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Posts Tagged ‘sedum’

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.  Sedum floriferum

Sedum floriferum

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.

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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.

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