Rain Gardens/Bio-Retention

The issue of dealing with storm runoff from impervious surfaces has put forth the need for best management practices. One of the methods for controlling runoff is rain gardens or bio-retention basins. These systems can be gardens of plants like wildflowers, trees, woody shrubs or grasses that utilize the microbial, chemical and biological properties for removal of pollutants from the storm water runoff. Because the process is relatively new, there has been some controversy over the soils used in bio-retention systems. Some engineers have determined that the soils must drain at a rate of 2 to 4 inches per hour to provide enough ponding time (24 to 48 hours) to remediate nutrients and other contaminates. In some locations standing water is not an option for safety or aesthetic reasons. Unfortunately the complexity of the soils and influx of silt into the system from the runoff can result in the clogging and failure of the system. Most systems using these parameters may result in higher maintenance or total replacement in five years. Other systems propose using higher amounts of compost in the media. If the nutrient content is too high the opportunities for the nutrients to be discharged into a creek or other body of water during the establishment period are greater. The other concern is the aesthetics of the plants. It is difficult to find a plant that can survive saturated soil for a period of time and then try to survive during the dry periods for an undefined period of time. A bed of weedy suffering plants does little to promote the use of rain gardens.


Carolina Stalite Company has been participating in rain garden trials using pre-consumer recycled expanded slate fines along with a 10% organic component for the past seven years. With a higher infiltration rate, all the trial gardens are still functioning with minimal maintenance and the plants continue to thrive despite the rainfall levels. The expanded slate provides additional air space deep in the system providing oxygen for the plant roots. Deeper plant roots provide the perfect environment for the microbes to do their job thereby reducing the need for retention time. The North Carolina Arboretum in Asheville, NC has just completed the installation of a dynamic bio-retention research facility using the Stalite media. The plants are showing impressive growth. It will be interesting to see how the nutrient concentrations reduce as the plants mature. Do not underestimate the value that plants have in this process not only for cleansing the runoff but also by providing an attractive alternative to stagnant holding ponds. If rain gardens remain attractive while at the same time providing a functioning system, more people will be willing to spend the money and effort to incorporate them into the landscape.


 Bio-berms for Protecting Existing Wetlands