California's Largest Green Roof

California Academy of Sciences
The new California Academy of Sciences building in San Francisco's Golden Gate Park opened last weekend -- with the largest green roof in the state.  The roof itself spans 2.5 acres, featuring seven hills covered with more than one million native California plants.

The undulating roof was designed by architect Renzo Piano, with the aim of integrating the building into the landscape of the park.  The only difference between the plants on the ground and on the roof is that the roof plants are all California natives.  Academy botanist Frank Almeda explained that his goal was "to choose native plants that were well adapted to the climate in Golden Gate Park and would provide much-needed habitat for native birds, butterflies, and other beneficial insects." He added that another goal was to "select species that would look attractive throughout the year, since a visually appealing roof is a much more powerful educational tool." 

Thirty species were tested on the roof of the old Academy before a final nine were selected -- all plants that can survive on the roof's steep slopes without artificial irrigation or fertilization.  The nine winners are:

• Self heal. (Prunella vulgaris) - shown left. Attractive to hummingbirds and bumble bees.
• Beach strawberries (Fragaria chiloensis).  Berries attract native birds.
• Sea pink (Armeria maritima) Flower attract moths and butterflies.
• Stonecorp (Sedum spathulifolium) Produces nectar for Hairstreak butterfly and threatened San Bruno elfin butterfly.l

• California poppy (Eschscholzia californica) - shown left. Produces nectar for bees and butterflies.
• Miniature lupine (Lupinus bicolor). Produces nectar for bees and butterflies.
• California plantain (Plantago erecta).  Host plant for butterfly larvae, including the endangered Bay Checkerspot butterfly.
• California goldfield plants (Lasthenia californica). Attracts a wide variety of native beneficial insects.
• Tidy tips (Layia platyglossa) attracts parasitic wasps and pirate bugs. Both feed on pest insects.

For the design of the roof, the architect collaborated with Paul Kephart, the top ecological designer at the Rana Creek wholesale nursery in Carmel Valley, CA.  Rana Creek developed special planting modules made from coconut waste products.  The modules are biodegradable, allow the roots of the plants to knit together, and allow air and water to move through the planting trays to the substrate layer.

Nearly 50,000 trays were installed on the roof domes, with 1.7 million individual plants.  Aside from providing a new wildlife corridor, the roof is expected to maintain a temperature 40 degrees cooler than an average roof.  In addition, it will absorb over three million gallons of water annually and prevent runoff from carrying pollutants into the ecosystem; keep temperatures inside the building 10 degrees cooler than a standard roof and reduce the need for air conditioning; and reduce noise inside the building by a whopping 40 decibels.

(images: top and bottom: California Academy of Sciences; Prunella: Robert H. Mohlenbrock, USDA/NRCS PLANTS Database; Eschscholzia: © Mark W. Skinner, USDA/NRCS PLANTS Database)