Rain garden installed to capture polluted stormwater runoff from an asphalt parking lot.

Rain gardens are popular stormwater control measures that are non-irrigated, planted landscape features designed to capture polluted stormwater runoff from impervious surfaces. They are built by excavating and creating depression areas within the landscape so that the stormwater can be captured and allowed to infiltrate (1). After excavation they are refilled with an engineered filter bed substrate and planted. An environment is created within the rain garden where adsorption, filtration, sedimentation, volatilization, ion exchange, plant uptake and biological decomposition occur (3).

Sand based filter bed substrates are generally recommended due to their slow drainage (2). In North Carolina, these sand filter bed substrates are often 85-88% by volume sand, 8-12% fines (silt and clay), and 3-5% organic matter (3). It is currently recommended to use pine bark for the organic matter which has low P content, low cation exchange capacity and does not bind many pollutants. However, there are potential alternative filter bed substrates such as slate, organic matter sources such as compost and methods of adding organic matter that can support plant growth and remediate polluted stormwater runoff similar to or better than the recommended sand filter bed substrates. The main objective of this research was to determine the effect from the addition of different sources of organic matter amendments to rain garden filter bed substrates on plant growth.

Two rain garden plants (Panicum virgatum L. ‘Shenandoah’ and Monarda fistulosa L.) were grown in thirty-two substrates that resulted from combinations of two filter bed substrates, two organic matter sources, two combination methods, and eight different combination amounts. The two filter bed substrates used were sand and slate. Both, sand and slate were amended with two different organic matter sources: pine bark and composted yard waste. Pine bark and composted yard waste were added as either a band in the depths of 1, 2, 3, or 4 inches or by incorporation using approximately the same amounts of organic matter in the amounts of 5, 10, 15, and 20% (vol./vol.).

Panicum virgatum ‘Shenandoah’ shoot growth in 100% slate
(left), slate amended with a 4” band of pine bark (middle),
and slate amended with a 4” band of composted yard waste
(right).

Rain gardens are one of the more utilized stormwater control measures because they are able to fit into many different types of spaces (small or large) unlike other options. They also provide numerous ecological benefits and if planted appropriately can be aesthetically pleasing. For this study, both sand and slate filter bed substrates created a suitable environment for plant growth. With the addition of composted yard waste, both species were larger than when pine bark was added to the sand or slate filter bed substrates.

Elizabeth D. Riley1, Helen T. Kraus1, Ted E. Bilderback1, and James S. Owen Jr.2

1 Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695
2 Department of Horticulture, Virginia Tech University, Virginia Beach, VA 23455

Literature Cited:

1. Dietz, M. E. 2007. Low impact development practices: A review of current research and recommendations for future directions. Water, Air and Soil Pollution, 186, 351-363.
2. Hsieh, C., and A.P. Davis. 2005. “Evaluation and Optimization of Bioretention Media for Treatment of Urban Storm Water Runoff.” Journal of Environmental Engineering 131.11: 1521.
3. North Carolina Division Environment and Natural Resources (NCDENR). 2009. Stormwater Best Management Practice Manual. http://portal.ncdenr.org/web/wq/ws/su/bmp