Planting Green Roofs For Extreme Heat and Drought

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Introduction - Planting for Climate Extremes

In a previous issue of LAM, I discussed different approaches to selecting plants for climate-resilient green roofs. For example, one approach is to select plants for green roofs that avoid drought and heat stress like many species of annuals and perennials that become dormant until favorable climate conditions return. Another approach is to select heat and drought-resilient plants that store water internally for later use during hot and dry periods. Succulents, for example, store moisture in their leaves, stems, or roots and some have developed reflective or thickened leaf surfaces to conserve moisture. Most succulents have shiny or waxy surfaces to reflect heat energy, are upright in form (to minimize solar radiance), and complete CAM photosynthesis at night instead of during the daytime to reduce water loss. Regionally adapted succulents can thrive in climates where heat and drought extremes are common such as the Desert Southwest, however, some succulent plants persist in humid or mild climates where thin soils and steep slopes form amplified or xeric growing conditions.

This sandstone glade is a main feature of Moss Rock Nature preserve in Birmingham, Alabama. Here, a mix of low-growing succulents, herbs and grasses thrive on this rocky surface. The rock surface absorbs and reflects heat, which causes stress for some forms of plants. Here, these heat and drought tolerant plants have adapted and found a permanent home. They also make great candidates for green roofs. Photo: Bruce Dvorak

Roof membranes exposed to sunlight heat up resulting in amplified air temperatures. Identifying plants for use on green roofs in any region might include a visit to a local nature preserve. In urbanized regions, it is not uncommon that some nature preserves have steep-sloped areas with shallow soils and fractured or unstable bedrock. Such lands are typically expensive to develop, hold little agricultural value, and remain in their wild condition. Landscape features such as glades, barrens, alvars, or rocky outcrops are ideal locations to find plants that might be adaptable to local green roofs (Lundholm 2006, Best, Swadek et al. 2015). Due to the physical characteristics of such landscapes, they include plants that grow in settings with amplified micro-climates that retain, radiate, and reflect solar heat. Plants that are adapted to these environmental conditions may be great candidates for green roofs.

Glades and Barrens for Resilient Green Roofs

This summer, I visited Moss Rock Nature Preserve, located in Birmingham, Alabama. I learned from landscape architect Sharon Deep-Nelson, PLA, about her work protecting Moss Rock Nature Preserve from becoming developed. I also learned that what makes Moss Rock Nature Preserve special is its sandstone glade habitats with many unique plants. Some of these plants are currently used on green roofs, however, with over 136 species of wildflowers and herbs in the preserve, there may be many more candidates appropriate for green roofs, but they are yet to be discovered! The glades at Moss Rock Nature Preserve could help inform green roof designers that are looking to make use of native and heat-adapted plants for green roofs in Birmingham, Atlanta, and cities throughout the southeastern United States.

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When I visited Moss Rock Preserve this summer in mid-July, I was happy to discover some familiar plants including the superstar plant fameflower (Phemeranthus calycinus). While exploring the glade, I found many clusters of native plants growing, blooming, and thriving, even though they were living in shallow soils and were surrounded by large slabs of stone. In glades and barrens, surface and air temperatures are often elevated above and beyond the surrounding forested environments. This is why these habitats have been identified as special places to identify plants for green roofs as habitat templates (Lundholm 2006).

Fameflower grows in Moss Rock Preserve at the glade, shown here (front left). At other locations of the preserve, cactus, agave, and other heat tolerant plants thrive. Some of these plants are likely candidates for use on green roofs in the southeastern United States where roof decks are sloped, have well-drained growing media and soil depths are appropriate. Photo: Bruce Dvorak

Fameflower (Phemeranthus calycinus)

During the early years of my investigating potential plant species for green roofs in central Texas, Ed Snodgrass, co-author of the book Green Roof Plants (2006) suggested I try fameflower (Phemeranthus calycinus) also known as Talinum and rock pink. Being new to Texas, I had no experience with the plant but was willing to give it a try. Little did I know that in 2011, Texas was heading into a historic drought in the spring when I first planted fameflower. By the end of the summer, College Station had been hit with air temperatures of 100-degrees F (37.7° C) for 69 days with only a trace of precipitation from early June to September. It was not a good time to begin green roof research on a roof without any source of water for irrigation. However, the goals of the experiment were to investigate plant species that may tolerate the Texas heat, without irrigation and on an extensive green roof (the roof had no access to water). To my surprise, fameflower not only survived, but it also produced multiple new plants the following year (Dvorak and Volder 2013). By the end of August, many of the fameflower plants were stressed, but they all survived.

