How to Secede on Green Roofs Without Really Trying
By
Ed Snodgrass, Plant Editor
May 4, 2009
Ask Ed Q & A Column
All Photos Courtesy
Ask Ed unless otherwise noted
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Aerial View
Prior to Succession; Photo Courtesy Adnams Brewery. |
What
happens on a green roof without intervention? Ecological
succession is at work in all climates at all times turning areas
scrubbed clean from fires or lava flows into meadows then shrub lands to
primary forests to secondary forests to climax forests. All
human-designed gardening and agriculture fights this natural succession
to keep the intended plants from being replaced.
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The Adnams
Brewery in Southwold, Suffolk, England; Photo Courtesy
Adnams Brewery. |
What does this have to
do with extensive green roofs? This question come to mind last
year when I visited the
Adnams Brewery warehouse facility in Southwold, England. I had
heard about this green roof from more than one friend.
John Lea-Cox, part of the green roof research team at the University
of Maryland, is related to the owners and had told me of his recent
visit to the brewery last year, and
Dusty Gedge of
London green roof fame spoke of the roof as well.
Vegetated with pregrown sedum blankets in
2006, Dusty lamented the missed opportunity to have made the roof more
biologically diverse as the loading restrictions only allowed a thin
sedum carpet on the roof.
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Installation in 2006: Men at (Sedum) Work; Photo Courtesy
Adnams Brewery. |
I made arrangements to
meet with the owner and facilities manager after the
World Green Roof Congress in London last September, 2008. I
was surprised when we crested a small hill and the warehouse came into
view that it appeared to be a grass roof, and not the sedum roof that
was originally installed.
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Grasses and
Sedums are Now Carpeting the Living Roof of the Adnams
Brewery. |
It turns out that the
warehouse was built on the site where a quarry once existed and species
of grasses were growing there in restrictive conditions very much like
green roof conditions. I don’t believe you could have gotten those
grasses to grow on the roof first, but those grasses certainly didn’t
mind being the second plant community to move in.
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View From a
Hill in September, 2008; the Roof is Beginning to Mimic the
Landscape. |
The conditions at the
Adnams warehouse were perfect for this succession. Very thin
substrates limit what plants can survive; the colonizers [in this case
the sedums] provided a good germination bed for the grass seeds and the
surrounding area. This is rarely the case and most often when
seeds fly in or are carried in by birds, those seeds create weed
problems. But it does beg the question, could we manage succession
on green roofs to increase the number and type of plants able to grow
and flourish there?
Mark Simmons at the
Lady Bird Johnson Wildflower Center once told me about something
called directed succession where a colonizing group of plants are put in
place solely with the objective of preparing a climate for a secondary
group of plants.
Maybe this is something worthy of a research effort.
Ed Snodgrass, Emory Knoll
Farms/Green Roof Plants
Send your questions or comments to:
PlantEditor@greenroofs.com
or
phone Ed at: 410.452.5880.
Visit
Emory Knoll Farm's website:
www.greenroofplants.com
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