Four Approaches to Making Living Roofs With Native Plants
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Artemisia frigida is native to well-drained soils of the high plains grasslands. It grows along with nearly 600 other taxa of plants at Red Rocks Natural Area near Denver, Colorado, and serves native bees. This and hundreds of other natives grow on living roofs at the Denver Botanic Garden. However, many of the thousands of plants native to North America have yet to be explored for use on living roofs! Photo: Bruce Dvorak
When Europeans first explored North America, they found a continent that was populated with First-Nations peoples, and the amazing landscapes that they sustained. In the mid-continent was an immense “sea of grass,” which was one of the most biologically diverse ecosystems in the world. Native Americans were the first stewards of the North American central prairies and it was the epicenter of their culture from generation to generation for more than 5000 years (Denevan 1992)! While the east coast of North America was largely vegetated with forests, the central, southern, and western regions were dominated by surface-fire grass-based ecosystems such as prairies, meadows, glades, barrens, alvars, scrublands, chaparral, savannas, and woodlands (Ricketts, Dinerstein et al. 1999). Today, however, grass-based ecosystems are some of the most threatened ecosystems worldwide, due to human influence, and mismanagement (Samson, Knopf et al. 2004). In most states where tallgrass prairie was the dominant ecoregion, less than 1 percent of the original native tallgrass prairies and oak savannas remain, in isolated patches (Diamond, Riskind et al. 1987, Packard and Mutel 1997). Today, conservation sites provide great value as outdoor classrooms where one may discover new plants to be trialed on green roofs (White 2006, Oberndorfer, Lundholm et al. 2007).
There is a great potential for and an essential role for making green roofs with native plants (Cook-Patton and Bauerle 2012, Lundholm and Walker 2018). I think it is essential, because if conservation practices are not taking place at the ground level, or if the native plant communities have already been significantly altered, then living roofs may provide a last chance to support the ecological heritage of the region. Further, some municipalities encourage roof-appropriate native plants, or biodiverse green roofs in their green roof guidelines. New research on the use of native plants is necessary to achieve these aims. In this issue of the LAM, I explore four approaches to make living roofs with native plants.
A scattering of unplowed prairie fragments (left) of the Grand Prairie near Fort Worth, Texas remains in isolated preserves such as this ridgetop remnant at the Tandy Hills Natural Area. Over 1500 species persist there including drought-tolerant grasses, yucca, and prickly pear cactus. This scene (center) near the Dallas/Fort Worth International Airport shows the radical potential for living roofs to rebuild ecosystem services once provided by native plant communities. The living roofs on Botanical Research Institute of Texas (right) in Fort Worth, established vegetation from a nearby prairie with the native Yucca pallida, which provides a larval host habitat for the Yucca giant skipper butterfly and Opuntia phaeacantha which attracts native bees, birds, butterflies and more. Projects such as this demonstrate how native landscapes can inspire ecoregional green roofs. Photos: Bruce Dvorak
In Ecoregional Green Roofs, we identify over 830 taxa of plants on living roofs that are native to North American ecoregions (Dvorak, 2021). Although there has been some research on the use of native plants on green roofs in some parts of North America, our findings reveal that there are now many ways to expand the potential pallet of natives on green roofs. The diversity and abundance of native plants on the green roofs we investigated varied greatly. We found that some green roofs had all locally native vegetation while others feature native plants, but also include exotics. We determined that there were four patterns of distribution of native plants on the green roofs we studied (Dvorak and Bousselot 2021):
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A green roof where all of the intended vegetation is native to the site, or nearby (local genotype);
A green roof where the intended vegetation is native to one or several adjacent ecoregions (i.e., vegetation from an alpine ecosystem that is planted on a green roof in a valley below);
A green roof where much of the intended vegetation is native to various but similar ecoregions of North America;
A green roof with vegetation from a combination of native and non-native (introduced/exotic) vegetation. These climate-adapted green roofs feature native plants but also include a significant number of non-native plants.
The following four case studies from Ecoregional Green Roofs demonstrate four patterns or ways of distributing native plants on living roofs.