Young fameflower plants grow here on a research green roof at Texas A&M University (planted 2022). In the wild, it grows and spreads in locations with shallow-well-drained soils such as talus slopes, glades, and barrens. This modular green roof tray has 4.5 inches (11.43 cm) of FLL-compliant growing media, modified for central Texas by a local producer. Shown here on a 104-degree day (40° C) in August 2023 preceded by nearly two months of 100-degree temperature days and scant precipitation these fameflowers still thrive. Photo: Bruce Dvorak

The summer of 2023 in North America experienced an El Niño of epic proportions, especially in central Texas. The city of College Station, Texas recorded 23 record high temperatures, had 69 consecutive days over 100 degrees F (37.7° C), and tied a record high of 112° F (44.4° C) degrees. In June of 2022, we planted extensive green roofs with fameflower, some in non-irrigated sections. In August 2023, I discovered that these plants were not just hanging in there, they were blooming, regardless of 29 days continuous at or over 104° F (40° C) maximum air temperature. The average high temperature for College Station in August is 96.6° F (35.8° C). On the research roof, the roof membrane surface was over 180° F (82.2° C) degrees at 11:30 am on a 100-degree day (37.7° C). The planted green roof modules had surface temperatures of 120° F (48.8° C). Fameflower is a perennial succulent with tuberous roots. It stores moisture below the surface, has tiny and shiny leaves, and requires well-drained substrates.

While it certainly holds true to its name, this heat and drought tolerant plant is not a focal point for green roofs beyond summer. Fameflower is cold hardy from zone 5 through 9 and dormant from mid-fall until mid-spring. During the winter, there is no plant canopy. It is a warm-season perennial succulent, and it should be planted with companions. One should expect to see little to nothing where fameflower grows from late fall to spring. However, once spring arrives, it begins to put on its summer foliage and a consistent show of purple blooms throughout the summer, including the hottest and driest of conditions. There are at least 16 species of Phemeranthus native to North America. To my knowledge, only Phemeranthus calycinus has been trialed on green roofs.

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Yucca

Yucca is a genus of evergreen shrubs with sword-like upright leaves that thrive in hot and dry climates. The 49 species of Yucca that originate from North America vary in size, shape and color and can be found growing across the United States (and Canada in limited locations). One of the smaller forms of yucca suitable for green roofs include Yucca filamentosa, also known as Yucca flaccida Adam’s Needle. Its variegated leaves make for an attractive plant foliage that can brighten up any green roof. It grows to 18-20 inches high, has a spread of about 20-24 inches and grows in U.S. cold hardiness zones 5-11.

My experiences with growing Yucca filamentosa 'Wilder's Wonderful' on green roofs have left me nothing short of amazed. In January of 2018, the university disassembled and removed our entire research green roof to the ground level so that it could replace the waterproofing on the building. The waterproofing was nearing the end of its life, and its warranty was about to expire. One hundred and forty-four modules were relocated from the roof down to the ground level. We had three Yucca filamentosas that were planted in 4.5-inch-deep (11.43 cm) extensive green roof modules, all of them remained unirrigated for four years. One of the yuccas survived and was replanted in the same module system on the revamped green roof research roof in June of 2022. In October of 2022 I shut down irrigation for these modules and have not irrigated them since then. The Yucca grows today and has survived the summer of 2023, without irrigation! This native yucca is known to serve butterflies including Giant-Skipper Butterfly, Megathymus yuccae and Cofaqui Giant-Skipper Megathymus cofaqui.

The Langford green roof research site hosted a mixture of drought and heat tolerant vegetation that is native to local habitats but also some are from habitats west or east of College Station, Texas. After one year of irrigation during the summer, this green roof transitioned to no irrigation. Seen here are Agave colorata (foreground), Opuntia cacanapa ‘Ellisiana’ (middle and background), Yucca louisianensis ‘Granted Wish’ (front right), Yucca filamentosa ‘Wilder’s Wonderful’, Yucca filamentosa (mid and back-center) which is native to west Texas, New Mexico, and Louisiana. Photo: Bruce Dvorak

Prickly Pear

Opuntia is a widespread genus with over 300 species worldwide and at least 61 species that are native to North America (Rebman and Pinkava 2001). Although they can grow to three to six feet in height (1-3 m) it can spread in shrub form about twice its height in the landscape. On green roofs, depending upon the substrate depth, they typically lie shallow and spread wide rather than attain full height. Prickly pears can grow in substrates as shallow as 4.5 inches (11.43 cm) without irrigation in Texas, it grows across the south and western United States on extensive and semi-intensive green roofs with and without irrigation (Dvorak and Bousselot 2021, Dvorak and Coseo 2021, Dvorak and Drennan 2021, Dvorak and Skabelund 2021).

