Growth. It’s a word commonly heard in relation to Austin. “A hundred people move here every day,” people say. They’re right — if they’re including all five counties that make up Greater Austin. In fact, as recently as May 2016, the U.S. Census Bureau placed Georgetown, Pflugerville and San Marcos — all part of the Austin-Round Rock metro area — among the fastest growing cities in the country. Georgetown, 40 miles north of Austin, is first on the list.
It’s not just Austin. Houston is predicted to take Chicago’s spot as home to the third largest population in the country by 2025, if not sooner. New Braunfels (near San Antonio), Pearland (a suburb of Houston) and Frisco (north of Dallas) are also on the fastest-growing cities list. They’re in the top seven.
And it’s not just Texas. For the first time in history, more of the earth’s population lives in cities than anywhere else. As the late Mark Simmons, former director of research and consulting for the Wildflower Center, succinctly put it, “We are now an urban species.”
No matter what urban area you live in, growth-related complaints abound: Traffic is unbearable; no one can afford to live here anymore; all the good jobs are taken. These are human concerns, and they affect human lives. But population growth affects the lives of plants too (which, in turn, affects human health). Room for green space on the ground is running out. According to Douglas W. Tallamy, author of “Bringing Nature Home,” development in the U.S. covers over 2 million acres per year — the size of Yellowstone National Park. But it’s not plant-rich parks filling these acres. It’s concrete. And people. And cars.
As human bodies pour in, claiming and developing and driving on the land, where does the green growth go? It’s a good question. And the Wildflower Center has been working on an answer: up. The green goes up.
Roofs With Bennies
Besides being beautiful and adding a living element to architectural structures — which is novel at the very least — green roofs provide a host of benefits. They cool the buildings they top, preserving the lifetime of the roof itself and saving energy (and money) on air conditioning. They slow down rainwater, which can help mitigate flash flooding. They reduce the urban heat island effect, a phenomenon where cities are much warmer than their surrounding rural areas. Green roofs also sequester carbon, filter particulate matter out of polluted city air, provide oxygen and serve as wildlife habitat.
In addition to these environmental benefits, architect Lauren Woodward Stanley, who served on Austin’s Green Roof Advisory Group and has a green roof on top of her own Stanley Studio, adds a few more pluses: “Education! Exposure!” she says, throwing “producer of tunas for prickly pear margaritas” in for good measure. A single green roof isn’t likely to provide all these benefits, but as John Hart Asher, an environmental designer at the Wildflower Center, says, “If you do it right, they can do a few of those things really well.” The aforementioned Simmons, who contributed a chapter to the 2015 book “Green Roof Ecosystems,” describes them as “imperative.” Asher agrees, noting, “There’s nothing that we can manufacture that delivers the ecosystem services provided by the million-year-old technology of photosynthesis.”
Native plants are a huge part of our green roofs’ success. They’ve got the genetic model to weather the climate of the region. They’re of this place.
Asher and Simmons collaborated on research at the Wildflower Center aiming to improve the success of green roofs in hot climates (which had notoriously failed). In fact, Simmons had a lot to do with convincing Central Texas architects, contractors and environmental designers that green roofs can succeed here. Casey Boyter, an Austin-based landscape designer and cofounder of green roof networking and education group, GRoWERS (Green Roofs: Working Expertise + Regional Solutions) calls Simmons “a big mentor as well as a peer.” She’s a strong believer in the power of collaboration, something Simmons helped facilitate among Austin’s green roof community.
Stanley, another co-founder of GRoWERS, approached Simmons in the fall of 2013 after a lecture he gave on using an experimental foam layer on green roofs, which ultimately led to their collaborating on the green roof atop the pool house at John Gaines Park in Austin’s mixed-use Mueller development.
Another local architect, Christopher Sanders — who worked with the Wildflower Center on its Admissions Kiosk, a kayak rental kiosk in The Woodlands, Texas, and a green roof on top of the Experiential Learning Center at Camp Young Judaea in Woodcreek, Texas — says, “I fell for [Mark’s] spell. I bought in hook, line and sinker. It made me think, ‘Why can’t we do this?’ He dispelled a lot of myths,” adds Sanders, citing Wildflower Center research as powerful backup: “It’s easier to sell when you’ve got good science behind your conversation.”