Slide Ranch Nature Center, Muir Beach, California (native to the site)
There are perhaps only a few green roofs like the living roof on the Slide Ranch Nature Center. As part of the Golden Gate National Recreation Area, the Slide Ranch property has a mix of mostly native and some non-native vegetation. The surrounding cliffs and coastal meadows are prime examples of the native coastal plant communities that go back thousands of years sustained by the Miwok tribes which used the cliffs for fishing, and meadows for harvesting its edible bulbs. Today, the landscapes immediately surrounding the Slide Ranch Nature Center include grazing plots for farm animals, vegetable gardens, chicken coops, and a few garden sheds. The Slide Ranch Nature Center is a farm-to-table learning center established in 1970 to “connect children to nature.” The living roof is located on a new building used to process food, hold staff meetings, or just hang out (Dvorak and Drennon 2021).
View of the Slide Ranch Nature Center multi-purpose building with a living roof. All of the vegetation was generated from seed harvested on site. Photo: Bruce Dvorak
The plant community on the Slide Ranch living roof includes small flowered needlegrass (Stipa lepida), California aster (Symphyotrichum chilense), coast buckwheat (Eriogonum latifolium), seaside daisy (Erigeron glaucus), wild strawberry (Fragaria vesca ssp. californica), yarrow (Achillea millefolium), yerba buena (Clinopodium douglasii), and Bluff lettuce (Dudleya farinosa). Cliff habitats and coastal meadows can be seen in the background. Photo: Bruce Dvorak
The living roof design team and the Slide Ranch administration and staff committed to making the living roof so that all of the elements came directly from their property. The 6-inch-deep substrate and all of the seed used to establish vegetation on the green roof are sourced from the site. The coastal meadow habitats served as a model plant community for the living roof. Seeds were collected from the property and preserved for establishment on the green roof (Dvorak and Drennon 2021). This means that plants or seeds grown from another state or country were not packaged and shipped to the site, and genetic material from outside the region was not introduced. Instead, the genetic material of the plants that co-evolved with the soils and native pollinators from that site was re-established on the rooftop meadow.
The vegetation is now well-established; however, the staff that maintains the living roof found that the shallow substrate dries out quicker than soils on the ground. The microclimate of the living roof substrate is drier and warmer than the soils that support the coastal meadow habitats below. So, in this case, due to the shallow soil, irrigation from harvested rainwater is used to keep the vegetation thriving and prevent the roof from becoming a fire hazard. Therefore, this all-native green roof is not an exact substitute for the coastal meadow habitats below. Regardless, it performs just as planned. It reduces the air temperatures of the habitable space below the living roof (kitchen prep and staff meeting space) and retains rainfall and serves wildlife (Dvorak and Drennon 2021).
Sonoma Academy, Santa Rosa, California (native to site and adjacent ecoregions)
Sonoma Academy is a private high school that believes and teaches the next generation that “nature is an asset.” Students and teachers learn in an environment where integrated design has become a new measure for normal. The green roof on the new Guild Commons building is not an added “extra feature,” it is integral (integrated) into the daily management of heating and cooling the building. It also collects rainwater and moisture from condensation for internal and external use, keeps photovoltaic panels cool, and it is a space for students to collect data and learn about how some of its native vegetation is an essential asset for an endangered butterfly (Dvorak and Drennon 2021).
This rooftop meadow has substrates 20 cm (8 in.) to 30 cm (12 in.) deep. The roof space is programmed with the biodiverse green roof (on top of conditioned space), solar panels, and a white reflective stone ballasted roof to capture water in a cistern for later on-site use. Photo: SYMBIOS
Sedum spathulifolium is native to Sonoma County, and is native to rocky slopes in the adjacent hills and grows on this living roof in a stone-based micro-habitat created by the shading of photovoltaic panels. This native sedum is also a host plant for the endangered San Bruno Elfin butterfly (Callophrys mossii bayensis). The Elfin uses only Pacific Stonecrop (Sedum spathulifolium) plants to complete its life cycle. The loss of Pacific Stonecrop is the extermination of the San Bruno Elfin butterfly! The www.calflora.org citizen scientists have located several Pacific Stonecrop plants at a nearby state park; however, the valley floor where Santa Rosa is located is home to prairie/oak savanna and not a natural habitat for this sedum. Although this stonecrop is native to Sonoma County, it is not native to this site. The use of sedum, in this case, was made fit by its appropriately situated partly-shaded and rocky habitat in the shade of the solar panels. It is there to be observed by students and teachers of Sonoma Academy. The “meadow” vegetation receives irrigation water from renewable sources (onsite-cistern) and protects the roof from becoming a fire hazard (Dvorak and Drennon 2021).
