By Cindy Gilberg
Rain gardens, a relatively new name for an old concept, have gained special interest in the past few years. Water is a precious resource. As it flows over impervious surfaces such as rooftops and pavement it carries pollutants, gains volume and velocity causing soil erosion and flash floods. Rain gardens offer a solution – they are shallow basins planted with wetland plants that serve to capture and slow down storm water runoff, mimicking natural wetlands. Plants take up water for growth and their root systems allow water to percolate back into the ground. Overall water quality improves as pollutants settle out in the rain garden.
It is important to understand that a rain garden is not the end point for the storm water. Ideally it is constructed perpendicular to the source (i.e. downspouts from rooftops) and between the source and traditional storm water culverts. Allow at least ten feet from other buildings. The water may be directed to the rain garden through a pipe or shallow swale planted with sedges (Carex sp.). Also called a bioswale, this slows the water before it enters the rain garden. An overflow point in the rain garden is located where the excess water can flow out, in the event of heavy rainfall, directing water towards storm water culverts. Small residential size rain gardens may require the help of knowledgeable designers, while larger, commercial or development size projects should enlist the help of a landscape architect and engineers.
Most wet areas in a landscape go through periods when they are very wet followed by periods when they’re more dry. Evaluate if an area is a constantly wet zone or does it periodically dry out. Heavier, clayey soils will hold water for longer periods than will well-draining soils. As the clay content of a soil increases so should the size of the rain garden. The rule of thumb for rain garden size is that it should be roughly 25% to 30% of the total square footage of the source of the storm water runoff (i.e. rooftops).
Missouri has a diversity of wetlands and therefore a diversity of native plants is available to gardeners that are both attractive and well-adapted for this alternating wet/dry situation. If the area stays wet, plants that are considered true aquatic plants, such as pickerel plant (Pontaderia) or hardy water canna (Thalia) are options. If the site goes through periods of wet and dry, there is a long list of native plants that will thrive in that condition. In any rain garden there are different zones of soil moisture – the wettest area is the deeper, center basin while bioswales and the upper slopes of the garden dry out quickly.
Note also whether the site is shady or sunny and adjust the plant list accordingly for maximum success. When planting, mulch is not necessary since it may actually float away during a major rain. If you are dealing with a slope, use a type of erosion blanket that is biodegradable to help hold the soil while the plants are getting established.
Sedges (Carex sp.), grass-like species, form the foundation of wetlands just as true grasses form the foundation of a prairie. Many sedges are rapidly spreading, making them good candidates for bioswales, large projects or erosion problems. For smaller rain gardens, slow-spreading, clump-forming sedges are recommended, such as Gray’s sedge (C. grayii) or palm sedge (C. muskigumensis). Just a few of the many sun-loving wetland plants are hibiscus (Hibiscus lasiocarpus), some coneflowers (Rudbeckia fulgida or R. subtomentosa), copper or blue iris (Iris fulva, I. shrevei) along with swamp milkweed (Asclepias incarnata), turtlehead (Chelone) and bluestar (Amsonia illustris). Among the shrubs that succeed in these conditions are buttonbush (Cephalanthus), spicebush (Lindera) and winterberry (Ilex verticillata). If you have a more shady situation, try plants such as squaw weed (Senecio aureus), cardinal flower (Lobelia cardinalis) along with some of the ferns like ostrich fern, sensitive fern or royal fern. There are sedges for both sun and shade, and these should be used in the planting as well.
Another landscape situation that can drive a gardener crazy is one in which water pools in low areas. Grass won’t grow there and most other plants won’t either. The answer is in remembering the famous horticultural quote “don’t fight the site,” also referred to as “right plant, right place.” Planting moisture-loving wetland plants rewards you with a beautiful and unique style of garden as well as providing an economical and sustainable solution to the problem.
Are you interested in learning more? A great place to start is The Native Plant School class, “Rain Gardens,” on June 12th from 1 pm to 4 pm, held at Shaw Nature Reserve. Another wonderful source of information is the Native Landscaping Manual “Chapter 2 – Rain Gardens” This can be viewed online at Shaw Nature Reserve or purchased at the SNR Visitor’s Center.
A new initiative, Show Me Rain Gardens, will be launched in the St. Louis area this year. It is a collaborative effort involving many partners such as Metropolitan St Louis Sewer District (MSD), Soil and Water Conservation, Missouri Department of Conservation as well as regional landscape architects and horticulturists. It will serve as a resource of information in addition to providing educational opportunities at all levels in the community.
Cindy Gilberg, horticulturist and Missouri native, founded and ran the garden center at Gilberg Perennial Farms with her husband Doug for 28 years, also teaching classes and workshops on gardening and garden design. She now focuses on garden design, consulting and teaching, and also works part-time in the Whitmire Wildflower Garden at Shaw Nature Reserve, emphasizing the use of native plants in home landscaping.