Seemed like a good idea at the time

I have been interested in green walls and green roofs on buildings for quite a while. 

You could almost say that I was spruiking them – with less enthusiasm for the green veneer popularised by Patrick Blanc, and rather more for the larger scale vegetation of the 'Bosco Verticale' by Stefano Boeri in Milan.

But lately, I have been pulled up short by a very sobering thought. How safe is all this greenery if there is a fire?
There is no question that there is a potential for unacceptable fire spread on the facade of any building, if the greenery has been allowed to dry out. Therefore it follows naturally that proponents of green walls suggest there should be no problem with properly maintained installations – even claiming that a well irrigated green wall may in fact work positively to limit fire spread.

You don't have to have a lot of technical knowledge to work out that by bridging the building's own fire separation detailing, plant material on facades already presents a high risk.  This is evident even before we factor in considerable financial pressure on building owners to minimise expenditure, on something that is inherently expensive to maintain.

Perhaps by recognizing the problem we will have the rapid development of systems that work to perform day-to-day irrigation, but are able to function as fire protection and suppression in the event of an actual fire.

I am far from the first person to recognise the potential problem. But as soon as I started to research it, I realized that as with all such problems, the people responsible for matching regulations to the risk become distracted by the legal frameworks under which they can act. An excellent example is the very succinct article by  Maarten de Groot in Fire Safety and Green Walls, published in Biotope City Journal. de Groote concludes that "green walls are fire safe" as long as built within the criteria he mentions.  Those criteria still leave me a little pessimistic, but there are reasons not to be. 

I am aware that at least one manufacturer of living walls, ANS Global, has put its products to the test and has been awarded first class classification for BS 476: Part 7: 1997 ‘Method for Classification of the Surface Spread of Flame of Products’.  The products were its living wall modules with a variety of plants within their containers to a total weight of 7kg, tested in mid-2016 for lateral spread of flame along the surface whilst in a vertical position.

Scott Anderson, director of ANS Global, is quoted as saying: "We are very pleased to have had such a good fire rating on our living wall system, it will not only give architects yet another reason to specify our living walls, but also it could slow down or prevent the spread of fire compared to other building products which ultimately could save lives."

He is right of course. We just have to compare living walls to the spectacular failures of some noncomplying composite panels.

Edit:
I don't want to change from a supporter of green walls and roofs, to the opposite. But in the last few days whole suburbs in northern California burnt to the ground in wildfires. I can't get out of my mind images of the same thing happening in a city of buildings covered by forests. I blame Hollywood.