Spotlight: The California Academy of Science’s Green Roof

How does a landscape architect cultivate nature without corrupting it? The question goes back at least to the 18th century, when the novelist Samuel Richardson wrote that the ideal was for the artist “not to level hills, or to force and distort nature; but to help it, as he finds it, without letting art be seen in his works, where he can possibly avoid it.” The undulating green roof that sits atop the new California Academy of Sciences building in San Francisco’s Golden Gate Park tries to strike a similar balance. Like the museum it shelters, it is designed to respect the natural world even as it appropriates it, serving at once as a wildlife habitat and a first-rate work of art.

When the roof (along with the building) opens to the public this month, it will be, at 2.5 acres, the largest such “living” structure in California. Conceived in 1999 by the architect Renzo Piano (who also designed the building), it was completed over six years by a team of specialists that included the environmental consultants Rana Creek; Frank Almeda, a botanist at the Academy; and the landscape-architecture firm SWA Group. The roof has seven signature hills, created to evoke San Francisco’s topography, and is blanketed with nine native plant species, which were chosen for their ability to attract pollinating creatures like bumblebees and hummingbirds, and butterflies such as the threatened Bay checkerspot. Like other green roofs, this one helps regulate temperature indoors and out—though the urban-heat-island effect isn’t a dire concern in San Francisco, where the mean annual temperature is about 58 degrees Fahren­heit. The roof is also designed to absorb 98 percent of all storm water, a decided benefit in a city where the sewage system is often overwhelmed during heavy downpours.

When Piano presented his first sketches to the Academy, he described his idea for the roof by asking his audience to imagine a huge elevated swath of the park, with the museum tucked underneath it. Executing the concept, however, wasn’t easy. One of Piano’s first demands was for an assortment of plant species with a particular kind of look: “He wanted it to be very monolithic, very neat and clean and green,” says John Loomis, of the SWA Group. But the plants that look good together and the plants that thrive together are not always one and the same. So Paul Kephart, of Rana Creek, experimented with 29 different plants before hitting on a selection that would promote biodiversity as well as meet Piano’s aesthetic requirements. “I wanted as much diversity as possible, and I challenged Renzo on this,” Kephart says. “He said, ‘Paul, this is all very interesting, but it has to be beautiful.’” After a few “spirited discussions,” the team chose four perennials and five colorful annuals that live well together, are low-growing (and thus “clean-looking”), and have extensive green periods.

A further challenge surfaced when Piano explained that he wanted to transport and install the plants without using petroleum-based plastic containers. Kephart responded by creating an innovative tray (soon to be patented) from coconut-husk fiber, a waste product from coconut trees. This BioTray is held together with natural latex and lined with 36 strains of fungi, which supply nutrients to the plants. Laid in large num­bers on the roof like tiles, the trays degrade within three years, leaving behind a colorful carpet of vegetation.

Piano’s grandest design gesture, the seven hills, also posed an obstacle: How to keep the soil from slipping down them? After an initial solution involving concentric circles was rejected by Piano’s office on visual grounds, the landscape designers devised a drainage system using a 24-by-24-foot grid of rock-filled wire baskets called gabions. In addition to allowing for drainage and strapping the soil in place, the gabions can be used as footpaths for the maintenance crews that traverse the garden’s steep slopes.

Today the roof is essentially complete. But, as with any living system, it will continue to evolve in unpredictable ways. “One of the most fascinating questions I get is, ‘What will this roof look like in five years?’” Almeda says. “People are always astounded when I say, ‘I’d like to be able to tell you, but I can’t.’” Like animals, plant species com­pete with each other for common resources, and it is not easy to predict which ones will win out. There has already been an unforeseen explosion of growth as birds and bees have dispersed foreign pollen and seeds on the site. “Wildlife will bring things to you that you may not want,” Almeda says. “And, if they bring a native species, just because it’s native doesn’t mean that we will keep it on the roof.” A few water-sucking willows, for instance, were evicted. “If we left them, they’d take the water from everything else and nothing would survive,” Almeda says. A noninvasive monkey flower, on the other hand, was allowed to stay.

The roof will also serve as an outdoor laboratory where scientists and students will study nature in action. Almeda recently made a scientific discovery of his own. Curious about the appearance of a cluster of mushrooms he found on the roof, he sent a sample to a specialist at San Francisco State University. As Almeda recalls, “He said, ‘Frank, I’ve never seen this mushroom anywhere in North America! It’s actually native to Europe, and I’d like to keep it to try to learn more about it!’” At a time when news concerning the environment is grim, that’s an encouraging symbol. “The Academy,” Kephart says, “is essentially a promise that we can restore biodiversity within the urban world.”

Green Architecture’s Grand Experiment

Part 1: The Building

Part 2: The Green Roof

Part 3: The Engineering