|Ice-frost around a green roof web supporting succulents|
And sometimes these unwelcome weather events can happen well above the freezing point.
Understanding the bio-mechanics of frost can help the living wall and green roof designer prepare for possible ice crystal stress on plants used in these systems.
In general, plants used in exterior living walls appear to be more resilient to cold than those flat on the ground. Dismantling one of my oldest walls last night, I was amazed to see one of the cactus plants, Disocactus flagelliformis, not only survive the cold but thrive. Amazingly, most of the literature on the web specifies a minimum temperature of 50 degrees F and clearly warns against frost exposure.
Last year we experienced several nights in the low twenties. However the cactus keeps on growing.
With my curiosity peaked, I researched frost, cold and plants on Google trying to sort through the thermodynamics of air movement, heat and cold transfer and the five different types of frost. It seems that as the ground layer of air cools, the warm air rises. So the vertical positioned plants on the wall could actually be several degrees warmer than those plants on the ground. There are several interesting stories of how orange grove owners use helicopters to keep warm air blown back down into orchards in California on occasional nights with freezing temperatures or where frost may become a threat.
Additionally and to my surprise I read where many plants on the ground support epiphytc bacteria growth of Pseudomonas bacteria, a gram negative bacteria that also acts as an ice nucelator.
From the available literature it seems that the presence of ice-positive Pseudomonas can actually cause ice/frost to form on the plant surface, even at temperatures well above freezing.
Frost damages the epithelial layer, in many instances killing the plant. Frost can act like a butcher knife on some succulents and cut the leaf surface to shreds, exposing the once protected vascular system to desiccating winds and debilitating sun. For those cold-tender succulents and living wall plants, an unexpected frost encouraged by unexpected Pseudomonas bacteria may ruin a tropical or semi-tropical living wall or even green roof.
And Pseudomonas bacteria is typically found almost everywhere. Sometimes there presence is persistent for some reason, possibly due to a per-existing environmental factor that may serve as a natural attractant to Pseudomonas. In these areas, frost my occur well-above the normal freezing temperatures.
Interestingly however, there is a ice-minus strain of Pseudomonas also, a mutant bacteria that also occurs naturally that does not possess the ice formation encouraging mechanism that its non-mutant sibling possesses.
Leave it to capitalism to go figure out how to profit on these two types of bacteria. SnoMax - made by Johnson Controls - see SnoMax's website, is a product made from the ice-plus variety and is used for making snow! On the other hand, FrostBan - see article - creates a crop resilient to frost. FrostBan was the subject of many GMO battles during the 1980's and early 1990's.
I don't have the testing equipment to see if my Disocactus cactus had the ice-minus Pseudomonas, or if the plant growing vertically with excellent air flow had just avoided the dispersal of common Pseudomonas.
But the fact that it survives the cold and continues to grow on the wall is another piece in the green roof and green wall plant database we are all developing.
There really is so much to learn concerning tropical and semi-tropical green roofs and living walls.