Rain Gardens as Living Stormwater Infrastructure: Planting the Rain Train
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A rain garden along a promenade soaks up runoff in the River North Mixed-use District in Denver, Colorado. This shallow depression was seeded with a low-profile mixture of plants native to shortgrass prairies. Native grasses include Western Wheatgrass, Tufted Hair Grass, Switchgrass, Indian Rice Grace, and Baltic Rush. Wildflowers include Plains Coreopsis, Prairie Coneflower, Black-eyed Susan, and Indian Blanket. The stone block directs runoff, communicates intentionality, and offers kids and maintenance workers a stepable surface. Eighteen inches of amended soil help infiltrate runoff from impervious surfaces and recharge groundwater. Photo: Bruce Dvorak
Rain gardens share a similar plant palette with green roofs and are very effective at soaking up stormwater. In this article I explore several rain gardens for their creative and multifunctional use of plants.
When Stormwater Detention is Not Enough
Raindrops never sit still. Gravity is always at work, pulling them somewhere. In urban watersheds, runoff from rooftops, parking lots, sidewalks, and streets is conventionally drained as rapidly as possible through a network of drains, subsurface pipes, and detention basins. This grey infrastructure bypasses the natural process where rain is intercepted by vegetation and soaked up by plants and soil to potentially become groundwater or baseflow to feed aquifers, wetlands, creeks, streams, and other aquatic systems. Contrary to popular belief, surface runoff is not the norm for nature (Patchett and Wilhelm 1999).
A sole reliance on grey infrastructure in urban watersheds can inflict ecological and economic loss and create hydrologic disorder. A perilous process begins to evolve when as little as ten percent of a watershed becomes covered with impervious surfaces (Booth, Hartley et al. 2002). With natural systems, rain takes a slow-moving path through the soil (Patchett and Wilhelm 1999). Impervious surfaces can cause flash flooding with the potential for the expansion and downcutting of stream beds. For plants and animals, the effects of impervious surfaces can be dire. Polluted and fast-moving water can damage and destroy riparian and aquatic ecosystems. Every year, impervious surfaces inflict significant damage in cities to personal property and public infrastructure and cause the loss of life to humans, animals, and plants. Left alone, these processes are irreversible and won’t resolve without human compassion and ingenuity.
During a class field trip, students, and faculty, from a landscape architecture studio at Texas A&M University snap photos of one of several major log jams on the Navasota River downstream from town. Before this watershed became urbanized, this section of the river was navigable. Log jams like these are caused solely by impervious surfaces. About fifty percent of this watershed is impervious and runoff is managed through detention basins. Trees along the banks of creeks and streams downstream of urban development become undercut and dislodged into the river and collect trash. This example demonstrates how stormwater detention basins don’t always solve the stormwater conundrum. Nature-based solutions like green roofs and rain gardens may need to become necessary components of urban watersheds. Photo: Bruce Dvorak
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Runoff as a Resource
Fortunately, more sustainable and attractive approaches to rainwater management have been developed. Entire new industries such as green roofs, large-scale rainwater harvesting systems, porous pavements, and rain gardens have been implemented in conservation-minded communities. This is a dramatic cultural shift. Societies are now beginning to understand rainwater as a resource. This means that runoff is now worthy of a garden! This approach includes a series of treatments and is sometimes called a “stormwater treatment train,” or “rain train,” because runoff flows from one infiltration design to another (Wadzuk and Traver 2012).
Rain trains capture, pool, and infiltrate runoff in a series of vegetated constructs that can include green roofs, detention basins, rain gardens, and bioswales. The plants featured in these designs are selected to clean water and return water to the ground or the atmosphere. This process mimics the natural water cycle. Plants play a large role in this process as they negotiate a path for the water into the soil and soak up water and pollutants. Plants with deep root systems are used and help make porous paths for excess water to find its way into aquifers and water tables and feed the baseline water levels of wetlands, streams, and rivers.
A pocket park at the Capitol Region Watershed District building and property includes an interactive exhibit to teach about nature-based solutions. Children can rotate the blue disc to pump water harvested from a cistern located inside the building and make it run down the face of the exhibit. The upper section of the exhibit represents urban elements that generate runoff. The lower section demonstrates how the roots of native prairie plants soak up rainwater. Plants native to Midwestern prairies have deep fibrous roots and are featured here as a pathway for water to descend. Runoff from the exhibit runs through the cast iron trench drain into a curbside rain garden. Photo: Bruce Dvorak
These nature-based solutions don’t completely replace grey infrastructure, but they complement it and can significantly reduce the scale of its application and prevent flooding. Combined or integrated solutions throughout the entire development make use of multiple low impact development (LID) designs and work together to reduce runoff completely. This approach is sometimes referred to as developing a ‘sponge city’, a concept that has caught on around the world.