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Sedums from any Mediterranean climate would very likely grow on green roofs in San Rosa, California; however, the school administrators and design team provided space for native plants of the region to serve the local endangered butterfly. This endemic butterfly was not endangered before the permanent settlement of the valley several hundred years ago. If the Elfin butterfly is to survive well into the future, it will be because conservation-minded people make it happen. This project sets a high bar to challenge educators to re-imagine what learning environments can be like, so students can further develop their relationship with nature and learn how nature is an asset.
(a) Native to Sonoma County, Nassella cernua and Nassella pulchra grow upright between the photovoltaic panels at Sonoma Academy. (b) Lightweight scoria volcanic rock and native Sedum spathulifolium cover the roof at the dripline of the photovoltaic panels. (c) The native Golden yarrow is in bloom with native grasses that are beginning to resume growth at the end of the summer dormant season. (d) Native vegetation planted in rain gardens at the ground level soak up runoff from an adjacent paved area. Photos: Bruce Dvorak
Palomar Medical Center, Escondido, California (native to various but similar ecoregions)
Located at the edges of the chaparral and montane ecoregions, the Palomar Medical Center building, landscape, and mechanical systems were designed to support positive and healing environments for people and the environment. The green roof vegetation is visible from all of the patient recovery rooms and through skylights from operating rooms below. Doctors, nurses, patients, and visitors all benefit from continual views of the “living wave”. This rolling roof deck is covered with coastal meadow vegetation native to western California coastlines (north, central, south), and the central US. The rooftop meadow features California fescue (Festuca californica), canyon prince wild rye (Leymus condensatus ‘canyon prince’), Idaho blue fescue (Festuca idahoensis), purple three awn (Aristida purpurea). These are native to California coastal prairies and meadows along central and northern regions of California. However, the featured plant on this roof is pink muhly grass (Muhlenbergia Capillaris), which is native to the south-central U.S. The design team added pink muhly grass to inflect hints of cheerful colors across the roof during the dry season. The habitat form/structure of a grass-based ecosystem is consistent with the natural vegetation and serves many species of wildlife including hummingbirds, bees, butterflies, and other insects. In addition to intentionally planted vegetation, coyote bush (Baccharis pilularis) is native to the region and has self-seeded onto the green roof, and is retained on the roof, as it is very attractive to native pollinators. Irrigation water is supplied from harvested rainwater and prevents the roof from becoming a fire hazard (Dvorak and Drennon 2021).
Pink muhly grass is native to prairies in the south-central US and is featured here on Palomar’s biodiverse and sloped living roof. The 4” (10 cm) deep substrate was provided by American Hydrotech. Photo: Bruce Dvorak
The inclusion of the biodiverse roof was a major investment for the hospital, and it has repaid itself in a short time. Palomar Pomerado Health spokesman Andy Hoang said “the green roof is more expensive than a traditional roof, but based on our analysis we will have recovered the cost through energy and water savings in a seven-year time period” (Breier 2010). In this case, native plants were included because of the prudent planning and the benefits of a collaborative design process, where the owner, architect, landscape architect, engineer, and staff were all engaged to develop the first concepts together. This process secured a place for native plants through a green roof that paid for itself, reduces energy bills, brings emotional healing to people, and supports habitats for native bees, hummingbirds, butterflies, and other wildlife.
SRM Development, Kirkland, Washington (combination of native and non-native)
Another approach to incorporate native plants onto a green roof is to mix natives with exotics. If a green roof provider offers a green roof with primarily exotic plants, then one way to upgrade green roofs to serve local wildlife is to provide some native plants that are proven to the region. This approach, is perhaps, within reach of many if not most green roof providers.
