Restoring green to cities, up in the air

The University of Haifa’s Green Roof Ecology Center explores creating natural habitats atop urban buildings.

A bee collects pollen from a flower. (photo credit: GILAD KAVALERCHIK)
A bee collects pollen from a flower.
(photo credit: GILAD KAVALERCHIK)
It may be impossible to plant an avocado grove on your rooftop, but a rooftop planted with bee-friendly herbs can feed the insects that pollinate avocado trees.
Prof. Leon Blaustein, ecologist and director of the Kadas Green Roofs Ecology Center at the University of Haifa, explains that green roofs reconciliate cities and nature by creating a natural habitat on urban roofs.
“We research what affects biodiversity and what biodiversity affects,” he says. “The center’s biggest goal is the encouragement of biodiversity and conservation of endangered species.”
Consider how green roofs might support not only pollinating insects but also the millions of migrating birds that regularly cross Israel’s skies. But wildlife aside, imagine the benefits to city dwellers of green spaces thriving and flowering eight or 20 stories up in the air.
Green roofs’ insulation keeps buildings warmer in winter and cooler in summer, reducing the need to turn on air conditioning, which in turn reduces greenhouse gas emissions.
“Green roofs reduce cities’ ‘heat island’ effect,” says Blaustein. “They suck up CO2 and reduce temperatures.”
A city with green rooftops could modify climate change and make summertime pleasant for everyone. Green roofs’ beauty is calming – good for the soul (and property values). And green roofs absorb noise, eliminating another everyday stress.
“One particularly interesting project of ours is integrating solar panels and green roofs,” says Blaustein.
“You might think that they compete, but we’re trying to understand if they have reciprocal positive effects. Solar panels make lots of shade. And because the sun moves, it’s not going to be the same degree of shade under the panel all the time. It’s the equivalent of a semi-open forest, where some areas get more shade, and some more sun. So we think that solar panels will increase the diversity of life on the roofs. We also think that green roofs will increase the electricity production of the photovoltaic cells. When the cells start getting warm,” he explains, “they become less efficient. Green roofs have lower temperatures than normal roofs. So we’re studying if solar panels create more diversity on green roofs and if green roofs create more electricity.”
Efficient electricity production equals money saved for the people living and working under that roof. Green roofs save money in other ways as well. For example, they require less frequent repairs.
“An urban green roof needs a number of layers to protect the roof from water and plant roots,” Blaustein says. “The layers preserve the roof, so it stays waterproof for longer.”
What seems like a passive green island on a roof is actually a complex, dynamic system involving all the elements. Like water. Green roofs absorb rainwater, a welcome effect where streets flood in stormy weather.
As for irrigating green roofs, Blaustein explains, “It’s a challenge to work in our hot Mediterranean climate. Israel desalinates a lot of water, but it comes at a high environmental cost. We want to use as little irrigation as possible.”
With a grant from the Environment Ministry, a master’s student at the center, Ariel Solodar, is researching how gray water (from sinks, washing machines and showers) may be used in irrigating green roofs. The green rooftop plots should clean the water, which itself provides some nutrients and irrigation to keep the green roof green.
The center uses only plants native to Israel.
“We’re studying what kind of diversity will reduce invasive plants,” says Blaustein.
Attracting bees and other pollinating insects is an important goal.
“We’re considering ‘bee hotels’ – wooden logs with holes in them, where bees can lay their eggs. We expect to attract some vertebrates, such as lizards and birds,” he says.
Is it realistic to hope that green roofs will eventually appear on Israeli homes? Possibly, if the government subsidizes them by discounting the land tax or taking other steps, says Blaustein. “Building a green roof for a household is a considerable investment. It will take some time before the investment pays for itself in terms of saving on heating and cooling,” he explains.
As to what a householder can plant on the roof: Where the building sustains a deep substrate and irrigation isn’t used, the professor advises planting perennials. Succulent plants such as native sedums and annuals that grow during the rainy season are appropriate for a shallow substrate. Rainwater or water from air conditioning can supply irrigation.
And what about harvesting food from green roofs? “I strongly favor the idea of agriculture on green roofs,” says Blaustein. “There are some places in the world where they actually have rice paddies on roofs. What one can do here is put in a bunch of pots. It’s not a green roof, but I support the idea of potted plants or extensive agricultural green roof plots because transportation of food to cities involves a lot of pollution.”
The Green Roof Ecology Center is initiating a collaboration with the Volcani Institute in researching rooftop agriculture.
Plans for greening the Knesset’s roof are only awaiting the end of the present shmita year. In time, the public will be able to tour green areas of the Knesset roof (large parts will remain covered with solar panels).
“We want to promote the idea that green roofs are good. They save money for heating and cooling, they’re beautiful, and they provide nature conservation” says Blaustein, adding, “It gives us research area and publicity.”
The project is part of the research thesis of PhD student Bracha Schindler in collaboration with the Green Knesset Committee, the flagship project of Knesset Speaker Yuli Edelstein and spokesperson Shmuel Chayen.
Another post-shmita project in the works is a partnership with PhD student Amiel Vasl and the Israel Electric Company, which has allotted the center space on their Jerusalem and Haifa rooftops. Space for research is essential, as projects demand experimental roof plots of comparable sizes.
“When you do environmental experiments, you need a number of replicates in order to see if there’s consistency. We’re experimenting with different sizes of plots. The smallest is a quarter of a square meter, while the largest is 16 square meters. We want to see the edge effect, where you determine if the edge of a plot is different than the middle of it. We want to apply what’s called island bio-geography theory, where the bigger the site size, the greater number of different species that thrive in it. Creating these plots isn’t cheap. The Electric Company is lending us space and paying for our research,” Blaustein explains.
The number of green roofs for research in Israel is growing in places such as Tel Aviv University and the Bird Observatory in Jerusalem.
Valuable input from experts in plant ecology and biodiversity support the center. The center manager is Dr. Shay Levy.
Blaustein acknowledges, “Our Green Roof Ecology Center not only exists because of a generous contribution by Peter and Dr. Gyongyver Kadas to create the center, but we have also benefited from the research partnership and consultancy of Dr. Kadas herself, who did her PhD research thesis in London on green roof ecology and biodiversity.”