The Living Roof: A failed experiment in native plant gardening

Living Roof, California Academy of Sciences, March 2011

When the California Academy of Sciences reopened in San Francisco in August 2008, its “living roof” was considered its most unique feature.  Thirty species of native plants were candidates for planting on the roof.  They were planted in test plots with conditions similar to the planned roof and monitored closely.  Only nine species of native plants were selected for planting on the roof because they were the only plants that were capable of self-sowing from one season to another, implying that they were “sustainable.”  A living demonstration of “sustainability” was said to be the purpose of the living roof. 

So, 2-1/2 years later, what have we learned from the living roof about the sustainability of native plants in San Francisco?  The results of monitoring the roof since June 2009, are reported on the “fromthethicket” blog about Golden Gate Park.

Two of three of the predominant species on the roof after 2-1/2 years are native.  The third–moss–is not.  It is described by “fromthethicket” as “varieties of early succession mosses, the types that commonly show up in disturbed soil.”

The monitoring project has divided the roof into four quadrants.  Non-natives now outnumber natives in two of the quadrants, those which are not being weeded.  Although natives outnumber non-natives significantly in the other two quadrants, non-natives are also growing in these quadrants.

California Academy of Sciences, April 2011

We had the privilege of meeting the ecology consultant who designed the plant palette for the  living roof for the academy and many other institutions around the world.  He would not be surprised by this monitoring report.  He advised the Academy to walk the streets of San Francisco and identify the plants growing from the cracks in the sidewalks.  These are the plants he advised the academy to plant because these are the plants that are adapted to current conditions in the city.  The Academy rejected this advice because they were committed to planting exclusively natives on the roof.

The designer also advised the academy not to irrigate the roof, because the point of the roof is that it is a demonstration of sustainability.  Again, the Academy refused because they knew that without irrigation most of the native plants would be brown during the dry season, roughly half the year.  They wanted the public to believe that the plants that are native to San Francisco are beautiful year around.

There is a lesson to learn here for anyone who is willing to learn from it.  The living roof is not natural because it is irrigated and intensively gardened (e.g., weeded, fertilized, replanted, reseeded), yet non-natives not only found their way there on their own, but are dominating it within only 2-1/2 years.  Native plants are not sustainable in San Francisco without intensive gardening effort.

Peter Del Tredici has been telling us this for several years.  He is a Senior Research Scientist at the Arnold Arboretum at Harvard University and a Lecturer in the Department of Landscape Architecture at the Harvard Graduate School of Design.

In a recent publication*, he advises the managers of public lands in urban areas to abandon their fantasy that native plants are sustainable in urban settings:

“The notion that self-sustaining, historically accurate plant associations can be restored to urban areas is an idea with little credibility in light of the facts that 1) the density of the human populations and the infrastructure necessary to support it have led to the removal of the original vegetation, 2) the abiotic growing conditions [e.g., temperature, salinity, moisture, etc.] of urban areas are completely different from what they were originally; and 3) the large number of non-native species that have naturalized in cities provide intense competition for the native species that grew there prior to urbanization.”

Sure, he says, we can grow native plants, but they require at least the same amount of effort as growing any other plant and are therefore just another form of gardening:  “Certainly people can plant native species in the city, but few of them will thrive unless they are provided with the appropriate soil and are maintained to the same level as other intentionally cultivated plants.”

He concludes that native plant advocates are making a “cultural value judgment:”

“…people are looking at the plant through the subjective lens of a cultural value judgment which places a higher value on the nativity of a given plant than on its ecological function.  While this privileging of nativity may be appropriate and necessary for preserving large wilderness areas or rare native species it seems at odds with the realities of urban systems, where social and ecological functionality typically take priority over the restoration of historic ecosystems.”

We hope that the managers of our public lands in the San Francisco Bay Area will soon catch up with the scientific literature as well as acknowledge the actual experience of years of failed “restorations.”  Aside from the waste of scarce resources, these efforts are poisoning our parks with toxic herbicides and destroying beautiful and healthy plants and trees to no useful purpose. 

* “Spontaneous Urban Vegetation:  Reflections of Change in a Globalized World,” Nature and Culture. Winter 2010, 209-315.

10 thoughts on “The Living Roof: A failed experiment in native plant gardening”

  1. Bravo … truly the difference between fantasy and reality.

    Nature is what is. Everything changes. We may not understand those changes or want to accept them. Nature is like a 30 foot wall of water rushing away from a calamitous sea.

    Earth is a part of the Grand Nature of everything.

    Maybe, for our own survival, we should stop warring against Nature.

    Is it survival of the fittest (who are the fittest) or is it, those that adapt, survive?

    Yeah, sure we can harness the atom.

    Maybe the true test of our survivability, is then we learn to live within the boundaries that are part of this environment.

    Look at any living organism, when any one part of that organism either fails or consumes more resources than all the other parts, the organism dies. This is the balance that works in all systems … learning to walk the Spiderweb.

    Our greatest strength also has the potential to be our greatest weakness.

