by Kevin Beuttell
Traditionally, stormwater was viewed as a burden on the landscape. Water was typically taken away through channels and pipes as quickly as possible to avoid flooding on site. Today, we know water and ecological quality can be improved when water is allowed to infiltrate, using it as a resource where it falls. It is now widely understood that rain gardens use the natural capacities of soil and vegetation to retain and cleanse stormwater as it infiltrates. Appropriate maintenance activities that ensure these landscapes maintain their ornamental appearance and critical environmental functions are less well known, however.
Many rain gardens fail to live up to their full potential because of inadequate maintenance. The responsibility for this situation lies as much with the designer as those directly in charge of maintenance. Ideally adaptive management would be the standard, but we must plan for the typical scenario of landscape crews that do not have the time, knowledge, or budget to carry out a comprehensive maintenance program. To ensure long-term success, it is critical to design rain gardens in ways that require minimal maintenance and help retain their attractive appearance.
The Maintenance Advantages of Grassland Plant Communities
Rain gardens modeled on grassland systems are particularly well suited to this low maintenance design approach. A plant community of herbaceous perennial grasses and wildflowers offers the following advantages:
- Vegetation maintenance consists of mowing and little else. Mowing is a simple non-selective technique that is applied to all the plants in the community. A single annual event of mowing down the previous year’s growth is all that is typically needed to clear the way for the current season’s growth to begin neatly and cleanly, and also keeps weeds under control.
Over time, a thatch layer may develop at the soil surface that keeps the wildflowers from self-seeding effectively, slowly leading to the dominance of the grasses. The presence of thatch has no effect on infiltration performance, but in order to keep the balance from an aesthetic and habitat point of view, supplemental wildflower plantings may be added every few years. Alternatively the thatch can be raked away immediately following the annual mowing.
- No watering or fertilization is needed. These plants thrive during summer droughts, maintaining their ornamental qualities without the need for supplemental irrigation.
- Healthy grasses actively promote and maintain a healthy soil environment. From a soil health and permeability perspective, grasses are the most important component of a rain garden planting. Most of the biomass of grasses is below ground in the roots, even at the height of the growing season. Approximately one-third of a grass root system dies annually, which helps to maintain a good soil structure and porosity (even through slowly accumulating sediment) by providing a continuous source of organic matter. Organic matter is a critical component of healthy topsoil. Organic matter contributes to soil aggregate formation, influences the amount of water available to plants, stores nutrients, and sustains the growth of soil microbes. Aggregate formation is particularly relevant to maintaining soil permeability, which is clearly relevant to stormwater infiltration.
Grassland communities in rain garden applications offer other benefits beyond maintenance as well.
- They create habitat. Designs that provide a diverse assemblage of native species create habitat opportunities for many other organisms.
- They optimize water quality improvement. Grasslands have more root biomass than woodland or shrub plant communities. Studies have shown that nutrient removal is directly related to root density.
- They are beautiful. Some of our most reliable, adaptable, and attractive plants come from our native grasslands. Rain garden aesthetics are critical. Because we cannot see the ecological functioning of the root systems, the water infiltrating through soil, or wildlife’s benefits from the landscape, it is difficult to include an ecological assessment in our judgment of landscape’s appearance. As a result, rain gardens are not economically or socially sustainable if they are not also attractive.
Managing the First Flush and Sediments
In many cities, rain gardens are planted in parking lot islands, thanks to their especially impervious surroundings. But parking lot islands are probably the most challenging environment for a rain garden installation. The first flush of runoff generated in summer can be extremely hot and carry oil and other pollutants. In colder climates like that of Massachusetts, deicing salts and their persistence in the soil are another concern. The first flush of pavement runoff carries a relatively concentrated amount of oils and other contaminants that accumulate on the surface of the pavement between storms. The combination of heat and pollutants severely compromises the quality of water directed into parking lot islands.
A small infiltration trench (potentially top-dressed with more ornamental stone) located between paved surfaces and the rain garden serves as an energy dissipation and bypass strategy to keep the first flush away from the ornamental landscape. The infiltration trench can also serve as a trap for sands and other sediments. By sequestering sediments in this limited and easily accessible area, their removal is a relatively easy part of an annual maintenance program. If a water-permeable geotextile fabric is placed under the first few inches of stone, the removal of sediments is even easier. The fabric can be lifted, and the sediments separated from the stone. Then new fabric and the cleaned stones are returned on top of the trench.
Public participation is another important consideration in rain garden establishment and maintenance. Involving local stakeholders is these activities offers one of the best opportunities to foster a sense of ecological citizenship. Rain gardens are restorative landscapes that use the natural capacities of soil and vegetation to retain, cleanse, and infiltrate stormwater. Public participation in rain garden establishment and maintenance amounts to public participation in nature. It is a restoration not only of natural processes, but also of the human cultural relationship with nature. Rain gardens in urban areas are particularly well suited to public participation in nature. It is here where the most people are closest to the most degraded landscapes. Even small projects like individual rain gardens create important opportunities for people to form relationships with their local environment.
About the Author
Kevin Beuttell, MLA, LEED AP, an Associate with Stantec in Boston, MA, has over 13 years of experience in developing sustainable design solutions for a variety of public and private projects. His focus is on site planning, stormwater management, and native plant community restoration. Kevin has designed and overseen the construction of various types of stormwater management systems including stormwater infiltration gardens, naturalized detention basins, stream-bank and shoreline restorations, green roofs, and porous pavements. He may be reached at Kevin.Beuttell@stantec.com.