Green Roofs project at Biosphere 2

Canadians Luba and Chris Miffly are scientists for a day. Leaning over one of 40 identical miniature houses, Luba aims an infrared thermometer at the base of a small, pink-flowering fairy duster on the roof.

“Honey, that doesn’t seem right. Why don’t you take that one again?” said Chris, with a clipboard and pen poised in hand.

The Mifflys are taking measurements for the Green Roofs Project at Biosphere 2, whose sprawling, science fiction-like structures rise from the hills of Oracle. Owned by the University of Arizona, Biosphere 2 is used to conduct scientific research, such as Green Roofs. The project’s goal is to determine the effectiveness of different combinations of soil, vegetation and irrigation for cooling the interior temperature of each miniature house. Eventually, the findings could be used to design green roofs for full-sized homes to reduce energy use, restore native wildlife habitat and combat the heat island effect.

Forty 4-by-4-foot houses stand waist high just south of the Biosphere’s towering rain forest enclosure. They’re painted dusty peach, yellow and rose. Their tiny windows each have a painted vignette of the nearby mountains. The flat roofs resemble meticulously landscaped planter boxes. Each contains a specific species of desert plant, and the soil is peppered with sensors that detect temperature and soil moisture.

Desert trials

 “Is it even appropriate to do it (build green roofs) in a desert?”

This question inspired the Green Roofs Project at Biosphere 2, said Mitch Pavao-Zuckerman. A University of Arizona assistant professor and Biosphere 2 researcher, Pavao-Zuckerman returned from Tucson’s annual Urban Heat Island Workshop in 2008 inspired by speakers who had successfully implemented green roofs, but questioned their suitability for the Southwest’s arid climate.

Some of the largest green roofs are found in temperate climates. The green roof atop the California Academy of Sciences in San Francisco covers close to 200,000 square feet—almost the size of four football fields. The green roof lowers the building’s interior temperature 10 degrees below that of a typical roof, according to the Academy’s website. In the Midwest, Chicago City Hall constructed a green roof that reduces the temperature inside up to 30 degrees in the summer, according to its website.

The Southwest presents unique challenges for green roofs. For one, existing roofs weren’t constructed to support the weight of water, soil and vegetation. A square foot of water alone exceeds 60 pounds, according to Professor Ron Stoltz, former director of the UA School of Landscape Architecture and Planning and part of the Green Roofs team at Biosphere 2. Roofs in Chicago are already fortified to sustain the weight of snow.

“What we’re worrying about is how much water is going to be retained in this super-saturated condition called our plant mix,” Stoltz said. The more water needed to keep the plants alive, the heavier the roof becomes and the more the building modifications would cost.

The desert presents another challenge to the design of Green Roofs. Tucson receives, on average, less than 12 inches of rain annually, reports the National Weather Service. Few plant species can thrive on such a small amount of water. Extensive irrigation can rack up a large water bill and add additional weight to the roof.

In places like Chicago, there’s enough precipitation to maintain a thriving rooftop garden. The city’s all-time annual low rainfall was 22 inches.

It’s all in the details

To combat the structural issues and the dry climate, the Green Roofs team at Biosphere 2 created the lightest soil mix that could sustain plant life. Researchers are experimenting with two coarse mixtures that resemble gravel more than nurturing plant food. Each contains more than 50 percent expanded slate, a crushed metamorphic rock. “It’s popped like popcorn under a high heat, so it’s actually very light,” said Green Roofs technician Becca Reiss.

The heavy mix is half slate mixed with half sand and organic matter, while the light mix contains 10 percent more slate. Each roof is blanketed with six to eight inches of soil mixture. The concoctions mimic desert soils and seek to maintain a balance between sustaining plant life and requiring the least amount of water.  Too little water and the plants wither; too much water and the project loses its sustainability.

“So we’re killing a bunch of plants up there,” Stoltz said in jest.