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A green roof on the offices of the Botanical Research Institute of Texas maintains desert prickly pear (Opuntia phaeacantha) and pale yucca (Yucca pallida). Planted in mass, these plants are growing in substrates 12.5 cm deep with irrigation only as needed. Photo: Bruce Dvorak

Near the top of the list of my favorite prickly pear species for green roofs is Opuntia ellisiana otherwise known as spineless prickly pear cactus. Many species of opuntia grow on green roofs, but ellisiana one is favored because it is easier to maintain. Although it lacks large spines, its tiny hairy spines can still cause irritation. It is recommended to handle this species with gloves.

False Aloe

Manfreda maculosa is one of my favorite drought and heat tolerant plants for green roofs in the south. False aloe is endemic to southern Texas and Northern Mexico. It is one of those plants that thrives in hot and dry climates, grows in shallow soils. My experiences with this plant are that it can persist through the extremes, hot and cold. On my research roof, I have several false aloe plants that were installed in 2011 and are still hanging in there, without irrigation. The same roof has clusters of fameflower. These 4.5 deep modules (11.43 cm) have not been irrigated since spring 2011. These plants have survived record heat, drought and near record cold temperatures. When the research roof membrane was replaced, unknown to me roofers stacked modules on top of each other for about two weeks, then set them back on the roof. The plants are still alive after a decade of growing without irrigation.

False aloe is a small and ornamental succulent with an orchid-like flower that attracts the Manfreda Giant Skipper butterfly (Stallingsia maculosus). This butterfly is a rare species, because false aloe habitat is not being protected and is becoming developed. However, with the positive experiences with growing this plant on multiple green roofs in Texas, it should be grown and planted on green roofs throughout southern Texas to help conserve the species and the butterfly. This is an example of a plant species that has a special niche in the ecosystem and could serve green roofs and conservation in a big way. False aloe is endemic to south Texas. My guess is that each ecoregion of North America has endemic plants that could serve green roofs and are well-adapted to climate extremes. It may take decades yet for new plant and green roof enthusiasts to discover the richness of each place.

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Summary

Glades, barrens and alvars are natural features that are not uncommon across the land, where shallow soils and steep slopes prevail. These habitat templates are prime hunting grounds for those looking to expand the green roof pallet with resilient vegetation.
El Niños can wreak havoc on green roofs if plants are not well-adapted to the regional climate. The native plants growing in glades, barrens and alvars adapt to amplified micro-climates and could offer a richer and more resilient biodiversity on green roofs.
It is not uncommon for some of these heat and drought tolerant plants to avoid drought by going dormant for part of the year. Companion plants may be needed to support their growth or fill in space during winter.
Some succulents have sharp spines, or needle-like pointed leaves. Select plant material carefully to avoid or minimize injury to maintenance workers or the public if the plants are located in accessible spaces.
Most regions have some endemic species that may be highly adaptive. Since they are endemic, they may not grow far outside their established climate zone.

A barrens habitat at Inks Lake State Park, located about 60 miles outside of Austin, Texas. A variety of heat and drought-tolerant plants grow in shallow and well-drained soils. Hedgehog cactus, prickly pear and yucca plants grow here with only natural rainfall, even during extreme climate events. These plants grow on green roofs throughout the central, western, and southwestern United States. Photo: Bruce Dvorak

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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.

New Green Roof Biodiversity Courses

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.

Acknowledgments

I would like to thank landscape architect Sharon Deep-Nelson, FASLA (elect) for providing information about Moss Rock Nature Preserve. Sharon worked for many years with the City of Hoover, Alabama to help save and protect this glade plant community in perpetuity as a nature preserve.

See the list of references below.

Planting Green Roofs For Extreme Heat and Drought

Introduction - Planting for Climate Extremes

Glades and Barrens for Resilient Green Roofs

Summary

A Terminal In a Garden: Exploring Bangalore’s New Airport

Building Resilient and Green Futures With the European Union’s New Climate-Resilient Construction Guidance Document