Hi allotrope,
First, I've never heard a Phal liking "cool" temperatures.. the coolest they may take might be around 15-16C!
What does caracterizes resistance to cold among plants is mostly their genetic make-up. Mostly/greatly it's their composition in lipid fatty acids and to what extent these FA chains are unsaturated. (Sorry here I go, there's no stopping me!). Plants regulate their membrane FA composition in concordance to temperatures (and also CO2 levels, water availability, etc.) ; the higher the temperature, the higher degree of saturation and inversely the lower the temps and the higher the degree of unsaturation.
Many plants can resist the cold if the drop in temperature has occured gradully (ie if they have time to change membrane FAs). To be able to desaturate (ie to make unsatared) FAs you need special enzymes called FA desaturases. There are different kinds of these enzymes. Those plants who can not resist the cold (by cold I mean above 0 temperatures and not for long periods, just a couple of days) perhaps lack these enzymes or cannot produce the unsaturated FAs in time and therefore cannot respond to the cold stress.
The above mentionned is the major reason of cold tolerance in plants (again by cold I mean above 0 temps and for a short period of time). Other factors I see might be different enzymes that are not functional at those temps, etc.
HTH
Naz

Hi
My Phals are regularly sujected to cool weather.The only time they get any protection is when the temps fall below 0
Or there is a high probability of frost. This is the only way I've found to get them o flower regularly. It's interesting that some plants show cold damage immediately
while others can take weeks .
One of the most cold sensitive lants I've found is Episia. They will curl below 50F!! 40's are fatal.
I have several understory palms that I'm not sure of
so I always keep them above 50.One,Kerriodoxa elegans took
28 for over 3 hours and didn't even slow it's growth.
I frequently find that tropicals that are exposed to cool weather often grow much better on a year around basis than those that have temps maintained. I find this particularly true of orchids and palms in general.
Can't tell you why but have found this frequently true.
Of course anything below freezing must be avoided.
Gary

Thanks Nazanine, your explanation seems to stir a vague recollection of this from my uni days. As far as Phal's defined as cool grower's, well it's what I found in some culture sheets. I'm no orchid expert (less than a year) but most seem to agree (Gary included ;o) that Phals need cool nights in order to bloom properly, and this requirement seems to be the definition for a cool grower. Most disagree as to the minimum temps but I've read that many in my zone put them outside during the summer, where low 10's is not unheard of in August.

:O 10 degrees!! if I even *mention* this to my Phals they,ll drop dead!!Theyre used to their 18C minima and 25 and plus in the day!
Here's a funny story, actually 2. Thats when I was still in Canada. Once I found a coleus in spring that I had planted the previous summer. The roots had survived the cold winter and it showed up in spring!
I also planted and lost track of some Sprekelia bulbs (S. Africa). Well they were out and blooming this spring! I find that there are lots of tropicals that can take the cold. The Sprekelia survived the winter where we had 6 ft frozen ground. Pretty cool isn't it!

The survival rate of sensitive roots/tubers plants shoot up significantly if there is a deep layer of snow that covers the mulch. It was onced explained to me that the snow has two functions, protects the soil from drying out (dry winter winds will suck the moisture out of the soil) and acts as insulation. If the snow covers a good layer of mulch, it will allow the mulch to sustain a high enough temperature to promote decomposition which in turns generates heat. If the snow isn't deep enough the temp is too low for effective decomposition and the ground freezes solid.

That said it doesn't explain why some plants survive freezing while others turn to mush. I suspect that the answer is similar to why some plants withstands cold temps better then others. If I recall some of them uses ethylene (gas) and glycol as antifreeze (no sure tho').

i thought is was also the % of salt in the cytoplasm and the cell wall structure?

as well as the enzyme thingie.
as a side note to that, i remember in college (15+ years ago) that my bact. prof was so excited cuz he had just come back from a collection @ yellowstone. he had these super heated bacteria that he was gonna look into enzyme function at these extreme temps.

froggy