This project features two buildings with living roofs owned by SRM Development and occupied by Google (Dvorak and Rottle 2021). The inclusion of some native herbaceous wildflowers and sedums came about because of the design team's larger goals to look beyond the project boundaries. The composition of vegetation is a blended approach where sedums, grasses, and wildflowers are arranged in a garden setting. Plants are irrigated via a smart irrigation system, delivered from harvested water.
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Vegetation features black-eyed Susan (Rudbeckia fulgida ‘Goldstrom’), which is native to the central and eastern U.S., however, other species of Rudbeckia are native to the prairies and hillsides of western Washington. Also, the wild onion (Allium schoenoprasum) which is native to the Pacific Northwest (native and exotic forms) thrives on the roof. The planting design also features a non-native grass, blue fescue (Festuca glauca), which is very similar to the native Idaho blue fescue (Festuca idahoensis). The base planting for this green roof is a sedum mat supported with a 4” (10 cm) substrate by DIADEM USA, Etera. The sedum mat includes a mixture of native plants such as Cape Blanco stonecrop (Sedum spathulifolium ‘Cape Blanco’), Oregon stonecrop (Sedum oreganum), Pacific stonecrop (Sedum divergens), in addition to nine exotic sedums. The sedums are native to rocky slopes and hills along the Cascade Mountains.
This private extensive green roof is accessible and was built to provide over 1000 employees a biophilic low-stress work environment. This roof garden receives maintenance weekly during the growing season by personnel trained to know which plants to keep, and which plants to send to the on-site composting facility. This living roof arfully blends native wildflowers into an exotic-native sedum mix. Photo: Bruce Dvorak
Summary
While the wild denizens of natural environments are counting on the humans that live there to conserve their kind, some people may debate if it is possible to do that with green roofs. However, this article demonstrates diverse examples of how owners and developers of schools, hospitals, nature centers, and large office buildings sought out designers and installers with special knowledge of the ecosystem services and native plants in their region. In this article, I set out to provide a range of examples of different ways native plants can become implemented into extensive green roofs. Through the use of native vegetation, green roofs can provide essential ecosystem services to:
Reconnect people with their natural and cultural heritage
Enhance ecosystem services by providing habitat for local wildlife
Connect living roofs with biodiversity living beyond the roof
Repopulate depleted historic ecosystems in dense urban areas
Refine the “art” of green roof design to accommodate regional characteristics
Demonstrate how an all exotic green roof can incorporate native plants
Provide guidance for municipalities seeking to make use of native plants on green roofs
Ninety-nine percent of the historic grass-based ecosystems are already long removed from many North American urban landscapes. This means that most of us lack a cultural connection to the beauty, significance, importance, and knowledge of what was lost with the demise of native ecosystems. As biodiversity continues to decline worldwide, we need to re-invest in rooftop plant communities that purify the air we breathe, clean the water we drink, sequester carbon, color our natural environments, improve our well-being and serve nature beyond the parapet. If the owners, designers, providers, and installers of green roofs develop an interest in conserving and expanding upon essential ecosystem services, then perhaps we can move beyond the one percent remaining habitat by growing it, with native plants on green roofs everywhere.
Bruce Dvorak is an Associate 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. Prior to his academic career, Bruce was involved with the design and management of several recognized green roofs including the Chicago City Hall Pilot Project and the Peggy Notebaert Nature Museum green roofs and green wall (with Conservation Design Forum). Bruce is currently a member of the GRHC Research Committee, and received the GRHC Research Award of Excellence in 2017. Bruce teaches green roofs and living walls in his courses in landscape architecture programs at Texas A&M University.
Special thanks to Kerrie Lee with SYMBIOS for providing photos, and for providing additional project details.
References
Breier M (2010) Green roof at new hospital is more than a cover. The San Diego Union-Tribune, San Diego.
Cook-Patton, S. C. and T. L. Bauerle (2012). "Potential benefits of plant diversity on vegetated roofs: A literature review." Journal of environmental management 106: 85-92.
