A rain garden or “bioretention area” is a planted space that takes advantage of rainfall and stormwater runoff in its design and plant selection. Usually, it is a small garden that is designed to withstand the extremes of moisture and concentrations of nutrients, particularly Nitrogen and Phosphorus, that are found in stormwater runoff. Rain gardens are sited ideally close to the source of the runoff and serve to slow the stormwater as it travels downhill, giving the stormwater more time to infiltrate and less opportunity to gain momentum and erosive power.
On the surface, a rain garden looks like an attractive garden. It may support habitat for birds and butterflies, it may be a formal landscape amenity or it may be incorporated into a larger garden as a border or as an entry feature. What makes it a rain garden is in how it gets its water and what happens to that water once it arrives in the garden.
Below the surface of the garden, a number of processes are occurring which mimic the hydrologic action of a healthy forest. Soils are engineered and appropriate plants selected for the rain garden. The garden is a small bioretention cell in which stormwater is cleaned and reduced in volume once it enters the rain garden. Nitrogen and phosphorus levels and overall sediment loads in the stormwater are reduced by the action of the plants and growing media on the water. Multiple rain gardens over an area will have a positive cumulative effect on both the volume and quality of stormwater run off.
Rain gardens are designed to be drained within four hours after a 1” rain event. This is achieved through the use of highly porous planting media and underdrains which carry the cleaned rainwater away from the garden. As a result, the plants selected for the bioretention cell need to be able to withstand both the extremes of flooding and drought. Plants on the upper edges of the garden are often xeric in their cultural requirement descriptions with plants lower in the garden being more adapted to floodplain conditions. Many riparian edge species are particularly well suited to the extreme environments of rain gardens.
Rain gardens with no underdrain typically hold moisture longer, particularly in the lower areas of the garden. Plants selected for this garden should be able to tolerate inundation for a more extended period of time. However, as in the case of the underdrained rain garden, the surface is drained within four hours, although the soil may be saturated. As in the bioretention cell, soils are amended with a very porous planting media, minimally to a depth of 8” and ideally to a depth of 2’- 3’. The lower the amount of soil amendment added when the garden is built, the more necessary it is to have plants adapted to prolonged periods of wetness. As with the underdrained rain garden, the plants on the upper edges of the garden will need to be more xeric in their cultural requirements than the plants in the lower areas.
In both types of gardens, the ground is excavated and the planting media is imported to the site. The imported planting media should be clean and weed seed free. A liner may or may not be used, depending on the local conditions.
Plants with deep fibrous roots tend to have a competitive advantage in a rain garden and provide the most cleaning and filtration benefits to the environment. Typical rain gardens are populated with natives or native cultivars because those are most well adapted to a locality, but other ornamental horticultural plants that are non-invasive but able to grow in the garden conditions can also be excellent choices.
Most of the examples of rain gardens that are available to review are populated with either herbaceous perennials, woody shrubs or trees. This does not mean that annuals are not a possible choice for such gardens; rather it means that the gardens constructed have been designed for habitat and low maintenance goals rather than purely seasonal aesthetics and color effects. Some annuals are good candidates for a higher maintenance version of a rain garden.
