“Green roofs may be a source of pollution” – Over Simplification of a Study Misunderstood or Grievous Offense?

November 14, 2013 at 2:18 pm

Wow, I haven’t gotten my ire up like this in a while!


Have you read the article in question yet by Harriet Jarlett “Green roofs may be a source of pollution” in Planet Earth Online of November 11, 2013 and its subsequent reprinting of November 12, 2013 on companion site Phys.org?

I have been back and forth with Dr. Andrew Speak, the researcher quoted, along with several of our industry experts in phytoremediation for their thoughts.  Lots of interesting commentary.   By the way, check out the comments on both sites above as well – they run the gamut from hitting the nail on the head to reading just plain funny.  Andrew himself posted yesterday on Planet Earth Online that “the title of the article has sent people off on the wrong track.”

This past September, Andrew contributed the Guest Feature Article “Green Roofs and Air Pollution in the UK” which referenced his recent research, which at the time was still in press.  After describing his work, he says both:

“The take home message from this work is that green roofs have a modest impact on urban air pollution compared to urban trees – but they DO have an impact.”


“My most recent study investigated the impacts on runoff water quality of a 43 year old green roof.  High lead levels were observed which were possibly from historic atmospheric deposition of pollution in the days before the phasing-out of leaded petrol, which accumulated in the green roof soil.” ~ Dr. Andrew Speak, University of Manchester

While acknowledging the above statements, the “Green roofs may be a source of pollution” article concentrates on the results of Andrew’s work with these greenroofs which were constructed in 1970.  We know that various air and water borne pollutants can be bound in soils (any soils) and later released at some point, that’s not the issue here.  But to say across the board that “Green roofs could become a future source of water pollution” without also stating that any other soil receptacle also could be is.

So I had to write the editors of both websites and thought I’d share it with you:

“We publish Greenroofs.com, and take offense to the completely misleading and irresponsible title of the November 11, 2013 article by Harriet Jarlett “Green roofs may be a source of pollution” in Planet Earth Online and its subsequent reprinting of November 12, 2013 on Phys.org.  Over simplification of a study misunderstood (or taken out of context at least) or grievous offense?

Greenroofs are not a source of pollution any more than a tree next to a highway (or motorway) would be or a river receiving runoff from a polluted field!  They may be conveyors or harvesters of such but these examples do not create pollution.  The source is elsewhere.

It would appear that Ms. Jarlett chose a titillating title taken out of context to gain attention to her article (which she certainly did!).  I believe Dr. Speak’s study and subsequent comments are the result from his study of older greenroofs – 43 years old – where pollutants from vehicles had accumulated (after 4 decades) in these older vegetated roofs only, derived from materials (as in leaded petrol) that are no longer being produced.

I believe the pollution accumulation issue in soils in general is much broader than just on greenroofs alone, and needs to be further researched.  Studies such as Speak’s and others are important to document the potential accumulation of pollutants on greenroofs – as in other receiving landscapes at grade – to establish countermeasures to manage them (specific plants, growing media additives, etc.).

In my opinion, Dr. Speak’s cautionary statement “We shouldn’t put green roofs next to busy motorways, or inner city motorways, basically anywhere where vehicle derived pollution could affect runoff in the future” is premature and unfortunately potentially harmful when other media outlets simply pick up on these inflammatory titles.

But to imply that greenroofs are a potential threat in “Green roofs may be a source of pollution” is simply as wrong as it is naïve.” ~ Linda S. Velazquez, ASLA, GRP, LEED AP, Greenroofs.com Publisher

Any thoughts?

~ Linda V.