contributed by Rhonda Evans
Rain gardens can provide fun “natural” garden space while providing the double function of retaining runoff onsite. As each lot at High Cove is developed, we are impacting the overall ecology of the High Cove property. A rain garden is one project that can help minimize that impact but should be considered within the overall plan for site development. When developing a lot, the goal is to retain as much of the natural functioning of the site as possible. The first and most important factor is to keep the building site footprint to a minimum and to reduce overall impacts to your site’s “community” of plants and organisms. In addition to retaining as many trees as possible, it is vital to keep the soil intact and the understory intact. Keep in mind that a rain garden is a band aid in comparison to retaining the soil and preventing the removal of the understory vegetation on site in the first place.
Assuming that you have followed the basic first principle of “don’t touch the understory” and kept your building footprint to a minimum, the next step is to estimate the amount of disturbance from the building footprint and the amount of roof, driveway and other runoff you anticipate from your site impacts. This is really an iterative process and watching your site as it is developed is critical to defining the size and location(s) of your rain garden(s). An ideal rain garden captures the water from impervious and erosional areas and provides a filter before releasing the water to downhill and offsite areas. The lots at High Cove have steep gradients making this particularly important for our small streams and natural areas.
To select your rain garden location, begin by observing the site during rainfall events. Where is water flowing from? (roofs, driveways, uphill) Where is the water going? Are there areas of erosion? Ideally, your rain garden should be located between the sources of runoff and the runoff destination. The North Carolina Cooperative Extension Service recommends the following design considerations:
–The garden should not be within 10 feet of the house foundation and should be downslope from the house foundation,
–Gardens should be located at least 25 feet from a septic system drainfield,
–Gardens should not be placed within 25 feet of a well head,
–Make sure to avoid underground utility lines, and
–The best location for the garden will be in partial to full sun (but there are a number of plant choices that can be made regarding sun and soil considerations).
There are also a number of soil and drainage considerations when designing and locating rain gardens. These comments are for High Cove and assume that you are locating your rain garden near your building site in a well-drained, relatively flat location. Rain gardens can be large or small and depend on the drainage area. For High Cove, I would recommend a few small rain gardens to be located in conjunction with rain barrels or cisterns in areas that would capture the first 2 to 4 inches of runoff from your roof, driveway and possibly the adjacent roadway. You can also use corrugated plastic pipe to route water from a distant downspout to a downhill sunnier area on your lot.
To estimate the drainage area, first figure out the square footage of the roof area, driveway area and any other impervious areas draining to the site. By dividing the total impermeable drainage area by 20, you will get a rough estimate of the garden’s area requirements for a water depth of 6 inches. (This will also help you determine if you would like to design separate rain gardens for separate impervious areas or maybe even a couple of smaller rain gardens that are terraced to capture the water.) Keep in mind too that we have steep slopes and may require an overestimate of size. Also, watching your site drainage over time will help you understand the location, dimension and sizing needs for your rain garden.
Once the location and size of the rain garden have been determined, garden construction can begin. The garden should be dug 4 to 6 inches deep with a slight depression in the center. The dug out soil should be used to create a berm along the downhill side of the rain garden. To prevent erosion, the berm should be mulched. For very well drained soils, adding compost to the top layer of the garden assists plant establishment and improves water retention. For compacted soils, adding gravel or mulch may also be needed. Rain gardens should have distinct entrances (uphill side) and exits (downhill side) to prevent erosion. For the entrance, a planted strip or rock border along the upper edge of the rain garden can be used to slow down the runoff water as it enters the rain garden (if piping downspouts to rain garden, make sure the pipe opens up into the rain garden and is not buried). For the exit, you will need to design a weir or nature will do that for you. The wier is an area of the berm that allows water to gently pass over it and is typically about a one-foot wide section of the berm that is several inches lower than the rest of the berm. With the steep slopes on the High Cove lots, using available rocks in the design considerations for the wier and berm will be useful to avoid future erosion.
Once the rain garden is constructed, larger container-grown native plants with well-established root systems should be used. Since a rain garden is designed to capture runoff, fluctuations in water levels will be greater than naturally and it is important to have a rapid establishment of plants. Planting a variety of native plants will ensure a long flowering season and give the garden depth and dimension. It is also important to use native plants that can tolerate a range of drought to wet conditions. However, understanding the range of conditions occurring at your site is also important for determining plant selection.
There are a number of native plant nurseries in the area that can assist with plant selection. The North Carolina Native Plant Society (http://ww.ncwildflower.org/natives/sources.htm) is also very useful. For further assistance with designing a rain garden, contact the North Carolina Cooperative Extension Office (http://www.ces.ncsu.edu/).