Exceptional Green Roofs from the West
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Howdy! so goes a proper greeting from Texas. I am honored to step into this column and continue the conversation about plants for green roofs. In this inaugural issue of LAM’s new and revamped all online format, I will discuss how green roofs can also undergo editing and revamping of the plant pallet over time, by discussing two examples from the new book I co-authored (Ecoregional Green Roofs) along with eight green roof experts from across the Western US and Canada including Dr. Jennifer Bousselot, Dr. Paul Coseo, Dr. Philippa Drennan, Daniel Roehr, Nancy Rottle, Lee R. Skabelund, Dr. Olyssa Starry, and Dr. Tom Woodfin.
In this article, and the next several issues I will focus on plants for green roofs located in the western US, Canada, and beyond. Similar content is also covered in Ecoregional Green Roofs: Theory and Application in the Western USA and Canada (Springer Nature, 2021). This book came about from my interest in the kinds of vegetation grown on green roofs located in climates that often experience high daytime temperatures, intense solar radiation and/or many weeks or months without precipitation. From June 1st to October 31st I made a five-month tour of 140 green roofs in the western US and Canada while on faculty development leave (sabbatical) to gather information for the book and to meet with co-authors.
One thing that I learned traveling across ecoregions is how many of the green roofs that I visited were replanted at some point in time, as the original planting of the green roof vegetation had become altered for various reasons. While many of these replanted roofs were in unexplored regions where plants had not been pre-tested, sometimes irrigation systems failed, or the selected plants didn't establish well, and sometimes the vegetation changed because the owner of a building wanted a change of scenery, as occurred on a green roof in Kansas City.
The original stand of native wildflowers can be seen beyond the patio and border located at the parapet screen. In the foreground, the original sedum roof is no longer visible as yellow blooms of lance-leaved coreopsis (Coreopsis lanceolata) and white blooms of foxglove beardtongue (Penstemon digitalis) now dominate this section of the roof garden. Many accent species such as spiderwort (Tradescantia virginiana) were introduced with its iconic blue pedals that attract bees, butterflies, and more. (Photo B. Dvorak, June 5, 2018)
One outstanding example of a green roof that was replanted to improve aesthetics is the former TWA Headquarters building located in downtown Kansas City, Missouri (Dvorak, Bruce, and Lee R. Skabelund. "Green Roofs in Tallgrass Prairie Ecoregions." Ecoregional Green Roofs. Springer, Cham, 2021. 83-142). After TWA relocated corporate offices to the east coast in the late 1960’s, the building was abandoned and sat empty for many years. In the early 2000’s the building was added to the National Register of Historic Places, and a developer purchased (2005) and updated the facility with the addition of extensive green roofs (2007) that features 10 cm (4 in) deep substrates and four species of exotic sedum in the main roof area. Taller native wildflowers were planted at the peripheral edge of the roof garden.
After several years of the owners enjoying the green roof, Maggie Riggs, who led efforts to maintain the roof garden, convinced the owners to expand the native pallet into the sedum roof. With appropriate irrigation and weekly maintenance taking place, Maggie took advantage of her long-standing knowledge of plants native to tallgrass prairies, and approached her client who set aside a small budget for annual purchase of vegetation for infill or expansion on the green roof. Maggie, over the course of several years purchased additional perennial wildflowers for the roof garden and plugged them into the existing extensive green roof. She then later adapted a technique of relocating seedlings of parent plants into new and desired locations on the roof garden. The effect is a kind of natural garden where loose drifts of native plants compete in groupings that are similar in height, but complement each other. Maggie trains her staff to work towards a meticulously maintained “wild” garden. The owners quickly became convinced that native plants can thrive and look great on their green roof, even in a shallow substrate (with irrigation and proper care).
