Designing and Planting Climate Resilient Green Roofs
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Rare and critically endangered Shaw’s Agave thrives on a green roof at the NOAA SW Fisheries Science Center green roof in La Jolla, California. This drought-adapted green roof is armed with native plants and an oversized rainwater harvesting system, all by Jeffrey L. Bruce & Company (Dvorak and Drennan 2021). Photo courtesy B. Dvorak, Oct. 2018.
In this issue, I will discuss ecosystem-inspired concepts that can be considered when selecting vegetation for resilient green roofs. In 2009, I first ventured into green roof plant research at Texas A&M University in College Station, Texas, where we (myself, Dr. Astrid Volder & students) planted 12 green roof modules—without irrigation. There was no water available on that roof and it was our only option on campus. People said, good luck with that, it will never work! Well, after several years of trialing a few dozen species without irrigation (once established), we found more than a few plants that survived on extensive green roofs growing in the Texas sun, heat, and drought: Opuntia cacanapa ‘Ellisiana’, Hesperaloe parvipolia, Phemeranthus calycinus, Agave parryi, Agave colorata, Hesperaloe parviflora, Manfreda maculosa, and others (Dvorak 2021). Some of these plants are not native to central Texas ecoregions, but are native to drier and warmer semi-arid ecoregions of west Texas and beyond. Shallow green roof substrates can dry out quickly and become warmer and drier than soils on the ground. So, one strategy for planting shallow green roofs might be to use plants that are native to an adjacent ecoregion where warmer and drier conditions prevail, especially, if the green roof is to receive infrequent irrigation or is designed to recover from drought.
While long-term climate changes can cause shifts in the natural vegetation over many years (Thuiller et al. 2008), it is not clear how plants may respond, shift or adapt during short-term extreme climate events on green roofs. The natural cycles of La Niña (cooling of Pacific Ocean) and El Niño (warming of Pacific Ocean) can frequently alter the jetstream and weather trends, which can cause stress for plants (Holmgren et al. 2001). The long-term establishment of plants on green roofs is certainly influenced by climate, including its dry and wet periods (Rowe 2015; Köhler 2006). This means that green roofs need to be planned for resilience, and a capacity to endure or recover from extreme weather events. Beyond selecting plants within local cold and heat hardiness zones, or planning for a reliable and sustainable source of water to irrigate green roofs (e.g. harvested rainwater in oversized cisterns, or greywater sourcing), several planting design strategies can be used that stem from the way natural systems are arranged and function: plant adaptation to diverse microclimates, and diversification of plant life cycles and forms.
Plant Adaptation to Diverse Microclimates
In natural ecosystems, plant communities are arranged by soil moisture regimes across a spectrum of topographic conditions and the soil-moisture gradients that form there (wet-to-dry). Seasonal rainfall and air temperature patterns establish and sustain plant communities over many decades. However, when the local climate becomes cooler and wetter than normal or drier and warmer than normal for more than a few years, ecosystems can begin to adapt through population dynamics when rich plant diversity is present (Gibson and Hulbert 1987). For example, if the climate turns warmer and drier for many years, species populations may shift as soils become drier and plants that favor drier and warmer soil conditions adapt. Thus, nearby plants that are naturally adapted to drier or wetter conditions or warmer or cooler conditions compete for space or replace plants that don’t survive a drought. The generation of new plants favoring the changed conditions may expand beyond a present boundary, based upon the altered microclimate of the soil. Thus, when a diversity of plant taxa are present, chances are that species that favor changing conditions will adapt and endure over time (Kiss et al. 2018).
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Plant Form Diversity
Ecosystems can sustain different forms of plants such as perennials (same adult plant persists through more than two years), annuals (a plant completes life cycle in one growing season), bulbs, and perennial or evergreen forms of succulents. Some plants grow low to the ground (groundcovers) while others are upright and vertical. Plant communities often include both prostrate and upright plants. Prostrate groundcovers protect soil from wind and soil erosion but they may become stressed during abnormal heat or drought events. Prostrate groundcovers take on maximum surface area exposed to solar irradiation while upright forms of plants minimize plant surface area exposed to direct sunlight. Some plants have shiny, hairy, spiny, or waxy leaves and can conserve moisture during drought and heat events (Figure 1). Knowing and making use of different life cycles of plants and different forms of plants can be helpful when selecting vegetation to endure through extended drier or warmer periods on green roofs (Rainer and West 2015).
