One of the sessions at the Native Plant Festival at Shaver’s Creek was on rain gardens. Time constraints and choice of material left large holes in the presentation through which, those of us who were not familiar with the particulars, quickly tumbled. Considering the amount of digging and soil shifting involved in constructing a rain garden it is doubtful that any of us are likely to be building one anytime soon but at least we, as gardeners, should know the basics of this latest fad.
The purpose of a rain garden is to divert the water that comes off roofs, through downspouts, across lawns, down driveways or streets and into storm drains or streams. During its journey this water can not only cause erosion but it picks up chemicals from pesticides, fertilizers, the latest home project or automotive adventure. Excessive runoff causes flooding and overtaxes sewage systems. A rain garden contains this run-off and allows it to be filtered by natural soil processes. On a typical suburban lot runoff can be reduced 25% by channeling the water from one downspout into a rain garden. Sediment is removed, volume of water is reduced and the plants reduce the amount of nitrogen and phosphorus in the run-off. The clean water then filters into the ground and ultimately into the water table or a stream.
There are two types of rain gardens; the under drained system uses very porous mediums and drains to prevent the cleaned water from filtrating into the surrounding soil. There are a number of reasons for limiting filtration into adjacent soil. One is volumes of water that are too copious for the soil to absorb. Another is that the surrounding soil would recontaminate the water because of the presence of pollutants. The under drained garden is designed to drain within two hours after one inch of rain has fallen. The second type of rain garden is the self contained one where the water filtrates into the surrounding soil. These are designed to drain within four hours after one inch of rain. The soil, of course, is not dry after these periods but the surface water is gone.
It would seem obvious to place a rain garden where water naturally accumulates. This is discouraged because these wet areas are indications that water does not filtrate into the soil. Suggested location areas are near rainspouts but at least 10 feet from the house so the water does not drain into the foundation. They should not be placed near wells or septic systems for the same reasons. A reasonably level spot should be used unless you are prepared to dig one end deeper or bring in fill. The bottom of the rain garden must be level. The spot should be in full sun for the benefit of the plants that will inhabit the garden.
The size and depth of the garden is determined by the amount of water that it is required to handle. There are complicated formulas to address the issue but the good old internet provides calculators that even take into consideration soil type and slope. There is one here http://raingardenalliance.org/right/calculator. If the source of water is a roof, which it normally is, the first step it to determine the square footage of the area that will be draining into the rain garden. That would be the area drained by one downspout unless the plan is to channel more than one into it. If there will be driveways, lawns, or sidewalks draining into it these must be taken into consideration as well. The simple formula is to figure 20% to 30%of the area draining into the downspout for the size of the garden. So a 30X30 inch roof with 4 down spouts would require a garden that covers 45 to 68 square feet. (30×30/4×20%) However this doesn’t take into consideration soil type and slope. When we use the calculator setting it for clay soil and a flat area we get an area of 113 sq feet. We must digress here to explain that the size is also dependent on the depth. It is advised that a rain garden on flat slope be dug 4 inches deep, that on a moderate slope 6 inches and that on a steep slope 8 inches. One dug 8 inches deep would works out at 56 square feet. This is what is needed to handle one inch of rain off of ¼ of at 30×30 foot roof. Making the garden too small or too shallow will result in flooding.
For aesthetic reasons rain gardens are usually given an irregular shape. The shape however has nothing to do with the efficacy of the garden. The depth and soil does. The soil in Western Pennsylvania tends to be heavy clay. It might be possible to dig 4 to 6 inches and still be digging top soil but in many areas at a depth of 8 inches the gardener will probably be moving clay. It is probably advisable to amend the soil with peat moss, compost or other materials that will provide better drainage and a better medium for plant growth. Since a rain garden is a basin there will be extra soil. This should be mounded around the edges to prevent overflow. Once the garden is dug it is advisable to check to see if it is functioning properly before planting. Water should drain in a few hours but definitely be gone in one day.
The rain garden will have three sections and the amount of wetness in each must be taken into consideration when choosing plants. The center zone will be the wettest. Plants for the wet zone will have to be able to tolerate “wet feet”. The next zone from the middle will require plants that can tolerate moist soil and the zone at the boundary will host plants that have more normal water requirements. For minimum maintenance the use of native plants is suggested. Recommendations from Penn State Cooperative Extension are:
blue flag iris (Iris versicolor), common rush (Juncus effuses), prairie cord grass (Spartina pectinate), golden alexander (Zizia aurea), swamp milkweed (Asclepias incanata), Virginia mountain mint (Pycnanthemum virginia) blue vervain ( Vervena hastata) and sneezeweed (Helenium autumnale).
wild blue phlox ((Phlox divaricate), mistflower (Eupatorium coelestinui), great blue lobelia (Lobelia siphilitica), ironweed (Veronica fasciculate), bee balm (Monarda fistulosa), and New England aster (Aster novae-angliae).
bee balm (Monarda fistulosa), false blue indigo (Baptisia australis), Culver’s root (Veronicastrum virginicum), foxglove beardtongue ((Penstemon digitalis), dense blazingstar (Liatrus spicata), Joe Pye weed (Eupatorium purpureum), Ohio spiderwort (Tradescantia ohiensis) and purple cone flower (Echinacea purpurea).
If the area is large enough serviceberry (Amalanchier laevis or Canadensis) can be planted in the outer zone and button bush (Cephalantus ocidentalis), summersweet (Clethra alnifolia), black chokeberry (Photinia melaanocarpa) and winterberry holly (Ilex veticillata) can be planted in the wetter zones.
A rain garden might be a partial solution for an area where surface runoff is a problem looking at it from the viewpoint of the home gardener. It doesn’t appear to be a solution to soggy spots in the yard or post rain puddles. It seems that it is more of an environmental endeavor to direct and clean water in an urban setting. A rain garden serves the same purpose as the natural vegetation and surrounding woodlands do in more rural areas. Most reference sources use terms such as “every little bit helps” basically conceding that a rain garden actually does very little to affect the environment. It is a pretty conceit and a means of using native plants that like a bit more water than is normally available.