The High Line is sexier, but Sidwell Friends could have more to say about the future of landscape architecture.
Posts Tagged ‘Green roofs’
Furthering the discussions about green roofs, plants, and maintenance: here’s an excellent issue of LeafLitter, the electronic newsletter put out by BioHabitats, Incorporated. BioHabitats does conservation planning, ecological restoration, and regenerative design, and Leaf Litter is a thoughtful and thought-provoking publication. This entire current issue, which focuses on green roofs and green walls, is a valuable read.
In it is an interview with Ed Snodgrass, who has become something of a pioneer in the the U.S. green roof industry. He grows and sells plants for green roofs, and has obviously put a lot of thought into the issues that every landscape architect should be aware of in designing green roofs.
You’ll also find interviews with The Center for Green Roof Research at Penn State; Dr. Robert Berhage (green roofs) and Bob Cameron (green walls) both discuss soils, mineral loads, water quality, plant selection, construction details, insulation, thermal loading, returns on investment, recycling of resources, biodiversity, habitat considerations, and the need for maintenance and maintenance budgeting. The Penn State guys and Ed Snodgrass don’t agree on all things, so reading all three interviews gives a multidimensional look at these really important considerations.
(One sobering bit: Dr. Berhage, in talking about reference books for green roofs, acknowledges Ed Snodgrass’s Green Roof Plants, but says that for the most part, the best books on the subject are in German. He points to a number of new American books and says “But in my mind, they are all pretty picture books. They are full of case studies of pretty buildings. You can learn a lot from that, but are they appropriate textbooks for anything other than a landscape architecture class? Probably not.”
His equating landscape architectural value solely with aesthetic value disturbs me. I know dozens of LAs who wrestle daily with the technical problems associated with designing a well-functioning landscape; somehow we have to remind ourselves and to make clear to the rest of the world that we’re not concerned only with big visual patterns, but also with the nuts and bolts of making living, growing places.)
We have a string of comments going on Toby’s sedum post, and now I’m going to add another question to the mix: is there evidence that sedum won’t escape from rooftops and become invasive in places where it may be unwelcome? It’s being used so heavily in green roof construction, and being pitched as a really great plant for these harsh places.
I know how it grows under easier conditions. A neighbor gave me a handful of Sedum floriferum from her garden about three years ago, and I tossed it onto a roughed-up bare spot in one of my perennial beds.
It now covers roughly nine square feet, and I pull it out like a weed when I want to dig something in to the bed. 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.
Out on the Charles River I see trash that has made its way down to Boston from towns upstream. The most minuscule scrap of litter tossed on the ground in Medfield, Wellesley, Newton, Waltham, or Watertown, for instance, will wash into a storm sewer and out to the river, where it bobs around until it’s picked up by man, bird, fish, or rodent, or until it drifts through the locks and out to Boston Harbor. (Check out the Charles River Clean Up Boat, a really good organization that works to stop the flow of trash into the river and harbor.) Water is one of the best conduits for litter — or plant — travel around.
So what’s to stop sedums from travelling like this, and establishing in areas where it’s not welcome? We know it’s aggressive, and use that aggressive nature to cover rooftops quickly. It seems a good idea to consider its maintenance regime, and urge care to avoid sedum escape during its planting, weeding, and removal.
Two advantages of green roofs are their ability to cool the building below and the air above, and to reduce stormwater runoff volumes, thanks to the transpiration of moisture through their leaves. The most common green roof plant are species of Sedum. Those plant can survive drought conditions in the rooftop environment in part because they transpire relatively little water through their leaves.
To what extent does the characteristic that helps Sedums survive, their low transpiration rate, negate their environmental benefits? Is the performance of a Sedum-planted roof significantly higher than that of a non-planted roof with the same construction and soils? If not, does the Sedum offer other benefits that still make its use worthwhile? If it offers no real benefits, and a higher-benefit roof is not feasible, is the only alternative a conventional roof, or is a plant-free soil-covered roof a realistic alternative?*
*Green roof people discourage the use of the word soil in favor of “growing medium.” In a world of where manufactured soils are more and more the norm for built landscapes, I am not sure there is a meaningful distinction.