Built in 1979, the Anza-Borrego Desert State Park green roof (forground) on the Visitor Center mimics the natural vegetation of the Anza-Borrego desert landscape (backgroud). Plants were spaced to mimic the natural spacing of plants in the desert landscape to conserve water, and make space for annual wildflowers. This green roof has the original waterproofing, 12 inches of substrate and wide variety of forms of vegetation. The endurance of this desert ecosystem green roof is a testament to the longevity and durability of green roofs with native drought-tollerant vegetation. Photo courtesy B. Dvorak, Oct, 2018
Nature-based Strategies for Green Roofs
On a green roof, it is wise to make use of regionally inspired nature-based strategies to keep a green roof vegetated even when the climate is abnormal. In addition to seasonal variances of climate year to year, longer-term cycles such as La Niña and El Niño can affect how plants adapt to green roofs (or not), and thus green roof designers need to know about time-tested approaches and good theory and practice for green roof plant selection (Dvorak and Bousselot 2021). I will illustrate these concepts by discussing a few examples covered in my new edited book, Ecoregional Green Roofs: Theory and Application in the Western USA and Canada (Springer Nature, 2021).
Annuals
Annuals are rapid-growing plants that complete their life-cycle in one or more seasons when rainfall is sufficient. They can be effectively used on green roofs as focal points, as “superstar” pollinators, used to attract birds, or as a strategy to include plants that can self-sow into bare areas. In nature, multiple species and can occupy the same space with annuals. Annuals work best on green roofs when they are incorporated into a mix with perennial forbs or succulents. Since most annuals complete their life-cycle in one season (spring or summer bloom), there may be no plant canopy or a shriveled-up/brown remnant of the plant that will eventually fall to the surface and reseed new plants next year. However, it may be possible to establish annuals that grow in different seasons in the same space. Annuals such as bluebonnets (Lupinus texensis) or Indian paintbrush (Castilleja) grow, bloom, and set seed during the spring, while summer and fall-blooming annuals such as Firewheel (Gaulliardia pulchella) may share the same space as bluebonnets. Gaulliardia also serves butterflies such as the bordered patch butterfly (Chlosyne lacinia) and the painted schinia moth (Schinia volupia), which feed upon its foliage. Black-eyed Susans (Rudbeckia hirta) are another summer (warm season) super-pollinator annual that can be used with early spring blooming annuals or perennial grasses (Figure 2). Some warm-season perennial succulents can also be used similarly. Phemeranthus calycinus, for example, is a warm-season perennial succulent that lies dormant fall through spring and grows and flowers during the heat of the summer and early fall and may be used with early spring bloomers like bluebonnets.
Annuals can serve as vital members of most native grassland and semi-arid ecosystems, and they can usually be sourced locally from reputable suppliers. One should seek guidance with the use of annuals to be certain that the proposed annuals are not invasive or aggressive on green roofs or the landscape. This approach is appropriate for green roofs with substrates typically at least 5 inches deep and with a reliable and sustainable source of irrigation, especially for warm-season annuals.
Perennial Grasses
Although most North American perennial bunchgrasses tend to be deep-rooted, they can avoid drought stress by shutting down the top growth when water becomes scarce, even on a green roof. A green roof outside Austin, Texas had been established for five years before repairs were needed on the pump of the rainwater harvesting system (Figure 2). Since the roots of plants on the green roof were mature and well-developed (with irrigation), the top-growth of the plants shut down but the roots laid dormant when irrigation stopped functioning. Once the irrigation system was repaired, the top growth sprouted back into a full canopy, after about 8 months of no irrigation. In nature, this is how bunchgrasses survive drought in deep prairie soils. On this green roof, long roots grow laterally and keep cool in the 10-inch deep custom-designed growth media (Dvorak and Skabelund 2021). It is not a good idea to grow grasses on shallow green roofs or without a reliable and sustainable source of irrigation.
A variety of plants native to Blackland prairies (left) grow on a green roof outside of Austin, Texas (inset). Its 10-inch deep substrate is sloped and irrigated with harvested rainwater stored in an oversized cistern (right). Constructed in 2013, the well-established vegetation became dormant (no top growth) in 2018 when the irrigation pump stopped functioning and needed repairs. The images shown here were taken four months after the irrigation system was repaired. The perennial vegetation returned to its former glory, and seeds of many annuals that lied dormant on the substrate emerged. The yellow blooms of black-eyed Susan (Rudbecki hirta) are one of 15 annuals on this roof. Other annuals include Callirhoe leiocarpa, Chamaecrista fasciculata, Coreopsis tinctorial, Gaillardia pulchella, Lupinus subcarnosus, and Monarda citriodora, most of which grow across the central and eastern US and beyond. Photos courtesy Bruce Dvorak.
