HOUSTON (CN) — Living roofs are a buffer against pounding hail, flooding rain and extreme heat. And done right, one expert says, they pay homage to their regions’ pre-concrete landscapes.
Landscape architect Bruce Dvorak managed one of the first municipal green roof projects in the U.S. He led the installation on Chicago’s City Hall in the early 2000s, the brainchild of then-Mayor Richard M. Daley who had been introduced to the concept during a trip to Europe.
But to walk among the rooftop’s 20,000 plants and beehives that produce 150 pounds of honey per year one must be part of a tour as due to practical considerations—no direct elevator access, lack of safety railing—the city chose not to allow unrestricted public access.
Luckily for the curious, there is no shortage of these spaces in America.
Now a Texas A&M University professor, Dvorak took a sabbatical in 2018 to explore how the industry had grown since its early days in Chicago. He took his wife and two homeschooled children on a long trip across the Western U.S. and into British Columbia, Canada.
“We hit the road in June 2018, put 22,000 miles on the truck, bought a travel trailer, sold the house. So we were on the road for five months. We visited 148 green roofs,” Dvorak told the Houston chapter of the Sierra Club on Thursday, discussing the fruit of his journey, his new book “Ecoregional Green Roofs” in which he and contemporaries did case studies with a special interest in roofs with native plant species.
“Most of the roofs were not really biologically diverse. Only a few of them had more than 20 native species. The green roofs with high plant diversity attracted a variety of wildlife – bees, butterflies, song birds, ducks, migratory birds,” Dvorak explained in the virtual meeting.
In fact, the professor said, the most popular plants for green roofs in the U.S. are sedums – low-growing plants known as “stonecrops” because they like dry, rocky soil – from Europe, western and northern Asia, Mongolia, China, Japan and Armenia.
“It’s really just a collection of exotic sedums and it’s a generic habitat that doesn’t represent any habitat on Earth,” Dvorak noted.
The inclination for sedums is understandable since living-roof plants are typically planted in just 4 to 10 inches of soil that cannot accommodate extensive root systems, and they must be hardy enough to thrive in direct summer sunlight. Native soil does not work, Dvorak said, unless it is gravelly and sandy, because it contains too many fine particles that will wash out and clog drains.
Despite the challenges, advocates say the benefits are substantial.
Besides their beautifying quality, they reduce flooding by retaining rainwater – many include rain-collection systems – and delaying the release of storm water down onto city streets, relieving pressure on sewage systems amid deluges. They make buildings cooler, decreasing the use of air-conditioning systems so they last longer.
They can also improve air quality by capturing pollutants and filtering noxious gases, according to Green Roofs for Healthy Cities, a Toronto nonprofit that teaches people how to build them.
Accompanied by the steady rush of vehicles passing on Interstate 10 and a helicopter buzzing overhead on Friday, Carlos Rebollar stood beside a garden on POST Houston, a downtown post office and mail-sorting warehouse refashioned into a food hall, concert venue and office space that opened in November, and told why he likes to visit the nascent rooftop park.
“I live in Second Ward and my plan today was to go out the ‘burbs. There’s a huge library out there where I get solitude. But my plans got mixed up. I was like ‘Where could I go that’s in the city that’s a few minutes away from my house, but where I could still get some solitude and some cool scenic views?’ So I came up here,” said Rebollar, 36.
“So this is definitely going to become a place where I come and do contemplation, meditation. I’m a pastor so I enjoy alone time and solitude to kind of gather your thoughts and recenter yourself. . . . When I saw this that’s the first thing I thought, like in the mornings when there’s not too many people I just come out here and observe nature, but yet in the middle of the city so I think that’s really cool, kind of that mix of both.”
While POST Houston’s entire rooftop is not a green roof in the traditional sense because it includes raised plant beds and it is not all soil sitting directly atop a membrane—material, such as synthetic rubber, used for all roofs that creates a waterproof barrier—the vegetation on its surface will likely increase its durability.
“Green roofs can be designed to survive a long time,” Dvorak told Houston Sierra Club. “There are green roofs in Germany that are over 100 years old. So they do a great job at protecting and waterproofing."
"A hailstorm came through Denver a couple of years back and they had billions of dollars in damage on waterproofing systems. But the buildings with green roofs didn’t have any damage because the plants and the soil protect the membrane from hail damage," he added.
Dvorak said one of his favorite green roofs in the U.S. exemplifies how they can become magnets for biodiversity.
An ecologist designed the 2.5-acre living roof atop the California Academy of Sciences in San Francisco, Dvorak said. It started with nine plant species but has expanded to 94, most of them Bay Area natives.
In 2014, the academy acquired the iNaturalist app through which people can document where they encounter wildlife and plants for uploading to maps that scientists and conservations use to track species. With the app, academy volunteers and staff have recorded their observations of more than 550 species of animals that live on or frequent its green roof, according to Dvorak.
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