Rain Gardens
A rain garden is a depressed micro-basin that has a layer of engineered growing media, planted with vegetation that can handle xeric (dry) and mesic (moist) soil moisture conditions. Mesic plants tolerate both temporary flooding (less than 48 hours) and droughty conditions. Vegetation native to the prairies and savannas of North America are excellent locations to identify xeric and mesic plants. These plants perform very well on rain gardens, bioswales, and green roofs. Similar to green roofs, rain gardens make use of an engineered growth media. The media is located at the lowest elevation of the rain garden and it is the primary path for runoff to percolate into the soil. Typically, rain gardens are located on soils with positive (moderate) drainage characteristics. Slow-draining soils with high clay content are not ideal for rain gardens and special measures may be necessary.
The design of rain gardens includes several important considerations. How much water needs to be sequestered? What kind of plants will grow in my region? What kind of maintenance requirements are necessary for rain gardens? How much do rain gardens cost? These important questions need to be addressed and some are beyond the scope of this article. More information on these topics can be found online.
Regarding stormwater considerations, rain gardens are typically designed to capture, infiltrate, and store at least the first flush, which is up to about one inch of rain (2.54 cm). The actual sizing and dimensioning of a rain garden and planning for an overflow for large rain events will need the attention of an experienced designer. Based on several online sources, the range of costs for installing rain gardens can vary from $5 to $25 per square foot (US Dollars). Many factors influence cost such as the scale of the installation, the size of plants installed (via seed or pre-grown plants), irrigation needs, drainage pipes, drainage layers, and the depth of the growing media layer.
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Rain Gardens in the Twin Cities
While attending the ASLA Conference in Minneapolis, Minnesota last October, I learned about several new and mature rain gardens worthy of investigation. I report here on two rain gardens, one new installation and one mature installation with multiple rain gardens installed more than a decade ago.
With dozens of freshwater lakes sprawled throughout the metro area, the Twin Cities of St. Paul and Minneapolis, Minnesota, offer property owners incentives to learn about and install rain gardens. St. Paul was an early leader in the promotion of rain gardens. Minneapolis has an equally active and seasoned rain program called Metro Bloom, which has been in place for over a decade and now has organized over 1000 rain gardens. Both cities also include rain gardens in public schools and have established a Boulevards Program where rain gardens and bioswales have been installed along major roads for over a decade. Research on the effectiveness of rain gardens has demonstrated their generally positive performance, however, not all rain gardens perform the same (Asleson, Nestingen et al. 2009). Attention to design details and maintenance are important for their success.
Headwaters of a rain train at the Capitol Region Watershed District headquarters building in St. Paul, Minnesota. Rooftop water drains to the ground and enters here through this trough. From here, water moves along to one of two rain gardens planted with native vegetation. Overflow is piped to another rain garden located below the parking lot. Photo: Bruce Dvorak
Capitol Regional Watershed District
The Capitol Region Watershed District (CRD) headquarters building is located near the headwaters of local streams and lakes that drain to the Mississippi River near the state capitol building in St. Paul, Minnesota. The district is a governing body that emerged out of grassroots efforts by local citizens to conserve water quality in the metro area. The region has 50 percent impervious surfaces. As part of a relocation effort, the new headquarters building was designed to demonstrate how nature-based solutions can be implemented with rainwater harvesting, porous pavers, and rain gardens. The rain gardens include multiple retention basins at descending elevations, creating a “rain train” to soak up runoff. The landscape is designed to treat and infiltrate up to 5.3 inches (13.5 cm) of rain within 24 hours (50-year storm event). The plants play a large role in the success of the rain gardens. Plants intercept water, soak up soil moisture, clean water, and engage other forms of life, such as pollinators, birds, and small mammals.
Alexandra Morrison, Stormwater BMP Technician with CRWD says that the plants grow in an engineered growing media. She said, “For rain gardens, they are typically constructed from 24 inches of excavated soil, then backfilled with 18 inches of amended soils (growing media). They use a mix of 80% coarse washed sand and 20% leaf compost.”
A pocket park connects to the neighborhood. Native plants adorn the rain gardens, sidewalk green spaces, parking lots, and a mini retention basin feeds from runoff from the pocket park. The rain garden supports a habitat hot spot for over 60 species of native plants, bees, birds, spiders, ducks, rabbits, dragonflies, and minnows (iNaturalist). Photo: Bruce Dvorak
In a series of rain gardens, runoff is first infiltrated at the top of the watershed and captures part of the rooftop runoff. The first treatment phase soaks water into the ground. If there is more water than it can handle, gravity pulls water through pipes to a rain garden below the parking lot.
There are 28 species of native forbs planted at the CRWD property, which includes five native grasses and 24 native perennial wildflowers selected to bloom during the spring, summer, and fall. This plant community provides ecosystem services to local wildlife year-round.
Native grasses include prairie dropseed (Sporobolus heterolepis), little bluestem (Schyzachyrium scoparium), blue grama grass (Bouteloua gracilis 'Blonde Ambition'), blue joint grass (Calamagrostis canadensis), and switchgrass (Panicum virgatum). Some of the popular wildflowers include butterfly weed (Asclepias tuberosa) threadleaf coreopsis (Coreopsis verticellata 'moonbeam') blue flag iris (Iris versicolor), kobold liatris (Liatris spicata 'kobold original'), obedient plant, (Phyostegia virginiana), black-eyed Susan (Rudbeckia hirta), New England aster (Symphyotrichum novae-angliae), and golden alexanders (Zizia aurea). Some native species with unique plant parts include rattlesnake master (Eryngium yuccifolium), prairie smoke (Geum triflorumand), and blue wild indigo (Baptisia australis).
