Beauty and utility are combined when a water-loving installation is installed according to best management practices.
When Cawrse & Associates Inc., a landscape architectural firm located in Chagrin Falls, Ohio, built its new office building in 2008, the company incorporated four “best management practices” into the site plan. These BMPs were implemented for storm water control and runoff quality and reduction, and were partially funded by a grant from the Chagrin River Watershed Partners (CRWP), a coalition of cities, villages, townships, counties and park districts united by their common concerns about flooding, erosion and water quality problems. The BMPs incorporated by Cawrse included a bioretention storm water basin, a bioswale, pervious pavement and a rain garden.
A rain garden is a man-made, shallow depression in a yard that can receive rainwater runoff from an impervious surface, such as a building rooftop, or from a paved surface. The rainwater from a storm event is collected from the paved area or rooftop and is conveyed over land, or by pipe, to the area of the rain garden. The Cawrse rain garden used rainwater solely from the new office building rooftop, and was conveyed by pipe from the downspout.
A reliable foundation
In general, the ratio of drainage area to rain garden area is 5:1 for a well-drained, sandy soil profile. For a compacted or clay soil that drains poorly, a drainage area to rain garden area ratio of 3:1 or 2:1 should be used. (Consult with your local soil and water conservation district for rain garden calculation formulas and charts.) The Cawrse rain garden’s size was calculated to require approximately 400 square feet of surface area, with an average depth of 8 inches. On-site soil was a heavily compacted clay. The contractor excavated 24 inches of the heavy soil from the rain garden and added 24 inches of a bioretention soil mix from Kurtz Brothers Inc. in Cleveland. This soil mix has a “loamy sand” USDA soil texture classification, and it is combined with up to 20 percent organic material. It has an infiltration rate of 1.5 to 2.5 inches per hour, and a pH between 6.5 and 7.5. Using the 8-inch-depth baseline determined for this project, Cawrse developed an undulating bottom to the rain garden as deep as 15 inches and as shallow as 2 to 3 inches. The idea was to give the rain garden a variety of elevations to provide a range of habitat for the plantings the company wanted to use.
The soil mix used is an extremely well-draining medium. Varying the depth of the bottom of the rain garden produced a soil range from dry to moist to saturated, while maintaining a minimum depth of 24 inches of the mix. Rainwater is brought into the soil mix by drainpipe from the downspout, the end of which is covered by filter fabric and large river stone. This fabric/stone installation acts as a reverse dry well, allowing the rainwater runoff to surcharge in the rain garden and immediately inundate the soil mix. Water saturates the soil mix, providing hydration for the plants, and the soil mix and plant roots filter impurities from the rainwater. The soil mix also slows down the rate of the runoff, holding it on-site for a longer period of time, rather than allowing it to run directly from the site.
A bare-bones view of the rain garden area shows the exposed subgrade and drainage stubs.
PHOTOS COURTESY OF CAWRSE & ASSOCIATES INC.
A rain garden’s primary function is to facilitate the recharging of groundwater by rainwater. Prior to the placement of the soil mix, the subgrade of the rain garden area was scarified, or broken up, to promote the percolation of the rainwater. This was done with the teeth of an excavator bucket. There is no subdrainage in the Cawrse rain garden. However, because this is within an area of pervious pavement, this rain garden does have an overflow. This was added in case of a heavier-than-normal rainfall event, which could cause rainwater to overflow onto the pavement. The overflow will drain the high water to a 6-inch pipe leading to the bioretention basin.
The soil mix was installed with little mechanical compaction. An excavator was used to distribute the mix after it was dumped into the rain garden subgrade, and only limited foot traffic during planting was permitted. This kept the soil in a maximized pervious state. This is critical to the performance of the rain garden, as rainwater must be able to flow freely – vertically and horizontally – through the soil. Compaction of the soil mix will significantly reduce the capability of the percolation.
A palette of plants
The excavator manipulates the bioretention soil mix into place, with no mechanical compaction.
The plants selected for the rain garden were primarily chosen for their tolerance of a range of soil moisture conditions, from wet to dry. Some designers make their plant selections from a palette considered to be “flood plain” plants: those that can tolerate periods of dryness punctuated by periods of inundation. In addition to their tolerance of soil moisture, the Cawrse rain garden plants were selected for full-sun exposure, stem color, leaf texture, leaf size and flower color.