So why was there such skepticism in the first place? Boyter puts it plainly: “People get intimidated by the climate.” More specifically: Extreme temperatures, prolonged drought and intense rainfall all present challenges — but it’s precisely because of these challenges that hot climates need green roofs.
Lost in Translation
The green roofs of interest to Wildflower Center researchers are called “extensive” green roofs. These have a thinner layer of growing media (less than 20 centimeters or 7.8 inches) and are subject to an amplified version of ground-level weather, including high winds, high thermal loads and varying humidity. The other type of green roof, known as “intensive,” is more like a roof garden with a greater variety of plants — even trees — planted in much deeper media (up to three feet). These require more irrigation and maintenance and, perhaps most importantly, roofs that can handle up to 150 pounds of weight per square foot. Extensive roofs are much more practical to install and easier to maintain in terms of water and fertilization. There’s simply “a greater potential for application” with extensive green roofs, according to Asher.
But extensive green roofs were initially designed at much higher — and cooler — latitudes, in cities such as Frankfurt, Chicago, Portland and Seattle. In Central Texas, Stanley says, “A green roof has to work a lot harder.” Asher agrees, noting, “Even though soil is a great insulator, any temperatures above 95 degrees in contact with the rhizosphere or the root zone, you start to experience mortality in plants.”
There’s also the problem of heat building up in drainage mat air gaps; drainage mats, which are basically roof-sized sheets of egg carton (same shape and size, no eggs), sit under plants and their growing media. Each egg-sized depression holds air that can heat up drastically on a Central Texas rooftop. That air gap, says Asher, was “a big problem” with green roofs in hot climates. With only a few inches of media, the air under the plants was simply getting too hot. To combat this, the Wildflower Center implemented the use of thinner, three-eighths-inch drainage mats.
The Wildflower Center also improved on earlier green roofs by developing its own planting medium, SkySystem™. Asher says most planting media prior to SkySystem was either composed of components that had a negative environmental association, such as sand (which requires mining), or they had green products in them, like coconut coir fiber, that weren’t locally sourced. “Where are the coconut plantations in Texas?” he asks. “They’re not here; they’re in the South Pacific, so there’s a carbon footprint association with that.”
Recalling Kentucky Fried Chicken’s Original Recipe motto, Asher says he “can’t go into everything about SkySystem’s herbs and spices,” but he does say the media consists of 100 percent recycled content — all of which is locally available. It’s a low impact development (LID) mix, including material that’s organic, like compost, and mineral, such as brick. “We have the inherent capability within our media that prevents these big temperature fluctuations,” he says. “If we can do that with recycled local materials, even better.” Currently, SkySystem is only used on projects contracted through the Wildflower Center, but Asher says a major green roof manufacturer is considering licensing the product, which would make it available for purchase by other developers and designers.
Another aspect of green roofs that didn’t translate well from cooler climes has to do with plant selection. Twelve years ago, when Simmons was first tasked with finding out why green roofs were not doing well here, he realized they were mainly planted with sedums (Sedum spp.), or stonecrops. But sedums have a hard time fixing carbon dioxide above 80 degrees Fahrenheit. During steamy Central Texas summers, even low temperatures hover near 80, meaning sedums’ stomata don’t function properly. As such, their photosynthetic cycle is interrupted and they eventually die. The Wildflower Center has solved “the sedum problem,” as Asher put it, by using native plants. “Native plants are a huge part of our green roofs’ success,” he says. “They’ve got the genetic model to weather the climate of the region. They’re of this place.”
Plants of Place
Now, native plants seem like a no-brainer — and that’s not just the Wildflower Center talking. “Because we are such a harsh climate,” says Boyter, “we have some pretty great plants to pull from. If they can hang out in an eighth of an inch of soil in west Austin, I’m pretty sure they can do okay in six inches of soil irrigated on a roof.” Of course, there are some native sedums in Texas, but not all native plants are cut out for rooftops. The Wildflower Center has seen greatest success with two approaches to native plants on green roofs: an herbaceous prairie mix and a succulent-and-annual-wildflower mix.
