excess water away during extreme storm events.
Choosing the Right Vegetation
Selecting the correct plants is paramount because the vegetation must tolerate both periods of saturation and drought. A functioning rain garden contains distinct moisture zones, and plants should be chosen specifically for their ability to thrive in these varied environments. The deepest area, Zone 1, is the basin floor where water pools and the soil remains wettest.
Plants for this wettest zone should be highly tolerant of standing water, such as Swamp Milkweed, Cardinal Flower, or Blue Flag Iris. Zone 2 is the slope or transition area, which experiences periods of saturation followed by drying. Native species like Joe-pye weed and New England Aster are suitable for these mesic conditions.
The third zone, the rim and the top of the berm, is the driest area, requiring plants that can handle typical garden soil conditions. Drought-tolerant native perennials like coneflowers and lupines, or a small shrub like Serviceberry, are excellent choices for this perimeter. Native plants are preferred for all zones because their deep, fibrous root systems improve soil infiltration rates and help the garden filter pollutants effectively.
Long-Term Care and Inspection
The period immediately following planting requires the most attention, as the newly installed vegetation needs regular watering until the root systems are fully established. Once established, the garden is designed to be self-sufficient and requires minimal supplemental irrigation.
Routine maintenance centers on inspecting the garden’s components. The inlet where water enters and the overflow spillway must be checked regularly for blockages from sediment or debris. Sediment buildup in the basin should be monitored and removed periodically to maintain the proper ponding depth and infiltration rate.
Weeding is important during the early stages to prevent invasive species from outcompeting the desired native plants. Seasonal cleanup typically involves cutting back dead plant material in late winter to make way for new growth. Applying fresh mulch, such as shredded hardwood or leaf litter, helps suppress weeds and conserve soil moisture.