Succulents
Sedums (Figure 3) are some of the most familiar and popular succulents used on green roofs. Since they store water in their leaves and roots, they can endure drought events and can thrive in well-drained shallow substrates. However, there are many hundreds of taxa of succulents that are native to North America and grow on green roofs, especially in hot and dry climates. Some larger forms of succulents such as Red Yucca (Hesperaloe), Prickly Pear (Opuntia), and Twisted-leaf Yucca (Yucca) will need at least 6 inches of substrate depth (or more) to thrive (Dvorak and Skabelund 2021). These plants can tolerate full sun, wind, high daytime temperatures, and drought. If a green roof is in the public eye, then spineless forms of succulents will be appreciated by those maintaining the green roof. Since succulent plants naturally adapt to climates where heat and drought are common, they may be able to survive on a green roof without frequent irrigation. Thus, when and if drought restrictions prohibit the use of potable water for irrigation, such plants may survive even with little or infrequent watering, when properly maintained.
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Bulbs
Bulbous plants are known for their thickened and fleshy roots where they store water and nutrients. Some bulbs grow only in moist, dry, or mesic habitats, while some grow in a variety of soil moisture conditions. They can be naturally adapted to areas with drought, and able to complete their growth cycle even during abnormally dry periods. Alliums (wild onion) perform well on green roofs across North America and have a long growth cycle. Other bulbs, such as Rain Lillies, may complete their growth cycle in several weeks. Bulbs such as irises can be used on green roofs, especially near drainages where water collects. Iris missouriensis has a rhizomatous root, grows in moist to wet soils, and on the green roof at the Berry Biodiversity Center at the University of Wyoming research green roof (Figure 3). Irises were planted near the roof drains where it forms large colonies in the consistently wet growth media (Dvorak and Bousselot 2021). Irises bloom from May to July and attract insects and hummingbirds.
The Berry Biodiversity Center green roof at the University of Wyoming has over 109 native plant species, including a native Sedum. There are a variety of forms of mostly upright plants including grasses, annual wildflowers, perennials, bulbs, and succulents. They were selected for their ability to adapt to a variety of dry to wet microclimates on the green roof and thus this green roof is likely to be resilient and adaptive for many decades to come. The open spacing between plants on this low mound mimics habitats where these plants thrive in local conservation reserves. The light-colored gravel mulch prevents heat gain, protects the 10 inches of growing media from wind erosion, and provides habitat for wildlife. Photos courtesy Bruce Dvorak.
Summary
Through the use of tactics and strategies found in nature, green roofs can be designed to adapt to the inevitable cycles and stresses that climate can impose. Green roofs can be designed to thrive, even when climate events challenge green roofs by using some time-tested approaches:
Select plants that are native to an adjacent warmer or drier habitat, or ecoregion for shallow-depth green roofs or south-facing green roofs that tend to be warmer and drier than local soil conditions.
Include at least 15-20 taxa of plants that are native to a variety of microclimate conditions. When a diversity of plants are selected, species that favor changing conditions (wetter or drier) may adapt and make a green roof resilient.
Include prostrate and upright forms of plants to allow vegetation to compete for its preferred niche on a green roof.
Light-colored mulch can be used to avoid excessive heat gain to the substrate and can help retain moisture in the substrate.
If irrigation is used, moisture sensors should be included in zones to help conserve water and prevent overwatering of the substrate.
Annuals can be used on green roofs to be “superstar” pollinators and to include plants that can self-sow into bare areas. Since many annuals complete their life-cycle during one season (spring, or summer), it may be necessary to include several species of annuals to have plants actively growing in multiple seasons.
Bulbous plants often feature colorful flowers and they are known for their thickened roots where they store water and complete their life-cycle during one season.
Succulents store water in their leaves and roots can endure drought events. There are many hundreds of taxa of succulents beyond Sedum that are native to North America, including many that are native to hot and dry climates. Ecoregional Green Roofs is a new book that explores over 830 taxa of plants native to North America, some of which were featured in this article.
In summary, it is easy to see the immense value in conserving natural systems, as they can provide insights into eco-friendly design, and inform about concepts that help make green roofs resilient and responsive to changes in climate.
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 Dr. Brad Rowe, Michigan State University for reviewing this article.
More Information
https://ahsgardening.org/gardening-resources/gardening-maps/
https://bullittcenter.org/2015/08/06/case-study-living-proof/
https://peasepark.org/peasecology).
See References Below
References
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