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Little bluestem grass lines the side slopes of a rain garden at Maplewood Mall. Native trees and shrubs are included in larger rain gardens such as this one planted in 2012. Photo: Bruce Dvorak
Maplewood Mall
Minnesota is known as the land of 10,000 lakes. Kohlman Lake is a 74-acre freshwater lake that lies downstream of the 35 acres of impervious surfaces at Maplewood Mall, in St. Paul, Minnesota. First built in 1974 and expanded in 1996, the mall’s impervious surfaces once directed 97 percent of rainfall directly into Kohlman Lake. Phosphorus from runoff from parking lots degraded the water quality of Kohlman Lake and it was declared an impaired water body (District 2008). In 2012, the owner of the mall teamed with the City of Maplewood, the Ramsey-Washington Metro Watershed District, and Ramsey County to secure a 7 million dollar grant to retrofit the impervious surfaces at the mall. The design and engineering included rain gardens, cisterns, tree trenches, and porous pavements. As a result, 90 percent of runoff is now captured and 60 percent of the phosphorus is retained! This multiple award-winning retrofit project is proof that even large-scale developments with impervious surfaces can be addressed through nature-based design solutions.
Over 1200 feet of rain gardens are planted with native plants used to soak up runoff from about four acres of impervious surfaces from parking lots at the Huntington Stadium in Minneapolis. The rain gardens helped this retrofit design effort to achieve LEED Silver Certification in 2009. The native perennial plant, Solidago (goldenrod) thrives here as well on green roofs in the Twin Cities and across North America. Birds eat goldenrod seeds, and it also supports butterflies, bees, and wasps with late-season nectar and pollen. Photo: Bruce Dvorak
The City of Maplewood publishes guidance for local rain garden designs and maintenance. Design approaches include low-maintenance rain gardens that are planted primarily with shrubs and a few trees. More diverse rain gardens such as some of the rain gardens at the Maplewood Mall are planted with grasses and wildflowers. These gardens need about two to three maintenance visits per year to trim back vegetation, remove sediment or debris, remove invasive species, and periotic replanting.
Summary
An effective, resilient, and attractive approach to managing stormwater in urban areas can be achieved through a combination of grey infrastructure and a widespread application of nature-based solutions such as rain gardens and green roofs. Planting approaches for rain gardens can include many of the same plants that can also work for semi-intensive green roofs. Finding the right plant pallet for your region may take some investigation. However, there are many hardy perennial grasses, wildflowers, shrubs, and trees that are proven in many regions to establish and sustain growth and reestablish rain gardens.
Plants for Rain Gardens
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To learn about how green roofs can support heat-tolerant plants and biodiversity, see the new Living Architecture Academy Course: Case Studies of Biodiverse Green Roofs, by Bruce Dvorak.
Bruce Dvorak, FASLA is a Professor at Texas A&M University in the Department of Landscape Architecture and Urban Planning, where he has been conducting green roof and living wall research since 2009. Bruce is a member of the GRHC Research Committee and founded a new Regional Academic Center of Excellence in 2022, the Southern Plains Living Architecture Center. Bruce received the GRHC Research Award of Excellence in 2017 and teaches green roofs and living walls in his courses in landscape architecture programs at Texas A&M University. His edited book, Ecoregional Green Roofs: Theory and Application in the Western USA and Canada (2021) provided inspiration and content for this article.
Acknowledgments
I would like to thank Alexandra Morrison, a Stormwater BMP Technician with the Capitol Region Watershed District in St. Paul, Minnesota. Alexandra provided the plant list for the CRWD offices project. Thanks also to J.C. Culwell, Associate Principal and Landscape Architect at Wenk Associates for sharing their plant list for the River North District rain garden project. Thank you also to Stephen Peck for reviewing this article and providing valuable feedback.
References
Asleson, B. C., R. S. Nestingen, J. S. Gulliver, R. M. Hozalski and J. L. Nieber (2009). "Performance assessment of rain gardens 1." JAWRA Journal of the American Water Resources Association 45(4): 1019-1031.
Booth, D. B., D. Hartley and R. Jackson (2002). "Forest cover, impervious‐surface area, and the mitigation of stormwater impacts 1." JAWRA Journal of the American Water Resources Association 38(3): 835-845.
District, R.-W. M. W. (2008). "Kohlman Lake Aquatic Plant Management Plan."
LINKS, M. "Minnesota Stormwater Manual."
Patchett, J. M. and G. S. Wilhelm (1999). The ecology and culture of water. Governor's Conference on the Management of the Illinois River System.
Wadzuk, B. and R. Traver (2012). Design, Construction, and Evaluation of a Stormwater Control Measure Treatment Train. World Environmental and Water Resources Congress 2012: Crossing Boundaries.