On the top of a commercial building in downtown Tempe, yucca plants and succulents flourish, cooling down the roof and providing some eye candy for hikers on “A” Mountain.
The Tempe Transportation Center, which opened in 2008, houses one of the few green roofs in Arizona, where the heat can be inhospitable and keeping plants alive isn’t always easy.
A green roof is when a building’s roof is covered with vegetation in some way. Green roofs can vary broadly, from simple moss coverings with shallow soil to large rooftop gardens with trees.
Bonnie Richardson, an architect with the city of Tempe, said the roof and other green features in the building collectively reduce electrical consumption by about 50 percent.
“Most of the time people ignore the roofs, but as cities get denser, you’re looking down on all this stuff that isn’t very attractive. So you can make it attractive, but you can also assist in cooling your building,” Richardson said.
The transportation center’s roof was the first green roof on a commercial building that Richardson could find in the Southwest, she said.
“People are afraid of it. Because, you know, if you don’t pay attention in Arizona, you can kill plants pretty easily,” Richardson said.
Richardson also views the green roof as a bit of a laboratory for different types of plants, including ice plants and rosemary, since there aren’t many commercial buildings with green roofs in the Southwest. The plants are all low-maintenance and require little water to survive, she said. The roof’s drip irrigation system waters the plants about twice a week in the winter, with more waterings in the summer, she said.
The roof slopes underneath and holds about a foot of lighter-weight soil and a plastic structure, Richardson explained. Water soaks into the soil and catches in the plastic structure. Any excess will drain off and be recycled, she said.
A standard commercial roof can reach temperatures of about 140 degrees on the hottest days in the Valley, but with the garden, the highest is about 80 degrees, she said.
“So only have to cool from 80 to 75 (degrees). You’re just not letting it get into the building anywhere, so you’re not using as much energy to cool it down,” Richardson said.
Paul Coseo, a professor and sustainability scientist at Arizona State University, researched different types of plants on small plots on the roof of the design building at the university to see which ones would be most successful in the dry desert environment.
Coseo moved to Arizona from the Midwest, where, as a landscape architect in Chicago, he worked on many green roofs. He noticed the dearth of green roofs in the Valley and started to research. He focused on nine common landscape plants, and found that four species tended to do best here: damianita daisies, lantana, Mexican honeysuckle and huntington carpet rosemary.
“They are viable, particularly if you use a lot of water. … I think here it’s critical, at least from my perspective, to think about the water use,” Coseo said.
It’s more difficult to have a successful green roof in Arizona because of the heat and the dryness, Coseo said.
Plus, much of the green roof technology first emerged in Northern Europe, which has a similar climate to the Northeast and Northwest parts of the United States, he said.
There’s a web of research on green roofs in arid climates, but much less knowledge on them in less arid places like Arizona, he said. Without solid research, it’s hard to know if the same benefits seen in arid regions – energy savings, stormwater reduction, wildlife habitat – translate to the desert, Coseo said.
“The question is still out on that. … There’s actually not much written about how they’re performing here. I think we just don’t know enough,” Coseo said.
But, if someone is considering adding a green roof to a building, there are social benefits, he said. As cities get denser, there isn’t as much space on the ground for greenery, and green roofs or rooftop gardens can provide gathering spaces and “make it more pleasant for people to be in those locations,” according to Coseo.
“Taking advantage of roof space is really critical in compact developments. … (Social benefits) are, I think, clearer benefits,” he said.