Sunday, April 16, 2023

Green Roofs in San Francisco

In a spring escape, I was in San Francisco in late March. We visited museums, parks, and restaurants.

Roof, California Academy of Sciences
Roof, California Academy of Sciences

I had not been to the California Academy of Sciences in more than 20 years. In that period, the building was completely rebuilt, with a green roof. I had been doubtful about green roofs--where people grow plants on the roof. I know they have been used for millennia in parts of Europe, but those are wetter climates than San Francisco. So I wondered about watering and keeping plants alive in shallow soils. Or, if the soils aren't shallow, the weight on the roof. 

Roof, California Academy of Sciences  
Roof, California Academy of Sciences 

Cal Academy's green roof was planted as the new building was finished, in 2008. When I saw it, it was more than a decade old, very well-established. They boasted of it; it cools the building, captures rainwater reducing runoff, and provides habitat for birds and insects, all without taking up space that is so precious in cities. 

It certainly looked good:

plants on the roof

grasses on the roof

 Healthy ferns

ferns in the roof garden

Flowers in March

monkey flowers     
 monkey flowers, Mimulus

Their sign about maintenance said there was an ongoing need to remove weeds that appear. I wondered--but found no information on--what other ongoing problems they coped with. It is a pretty general ecological principle that every solution causes new problems; the trick is to choose wisely.

 This is a working roof, venting heat from the building, channeling and capturing rain water. The signs say it captures 98% of the rainfall, an important thing in a city where runoff can be a problem. The plants surround sky lights and solar panels

roof with solar panels

and sensors for monitoring wind, rain, and temperature.

Cal Academy roof

The Cal Academy roofs were built as hills and so are more complex than most green roofs. But the little hills mean some places are relatively shady, others more exposed, fostering a more diverse community of plants. Since Cal Academy is a science museum, they are of course studying the impact, though I could find neither publications nor press releases on developments since 2008.

hills on Cal Academy roof
Hills of the Cal Academy roof

A roof like this is a big project; the roof  has multiple layers of material to allow it to grow plants without leaking water. It also needed to be strong to support the weight. San Francisco has multiple suggestions for green roofs (link), but it is evident that building to include a roof garden is easier than retrofitting an existing roof. A green roof is more expensive to build than a traditional roof, but it also insulates better, controls water better, and provides a variety of other benefits to the owners and the community. 

roof on Cal Academy

Advice for choosing plants for a green roof recommends heat and drought tolerant shallow-rooted plants. Cal Academy tried many, producing the mix you see today. A lot are California natives, 90 species the sign said, for example, California poppy, monkey flower, California asters, and yarrow. But roof gardeners also especially liked succulents such as sedums, tough and shallow-rooted, even though most are non-native.

As this sign reports, there is no shortage of attractive plants that grow well on the roof.

flowers of the roof sign

They describe a number of animals which use the roof, from hummingbirds to bees and butterflies. I didn't see interesting animals, but I wasn't there very long.

animals of roof-top gardens sign

Back at my hotel near Chinatown, my 9th floor room had a balcony just big enough for a pair of lounge chairs and a few plants in a box. Honey bees were visiting the rosemary. From where I stood, I could see no other green plants in any direction, let alone flowers. I was impressed; if you provide flowers, urban bees will find them.

honey bee on rosemary, 9th floor
 Honey been in middle of flower on rosemary.

One more photo from the California Academy of Sciences roof

Cal Academy roof


Comments and corrections welcomed.

Kathy Keeler, A Wandering Botanist







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