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At the backside of the accessible roof garden, this hidden extensive green roof provides ecosystem serves over top of occupied offices below. Here, the prairie vegetation is free to compete and only occasionally see human interference. This wilder section of the green roofs includes the same species located at the roof garden, plus the addition of little bluestem (Schizachyrium scoparium), prairie dropseed Sporobolus heterolepis). (Photo Bruce Dvorak, June 5, 2018)
Upgrades to the original sedum green roof also included interplantings of: American pasqueflower (Anemone patens), Wild red columbine (Aquilegia canadensis), Common milkweed (Asclepias syriaca), Butterfly milkweed (Asclepias tuberosa), Blue false indigo (Baptisia australis), Purple poppy mallow (Callirhoe involucrata), Lance-leaved coreopsis (Coreopsis lanceolata), Purple coneflower (Echinacea purpurea), Dotted blazing star (Liatris punctata), Dense blazing star Liatris spicata), Beardtongue (Penstemon digitalis ‘husker red’), Foxglove beardtongue (Penstemon digitalis), Aromatic aster (Symphyotrichum oblongifolium), and Spiderwort (Tradescantia virginiana). Based on my visit with Maggie Riggs, these plants all thrive on the TWA green roofs.
Another green roof that has seen consistent planting upgrades is the EcoCenter education building owned by the Port of San Francisco at Heron’s Head Park (Dvorak, Bruce, and Philippa Drennan. "Green Roofs in California Coastal Ecoregions." Ecoregional Green Roofs: Theory and Application in the Western USA and Canada: 315). This green (living) roof is an integrated part of a LEED Platinum –Zero Net Energy building, and was built to teach and foster a sense of environmental values for local children and local residents.
This green roof has a 5 cm–20 cm (2 in–8 in) variable-depth substrate (made from local materials), which helps insulate the building, manage stormwater and provide habitat for local wildlife. The initial installation of vegetation took place in 2010 and included thirteen taxa that were pre-grown at the nurseries at the properties of the Literacy for Environmental Justice, and include: Achillea millefolium, Armeria maritima, Athyrium filix-femina (lady fern), Blechnum spicant (deer fern), Coreopsis maritima, Dudlyea hassei, Fragaria vesca, Heterotheca San Bruno Goldenaster, Heuchera maxima, Koeleria grass, Mimulus aurantiacus, Salvia spathacea, and Sisyrinchium bellum. Three years later Habitat Gardens (design and installation) added additional plants that were pre-grown in pots including a mix of native and some exotic plants: Armeria maritima Splendens, Mimulus aurantiacus, Mimulus hybrid ‘Georgie Red’ Epilobium canum, Sedum telephium, Festuca glauca, and Salvia spathacea, and other additions over time.
This section of a north-facing biodiverse green roof in San Francisco meets city guideline recommendations for the inclusion of native plants. Shown here upfront left is woodland strawberry (Fragaria vesca) and right Catalina Island live-forever (Dudlyea hassei), center left yarrow (Achillea millefolium) and to the edges tufted hairgrass (Deschampsia Caespitosa) which are common to upland habitats of the bay area. The evergreen shrub on the left is manzanita which grows in an isolated section with a deeper substrate (30 cm/12 in). Driftwood branches (lower right) and a small shallow-elevation water pond are provided to attract birds and wildlife. Water for irrigation is provided from harvested rainwater cisterns located at the ground level. The gravel and stone are sourced from the gravel quarry adjacent to the property (upper right). (Photo: Bruce Dvorak, October 3, 2018)
This green roof also receives maintenance under the careful eye of a lead manager, Tai Trang, who trains volunteers to assist with maintenance activities. Seedlings of parent plants established on the roof are re-distributed to desired locations on the roof, to infill bare spots or enhance the appearance of the green roof. Timing of the transplanting is critical, as young plants need time to establish their root systems before stresses of high daytime temperatures can stress plants not fully developed.
These examples demonstrate how green roofs can and should be inspected annually to see where and what kinds of vegetation might need to be replanted. Sometimes, only a small budget is necessary to purchase plants to keep green roof ecosystems healthy, and sometimes only a knowledgeable caretaker is needed to educate workers how to redistribute seedlings or cuttings. Whatever approach is taken, the long-term care of green roofs is dependent upon a watchful eye, and someone to champion the effort!
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.