Denevan, W. M. (1992). "The Pristine Myth: The Landscape of the Americas in 1492." Annals of the Association of American Geographers 82(3): 369-385.
Diamond, D. D., D. Riskind and S. Orzell (1987). "A framework for plant community classification and conservation in Texas." The Texas Journal of Science.
Dvorak, Bruce, et al, (2021) Ecoregional Green Roofs Theory and Application in the Western USA and Canada Springer, Cham, Switzerland.
Dvorak B and Drennan P (2021) Green Roofs in California Coastal Ecoregions. In: Dvorak B (ed) Ecoregional Green Roofs: Theory and Application in the Western USA and Canada. Springer International Publishing, Cham, Switzerland.
Dvorak B and N. D. Rottle, Green Roofs in Puget Lowland Ecoregions, In: Ecoregional Green Roofs: Theory and Application in the Western USA and Canada, edited by B. Dvorak, Springer International Publishing, Cham, Switzerland.
Lundholm, J. T. and W. Walker (2018). "Evaluating the habitat template approach applied to green roofs." Urban Naturalist S 1: 39-51.
Oberndorfer, E., J. Lundholm, B. Bass, R. Coffman, H. Doshi, N. Dunnett, S. Gaffin, M. Köhler, K. Lui and B. Rowe (2007). "Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services." Bioscience 57(10): 823-833.
Packard, S. and C. F. Mutel (1997). The tallgrass restoration handbook: for prairies, savannas and woodlands. Washington DC, Island Press.
Ricketts, T. H., E. Dinerstein, D. M. Olson, W. Eichbaum, C. J. Loucks, K. Kavanaugh, P. Hedao, P. Hurley, D. DellaSalla and R. Abell (1999). Terrestrial ecoregions of North America: a conservation assessment. Washington, DC, Island Press.
Samson, F. B., F. L. Knopf and W. R. Ostlie (2004). "Great Plains ecosystems: past, present, and future." Wildlife Society Bulletin 32(1): 6-15.
White, M. (2006). Prairie Time: A Blackland Portrait. College Station, TX, Texas A&M University Press.
Case Study Credits
Botanical Research Institute of Texas
Building Owner/Client: Botanical Research Institute of Texas (BRIT)
Green Roof Design Team Lead: Tony L. Burgess (Texas Christian University)
Brooke Byerley Best (BRIT)
Architect: H3 Hardy Collaboration Architecture
Landscape Architect: Balmori Associates (landscape master plan)
Installation Contractor: American Hydrotech, Inc.
Maintenance: BRIT
Project completion: July 2010
Green roof area: 1083 m2 (11,400 ft2)
Slide Ranch Nature Center
Building Owner/Client: Slide Ranch Nature Center
Green Roof Design Team Lead: SYMBIOS
Architect: Mark Cavagnero
Landscape Architect: Rana Creek (landscape master plan)
Installation Contractor: SYMBIOS
Project completion: 2017
Green roof area: 33.6 m2 (740 ft2)
Sonoma Academy (Janet Durgin Guild & Commons)
Building Owner/Client: Sonoma Academy
Green Roof Design Team: Rana Creek (lead), SYMBIOS
Architect: WRNS Studio
Structural Engineer: Mar Structural Design
Landscape Architect: RHAA Landscape Architects
Installation Contractor: SYMBIOS
Project completion: September 2017
Green roof area: 564 m2 (6,074 ft2)
Palomar Medical Center
Building Owner/Client: Palomar Medical Center
Green Roof Design Team Lead: Spurlock with Rana Creek as consultant
Architect: CO Architects with Stantec
Landscape Architect: Spurlock Landscape Architects
Installation Contractor: ValleyCrest Landscape Companies
Project completion: 2012
Green roof area: 5574 m2 (60,000 ft2)
SRM Developmen
Building Owner/Client: SRM Development/Google
Green Roof Design Team Lead: Thomas Rengstorf & Associates (TRA)
Architect: DLR Group
Structural Engineer: DCI Engineers
Landscape Architect: Thomas Rengstorf & Associates (TRA)
Green roof systems: DIADEM USA, Inc., Etera
Project completion: 2015
Green roof area: 1593 m2 (17,150 ft2)