The rain garden also heavily relied upon the use of Ohio native plants, although most of these were not straight species, but cultivars selected mostly for improved disease and pest resistance. The added benefit of using native and adapted-native plants is that of less maintenance: less fertilizing, less pruning, less spraying for insects and disease, and so on.
With the soil mix incorporated and roughly shaped, drainage pipe has been placed.
The preliminary plant palette was developed by Cawrse and then forwarded to Bill Hendricks at Klyn Nurseries in Perry, Ohio. Hendricks reviewed the plant list, made a few changes and provided the plants.
Beginning with the woody-stemmed plantings, Cornus amomum (silky dogwood) and Myrica pensylvanica (northern bayberry) were sited along the outer edges to screen adjacent paved areas and the U.S. Geological Survey monitoring equipment required as a part of the grant from the CRWP. The stem color of the dogwood is outstanding, and it can survive in heavy, wet soils where other shrubs would fail. The native bayberry gives the rain garden decorative gray berries well into winter. Itea virginica ‘Little Henry’ (Little Henry sweetspire) provides an early summer show of white flowers and dramatic red-purple fall color.
River rock and fi lter fabric have been placed. Final shaping is taking place, and plants are waiting for installation.
Heptacodium miconioides (seven-son flower), while not native to Ohio, was used as a specimen tree at the edge of the rain garden. The exfoliating bark and white flowers, which are followed by red calyces, provide a nice focal point.
The selection of grasses was limited to Carex muskingumensis (palm sedge), which was located on a side slope. This Ohio native can take sun or shade and does very well in wet locations. It has formed a nice mass and borders a companion planting of Phlox paniculata (‘Little Princess’ garden phlox). This phlox is an adapted native and was selected for its pink flowers and mildew resistance.
Plants have been newly installed in the rain garden. On opposite page, approximately 6 inches of rainwater inundates the river rock, where iris, hibiscus and bee balm are readily visible.
Around the river rock inlet, various types of Louisiana iris were planted, including ‘Bold Pretender’, ‘Hurricane Party’, ‘Clyde Redmond’ and ‘Scamp’. These iris are hybridized American species native to wet and boggy areas, and they can grow in water up to 6 inches deep. This and another iris planting are the lowest points and frequently pond water during rainfall events. The second planting of iris is separated from the inlet area by Hibiscus moscheutos ‘Lord Baltimore’ (‘Lord Baltimore’ hibiscus or rose mallow), which is the most noticeable during summer when it develops huge, dinner-plate-size red flowers. These blooms, and the fact that Lord Baltimore and its related Eastern U.S. hybrids can grow in water up to 6 inches deep, far outweigh the plant’s late break in dormancy.
Hibiscus, Joe Pye weed, Jacob’s ladder, sweetspire and iris thrive in the rain garden; even a small patch of New England aster is happily situated between the hibiscus and iris.
Bracketing the hibiscus, plantings include Eupatorium maculatum (‘Gateway’ Joe Pye weed), an adapted native plant that does a great job attracting butterflies, Lobelia cardinalis (red cardinal flower) and L. siphilitica (blue cardinal flower), both native species thriving in moist to wet soils. Aster novae-angliae ‘Purple Dome’ (‘Purple Dome’ New England aster), a favorite in the rain garden, provides a mass of purple flowers with yellow-white centers. Polemonium reptans ‘Stairway to Heaven’ (‘Stairway to Heaven’ Jacob’s ladder) is tucked into the remaining spaces, providing a unique variegated leaf and equally unique bloom. This is combined with Monarda didyma (scarlet bee balm) for a festival of flowering.
A bird’s-eye view and cross-section of the rain garden illustrate construction specifi cations.
These plants have thrived in the rain garden and have filled in the space, going into their second full growing season. The success of the rain garden can be measured in several ways. First, all rooftop rainwater has been absorbed by the ground and the plants. No rainwater has surcharged the overflow, and with the full-depth concrete curbing surrounding the rain garden on three sides, none has filtered through to the pervious pavement subbase. Second, the plantings have provided a variety of leaf and stem color, flowering all through the growing season, as well as a new habitat for birds, butterflies and small animals. Last, it has provided a living classroom for students, professionals and laypeople who wish to learn about rain gardens and how to benefit from their implementation.
Dale Burrier is president and a principal landscape architect with Cawrse & Associates Inc., Chagrin Falls, Ohio. A graduate of The Ohio State University, he joined Cawrse in 1984, and has over 28 years of experience in land planning, landscape design, construction specification writing and golf course community design. He can be reached at email@example.com.