At Biosphere 2, Reiss wears a floppy-brimmed tan hat to shade her face from the sun. Gravel crunches underneath her tennis shoes as she walks up and down the rows of houses, taking measurements. Reiss uncaps a PVC pipe embedded vertically in the center of one of the roofs and reveals tiny data loggers called iButtons. They dangle on fishing line and record the interior temperature of the houses every half hour, Reiss said. Below the soil’s surface, probes measure soil moisture.

The plants rely on one of two watering regimens. The standard schedule is three times daily for four minutes, while the smart watering regimen dispenses the appropriate amount of water using the Rain Bird irrigation system to measure evapotranspiration.

“Evapotranspiration is a measurement of how fast water moves through a plant,” Stoltz said.

On particularly hot, dry days the rate of evapotranspiration is higher and plants need more water. The University of Arizona uses a similar system to irrigate the main campus.

“What would be fabulous is if we could grow with no supplemental water once the plants are established,” said Margaret Livingston, a professor at CALA. She hopes that after two years, the plants could survive without any irrigation at all.

Livingston was responsible for plant selection and chose three species, all native to the Sonoran Desert. Now nearly a year old, the plants wave gently from the rooftops in the diminutive neighborhood.

Octopus-like red yuccas point their long, slender leaves in all directions. Their stalks bear the remnants of flowers that have dried into hardened pods. Next door, dogweed bloom with daisy-like yellow flowers. The fairy duster’s wispy pink blooms live up to its delicate name. The leaves are miniatures of those found on a mesquite tree.

Livingston selected these plants for heartiness more than aesthetics. They resemble a desert landscape, not a meticulous English garden. “We joke that it’s more of a tan roof than a green roof,” Livingston said. 

 Native plants like these help replace local habitat lost to urban growth. Songbirds, hummingbirds, bees, butterflies and lizards might call a green roof home if the concept is applied on a larger scale, Livingston said.

 “The green roof as it’s designed now, in most cases, is an island in the sky,” Stoltz said, “but I think the future is when we see the urban wildlife habitat up on the roof.”

Citizen science in action

The Green Roofs experiment fits into Biosphere 2’s goal of involving people like Luba and Chris Miffly in science. Every tour that goes through the Biosphere is given the option of stopping to record measurements that will be included in the project’s final tabulations.

“It’s a good grabber,” Pavao-Zuckerman said. “It pulls people in, because it’s something that’s potentially related to their own home.”

Participants wield temperature-sensing guns, prod pointy metal thermometers into the soil and record measurements on clipboards.

Results so far show that the surface temperature of the planted roofs is almost 20 degrees cooler than that of the dark-colored control roofs. The roofs planted with red yucca have on average the coolest temperatures.

“We have a very keen interest in it [green roofs]. It seems like a great idea,” Luba Miffly said.

About 2,000 people engaged with Green Roofs from December 2010 to May 2011, Pavao-Zuckerman said. “Their willingness to spend another 15 or 20 minutes in the sun collecting data is impressive.”

Building a future in Green Roofs

By spring 2012, visitors will be able to track their data and suggest hypotheses from home with a new app. The Green Roofs team is collaborating with UA’s School of Information: Science, Technology and Arts to create both an application and a Web portal that will allow data to be uploaded to the Biosphere 2 Green Roof website.

“People can, in theory, have a longer and a more enriching interaction with the science,” Pavao-Zuckerman said. “Then it’s more than just the 10 minutes they spend collecting data.”

The Green Roofs experiment is set to run for another year and a half, Pavao-Zuckerman said. At this point the team will consider adding new species.

“I’d like to try to use the casita village at the Biosphere as the next phase of the experiment,” Pavao-Zuckerman said. These guesthouses would show the effects of green roofs on a larger scale. Researchers could compare temperatures before and after installing green roofs.

Ron Stoltz envisions an even larger role for green roofs. “The future for us and our thinking is not a green roof but is complexes of green roofs.” The research done at Biosphere 2 will help bring that vision one step closer in the desert Southwest.

(Editor’s Note: Mariana Dale is a sophomore with the University of Arizona School of Journalism).