There are pluses to each approach. The herbaceous mix is easier to install, cheaper and usually encounters fewer weeds after establishment. It can feature a wide variety of grasses and wildflowers, from buffalograss (Bouteloua dactyloides) and blue grama (Bouteloua gracilis) to little bluestem (Schizachyrium scoparium) and purple threeawn (Aristida purpurea); wildflowers such as prairie verbena (Glandularia bipinnatifida), black-eyed Susan (Rudbeckia hirta) and horsemint (Monarda citriodora) are mixed in for color and diversity. On top of a small kiosk (like the Wildflower Center’s) or spread across an entire home (such as Edgeland House, an award-winning private residence on Austin’s Colorado River), the herbaceous mix looks fuzzy and full, and its color and thickness change with the seasons. It also helps Central Texas’ quickly diminishing blackland prairie, one of the most endangered habitats in North America, gain ground — or roof, as it were — one plot at a time.
The succulent mix, while more susceptible to weeds, can survive with less water. It also offers more variety in shape than a prairie scheme. The smooth paddles of spineless prickly pear (Opuntia ellisiana) join spiky red yucca (Hesperaloe parviflora) and spotted false aloe (Manfreda maculosa), while the basal leaves of annual wildflowers — usually Texas bluebonnets (Lupinus texensis) and plains coreopsis (Coreopsis tinctoria) — provide green mulch in winter. This assemblage also creates an upper and lower canopy. On a recent walk to the Wildflower Center green roof research plots, a manfreda was seen growing in the eastern shade of a prickly pear. Center Environmental Designer Michelle Bright says there’s “not a pattern; plants will pick where they feel comfortable.” She also calls prickly pears the “superstars” of green roofs because even their fallen pads root in and grow.
Many of these superstars adorn a green roof Bright helped design for a medical office building and parking garage at The University of Texas at Austin’s forthcoming Dell Medical School. Her work is also behind the Living Wall Project at Goldsmith Hall, home to the university’s School of Architecture. This much-lauded feature, which is the product of a five-year collaboration between the Wildflower Center and architecture professor Danelle Briscoe, adds green space to a vertical, rather than horizontal, plane (read more on page 28). But as a west-facing wall in Texas, it’s going to get its fair share of heat and sun, which is why green roof technology — including carefully selected native plants (such as little bluestem; prairie verbena, Glandularia bipinnatifida; and red yucca) and cells that can hold enough soil to deter desiccation — was applied to its design.
In some situations, like the green roof on top of our own Wildflower Café, both plant mixes are used: One side is herbaceous and the other succulent. The herbaceous side of this particular green roof was planted with Habiturf®, a mixture of native turfgrass species developed by Wildflower Center researchers to perform well and conserve resources in hot, dry regions. In any case — prairie or succulent, vertical or horizontal — Asher says the Wildflower Center “has a very specific mission to solve problems with native plants or ecosystems.”
When It Rains It Pours
As most hot climate gardeners know, even drought-hardy natives need some water to survive. So there is the issue of making sure plants on green roofs get enough water to thrive and provide the very benefits they’re designed for.
According to Asher, an average Central Texas rain pulse is about one-quarter of an inch. He says Wildflower Center research for the City of Austin demonstrated that our green roofs were good at holding onto those quarter-inch events. “Any more and it’s just going to become saturated like anything else.”
But green roofs are often designed with heavy downpours in mind, too. Both the Camp Young Judaea and John Gaines Park green roofs have a rain-gathering aspect designed right alongside the planted section. These “rain-roof ” surfaces gather water that is collected in cisterns and then pumped back up for irrigation. An inverted butterfly roof, which can be seen on the Wildflower Center’s Auditorium and as part of Camp Young Judaea’s watercollection setup, can greatly facilitate rainwater harvesting. Water can — and often does — come from HVAC condensate as well.
