We always recommend that green roofs are installed with a watering facility to cover unusually hot, dry spells. But how necessary is it really? What are the consequences of having no irrigation facility?
In a hot dry period that lasts a week or so, a lot of wildflower roofs can begin to look decidedly brown, and many un-watered sedum roofs start to turn red. Does this matter? Well, it certainly does if the roof is overlooked and designed to be an aesthetically inspiring space. Views of well-managed green space are claimed to increase productivity and health. Views of dead or dying plants are not.
It is worth noting that some of the environmental functions of the green roof will be severely diminished if the plants are not healthy. The air temperature control and water detention provided by transpiring plants is dependent on good, working greenery.
It can certainly be argued that for biodiverse roofs that are primarily devoted to wildlife, which in many cases may not be overlooked, that it is perfectly acceptable to let vegetation dry out. Some of the spent brown flower heads of the Petrorhagia and Gypsophila that have been left uncut have provided a cool, damp patch in which some Thymus has taken off with some lush green leaves, currently hidden from the worst of the sun.
The more established Thymus looks like it would struggle to come back (I am confident that I will be posting twitter photos in a month or two proving that it has). However the Delosperma nearby that looked like it was starting to give in has now perked up, enjoying the moisture-starved environment.
Across on the wildflower rig, resilience is taking the surprising form of chives (Allium schoenoprasm) and the slightly less welcome form of invading grasses. I have no doubt that most species will be self-seeding or hibernating for successful re-emergence, however our normal flurry of flying pollinators seem unexcited by this future promise in their current search for nectar.
My conclusion is that watering facility should always be added to a green roof, because healthy green plants will perform many desirable functions, including pollinator support, better than struggling ones. They will also be better equipped to hold their own against unwanted visitors, especially in the case of wildflower roofs. However I would also note that from the point of view of a naturally self-sustaining eco-system, diverse plant communities can be much more resilient than they may first appear.