    What are we learning as Co-Creators … you and me?

  2. “…these efforts are poisoning our parks with toxic herbicides and destroying beautiful and healthy plants and trees to no useful purpose.”
    This is not a true statement. These efforts make me feel self-righteous, virtuous, and as secure as an ostrich with its head buried deep in the sand. That’s THREE useful purposes.

  3. What a welcome addition to a scarce but necessary literature! Aggressive “nativist” gardeners from our city Rec and Parks Department and others here in San Francisco have destroyed vast swathes of Mt. Davidson Park, Mt. Sutro, Glen Park, Golden Gate Park, tearing out 150-year-old giant trees, digging up foliage, removing wildlife habitat underbrush, all in the name of their wrong-headed anti-immigrant crusade.

    Yet it seems that the so-called “native” plants they put in do not grow well here, will not grow, without intense irrigation and poison sprays and other expensive cultivation. It’s because their ancestors may have been “native” back in their chosen year of 1776, when this place was a sand-dune, but their descendants are no longer. Times have changed. Stop destroying our healthy parks, people. We have laws against killing trees, removing plants — “don’t pick the flowers”.

  4. The author does not consider that disturbed soils can be restored to their more natural functioning state by replacing invasive plants with native species. This process takes a few years but the benefits are increased biodiversity,
    Better soil function resulting in improved air quality and hydrology and pollination. Ecological services are protected through the restoration of native plant species in highly disturbed areas where most people live. This process does take a few years and it does require hand removal of invasive species. Roof gardens are contrivances and non-native, non-invasive species seem more appropriate. However, when planting in terra firma
    Webmaster: Yes, we are aware that plants sometimes alter the soil. In fact, both native and non-native plants are equally capable of altering the soil. This is therefore not an argument in favor of native plants. It is, however, an argument against massive native plant “restorations” that would require the import of new soil to be successful, as this would be prohibitively expensive.

    The expense might be justifiable if the benefits you predict were in fact the end result. However, there is no evidence that eradicating non-native plants produces greater biodiversity. Since non-native plants dominate the landscape, their eradication would reduce biodiversity. The old saw that the mere existence of non-native plants causes the extinction of native plants is discredited by scientists. With the exception of islands there is no evidence of such extinctions. (See “Mark Davis: ‘Friend to Aliens’”)

    We are unaware of any “improved air quality and hydrology and pollination” resulting from the eradication of non-native plants. That is a counter-intuitive prediction. The destruction of plants and trees releases tons of sequestered carbon into the air, contributing to greenhouse gasses that contribute to climate change. The removal of non-native plants and trees frequently causes erosion because of the loss of roots that absorb water which otherwise runs off the land. And pollination services to agricultural and orchard crops are provided primarily by the honeybee for which non-native plants and trees are their primary source of nectar. If you can provide references for any of these claims, we would be glad to read them.

    Thank you for your visit and for your comment.

  5. Since you don’t have the confidence to post your identity, I thought it only fitting to return the favor. Your argument would have much more strength if you were not anonymous.

    You claim on your home page to provide citations, yet in this article you fail to tell us who “We had the privilege of meeting the ecology consultant who designed the plant palette for the living roof for the academy and many other institutions around the world.” is referring to. The “ecology consultant” does not have a name? By then throwing out Peter Del Tredici’s name it would appear as though you wish your reader to assume that he was the “ecology consultant” you refer to.

    On another note, you mention this third predominant species on the roof as “a moss” and then tell us it is non-native, yet offer no proof of this. Citing another blog with no standard of journalistic integrity is not a valid source. You might want to present the scientific name of this moss to prove your point, though I do not believe, judging by your writing, that you actually know the name, or you would have presented it. There are many different species of mosses out there, some native, some non-native. Many moss are very cosmopolitan in their range and therefore may occur natively all over the world. The actual quote from InTheThicket is as follows: “Nobody knows where the mosses came from, but they appear to be varieties of early-succession mosses, the types that commonly show up in disturbed soil.” I challenge you to show me where this says they are non-native.

    I think it is important for anyone reading this blog to be reminded that it is just that, a blog. It is written by an anonymous source who can say whatever they like with no fear of reprisal other than these comments, and with no need to live up to journalistic standards of any kind.

    Webmaster: The ecology consultant for the living roof on the Academy was Paul Kephart of Rana Nursery. He is the person we are quoting in this article.

    The mosses on the living roof are identified on the Academy’s website: This link wasn’t available when we wrote the article about the living roof, but when the list of the mosses became available to us, we took the list of 4 mosses in this report to the Jepson Herbarium at UC Berkeley to confirm that our statement was accurate. The staff at the Herbarium looked up these mosses for us and described them as “cosmopolitan,” which means that they are widespread and cannot be considered either native or non-native, as you seem to acknowledge in your comment.

    As for our anonymity, we believe our post about the apparent legal threat to the Save Sutro webmaster ( explains our motivation. We are confident about what we say on Million Trees because we say almost nothing for which we cannot provide a scientific reference.

    Thank you for your visit and your comment.

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