Usually more irrigation is needed as a green roof gets established; then watering switches to a timed drip or, in more sophisticated arrangements, done only when soil sensors indicate that plants need it for survival. The Dell Medical School green roof aims to use such sensors to potentially not water at all. The Wildflower Center research plots that this particular roof is based on haven’t been irrigated since April 2014, and they’re doing just fine. The same plant mix — coupled with a slightly deeper 12 inches of soil (thanks to the structural integrity of the parking garage below it) — could make it possible for this roof to be a system that maintains itself. Markus Hogue, an irrigation specialist with The University of Texas at Austin’s Facilities Services who Bright calls “a water conservation guru,” will help implement the soil sensors. The fact that he was able to reduce water use at the university by 66 percent using the same soil-monitoring technology (in conjunction with more efficient nozzles and live weather data) is encouraging.
Even an herbaceous roof using drip irrigation is very conservative in terms of water use. “We run the drip for three minutes three times a week,” says Asher. “Each emitter only drips .42 gallons per hour.” Stanley says of her studio’s green roof, “You can accept the effects of a hot dry season and let everything brown and go dormant, or rise to the challenge of keeping irrigation going through tough seasons to maintain some green.” If you go with the former, Bright says, “You’re gonna get brown crispies,” which can be dormant herbaceous plants or dead annuals (which should be left long enough to seed).
At their most exciting, green roofs also create opportunities for amazing architecture. Daniel Loe of Bercy Chen Studio LP, an architecture and construction firm that worked with the Wildflower Center on the breathtaking Edgeland House, says native plants provide advantages “beyond the practical realities of lower maintenance and less water consumption.” He says, “There’s a pride of place generated as a result of using native materials. It’s a way of saying, ‘This house could exist in no other location than where it is.’”
Among other incentives, green roofs just look cool. “Who wants to be around an ugly hot roof ?” asks Asher. Stanley agrees that her own green roof provides “a place to watch the stars and the street, a habitat for butterflies and other critters, a bluebonnet and daisy showcase … and through it all, a place to show others who are curious and enticed by green roofs” — which circles back to her earlier comment about education. Sanders believes in the potential of green roofs to raise awareness too. “It’s important that the green roof be visible,” he says. “The green roof changes with the seasons. It’s beautiful. I wouldn’t want to waste that opportunity.” But it’s not just eye candy. He says it’s a way to make others think about our consumption of water and power. Loe agrees, “As designers, builders and citizens, we have a moral obligation to limit our destructive forces on nature,” he says. “Green-roofs are one means by which the man-made and the natural environments can coexist.”
So, can you try this at home? It depends. Sanders says his firm has only worked on green roofs as part of new construction, and pretty much all experts agree that retrofitting can be prohibitively expensive. Asher notes that, if you’re going to design a new house, it’s not that much more to put in a green roof. While it is going to cost more per square foot, it will have a longer life and save energy costs cooling the interior, so it’s a trade-off. Boyter also mentioned the possibility that your house was designed to have another story that was never built. In such cases, the intention for another floor can provide enough load-bearing capacity for an extensive green roof. She says it’s worth investigating your structure if you’re seriously interested. “If you have the ability to do it,” Asher asks, “why not?”
Alive and Thriving
Green roofs in hot climates have come a long way in recent years. Asher says he’s seen a dramatic spike in demand for green roof projects since he began at the Wildflower Center in 2009. Back then, “Green roofs were this cool thing, but they weren’t working,” he explains. Thanks to research and experience, the Wildflower Center has been able to say — and, more importantly, prove — “This works.”
Lady Bird Johnson once asked, “Will your cities be places to thrive in, or to escape from?” Standing on top of a Dell Medical School office building, with downtown Austin and the Texas State Capitol lingering in the distance, I watch men in bright turquoise helmets and fluorescent yellow vests plant prickly pears, red yucca and false aloe into permanent homes on the roof of a major Austin development, and I realize plants and humans can both thrive here. All we have to do is look up.