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


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.

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.

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.

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.

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


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