alessandro, that's an excellent question. I have no idea. Here's what I recently posted in another forum though...
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"The ability to reduce the freezing point of water that's inside the tissues of that plant and also, once that water freezes, to allow that plant to survive freezing temperatures [helps]," Dr. Francko said. "It can be frozen solid and still be viable."
We all know that cold and dry is a better combination than cold and wet...but why is this the case? For a while I kind of figured that it had to do with reducing an orchid's susceptibility to rot. This could certainly be true...but in terms of freeze damage...an orchid that's been dry over a certain amount of time...or "dehydrated"...will have less water in its cells. With less water in its cells the less damage it might sustain during freezing.
Practically speaking, while it might be a challenge to keep a mounted orchid hydrated during summer...that same challenge might work to its advantage during winter.
I've been meaning to conduct a simple experiment to see how well this works out. Basically, I would take two leaves from the common Jade Plant (Crassula ovata)...a freshly separated leaf and a leaf that had been removed a week earlier...and put them in the freezer. Not sure how long I'd leave them in the freezer though. Then I'd compare the extent of the cold damage in both leaves.
What do the Jade Plant and many orchids have in common? Crassulacean Acid Metabolism (CAM). It's basically a technique that plants from arid climates use to reduce water loss during the day. As the name implies...this method of metabolism was first observed in the Crassula family. A side effect of CAM is succulence.
While most orchids do not occur in arid climates...many of the epiphytic ones can be considered to occur in arid microclimates. In fact, the percentage of CAM orchids occurring in a given section of forest is a good indicator of where that forest falls on the moisture continuum.
A fun tidbit for your friends that are really into cactus/succulents is that the Orchidaceae has more CAM species than any other plant family...including the Cactaceae.
All things being equal...in terms of cold damage, I'm definitely leaning towards a CAM orchid being safer mounted on a branch without any moss than in a pot.
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What I recommend is making a list of all the subtropical and tropical plants that grow outside in your area. Look up the distribution of each of those plants. Chances are pretty good that some epiphytic orchids share the same distribution ranges. It doesn't guarantee that those orchids will be as cold tolerant...but it is some supporting evidence.
You should also contact my friend
Tomas in Rome, Italy. He's been growing quite a few orchids outside year around for some time now. Here's how Florence compares to Rome...
Sources: Orchid Culture,
Weather Reports,
Temperature Charts
Linear Mode
