Wednesday, March 2, 2011

Green Roof Sedum Plantings

I am fascinated by the Sedum (Hylotelephilum as some botanists suggest).  They are hardy, drought tolerant, can switch between C3 and CAM photosynthesis and several native species provide food for endangered Lepidoptera species including the Grey Chi and San Bruno Elfin Butterfly.

There are many native Sedum species considered native to  the US.  Some are even protected by law as threatened or endangered as in;
Green Roofs have been shown over the long term to provide great opportunity for the re-establishment of endangered species.  Green Roofs may provide an important opportunity for those threatened or endangered Sedum species to recover.

There is a wonderful American Society of Landscape Architects article on the web focusing on a Green Roof with several popular Sedum species as part of green infrastructure installed to help the Rock Creek Watershed.

The Sedum's used in the project were;
  • S. sexangulare
  • S. album
  • S. spurium
  • S. rupestre
  • S. tectractinum
  • S. floriferum
  • S. kamtschaticum, and possibly others.
From a habitat, stormwater, 'sense of place' and heat island effect, this project was timely and ecologically important, and so too were the Sedum plantings.

Yet as you will read, many of these species are potentially invasive and have been recorded on state and organization invasive plant lists.

Importantly however, Sedum should not be singled out as the only invasive or potentially invasive plant when discussing green roofs.  There are many potentially invasive plants that could be associated with green roofs.  Last year we posted a brief article on the use of Nandina spp. on the Gainesville, Florida Regional Utilities Green Roof (thankfully the Nandina was subsequently removed from the planting schedule).
    Here in Florida, Sedum have difficulty with Southern Blight Fungus and tend to dampen off during the long hot and humid summers.  In Jacksonville, S. reflexum appears to do the best on living roofs though it too suffers from the humidity.

    Sedum propagates easily according to botanical literature and others, as suggested in the Emory Knolls Farms website.  In fact, spreading cuttings and clippings of sedum leaves and stems is suggested as a quick approach to filling a green roof with more plants.

    However, some Green Roof companies raise a red flag about invasive species, such as S. sarmentosum where as the Roofscapes, Inc site indicates;

    "The only Sedum that might be considered invasive is Sedum sarmentosum, also known as Star Sedum,Gold Moss, Stringy Stonecrop, or Graveyard Moss. It has green spearhead-shaped leaves, lemon yellow flowers, and long tendrils. We do not recommend using this species."

    But it is not just green roof plant nurseries that promote Sedum's ability to grow into full size plants from the tiniest of cuttings, other ecological or nature websites suggest the same.

    An interesting article and discussion of Sedum and Green Roofs can be found on the Taking Place website where the article entitled Sedum2 mentions of S. floriferum (one of the Sedum species used above mentioned watershed project);

     I’m careful to throw the weeded stems away in the garbage, because the tiniest little bit of stem or spare leaf will root in where it lands and start growing.  It’s not a problem (so far) here, because I do maintain my garden; I wonder about pieces of sedum escaping from roof gardens or green roofs, and rooting in, say, along a river bank or in pavement cracks.  It’s attractive, but it can cover and smother other plants if allowed. 

    Importantly, S. sarmentosum has been considered an escaped and invasive species in Japan, subject to numerous studies.

    Any plant that roots quickly from leaves and stem cuttings could potentially escape a roof.  Birds carry plant material from the roof on a daily basis.  Here in Florida as well as other windy areas, wind is a strong potential vector of plant DNA.  Fern spores are known to have been carried across the Pacific Ocean on winds, introducing Asian ferns to the Americas.

    I suggest, planted anywhere Sedums will escape and start to naturalize.  How bad this naturalization will become is another question.

    As a plant biologist my own opinion is Sedum will initially only naturalize within the disturbed environments of the Urban Core of cities such as Jacksonville here in the southeastern US, because of the humidity and Southern Blight fungus issues.  I personally believe it will have a hard time becoming established in the Florida swamps.

    Not so the case elsewhere.

    Here in the US there are many red flags and concerns now being raised concerning Sedum use and the potential for invasive activity.

    For example;
    Other Countries too, view Sedum species as an invasive plant, including;

    Many Not-For-Profit organizations and preserves also consider Sedum to be potentially invasive. A few include;
    Yet Sedum will continue to be used across Green Roofs here in the US and the World.

    The issue is not really about Sedum.

    The issue is one of native plants and exotic, introduced landscape plants.

    Do we feel comfortable with the practice to replacing native plants with introduced species because the introduced species are easier to use, hardier and more resilient (maybe because of the lack of natural predators)?

    I believe Sedum can be managed appropriately on Green Roofs.  Rooftop growing areas can provide habitat for those native Sedum too, offering protection for those endemic plant species threatened by monoculture takeover of exotics.

    The key component to successful exotic Sedum usage is awareness of its plant biology, maintenance practices, local exotic and invasive species management plants, state and federal laws and proper coordination with state and local Native Plant and Invasive Plant organizations.

    Many of the lists above are not law.  Sedum is not illegal to use.

    The lists above caution us to take account of our practices.

    Changing the constituency of Nature can be a dangerous act.

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