Ah,minus 30?People actually live where you can get that cold?Well,maybe your tufa made it thru(and,looking good!)but this warm blooded critter wouldn't!I think we saw -10 one night,but a nice fire in the stove took the chill off.
Out here,tufa and other concreting is pretty well year round.If you're lucky enough to have a great spouse like mine who said"Sure!Let's put a studio in the new home..as long as I get a beer and wine room."
Sigh...what a woman.Keeping me warm in my craft area,and well fueled with brews and wines.Does it get any better?

I'm tempted to ask -30C or -30F, but I guess it doesn't make a whole lot of difference! How cold is almost irrelevant, even -100 wouldn't do a whole lot of damage. The main damage is caused by moisture within the structure freezing and expanding. Repeated freeze-thaw can destroy porous materials like hypertufa. Containers full of a moist material like soil can simply be broken apart if they freeze solid. Your container is a good shape. You could probably fill it with water and freeze it without damage. Some of my more porous recipes are showing a lot of flaking and cracking after just one winter.

I have several pieces I have left out for three Wisconsin winters now. They go through freeze and thaw daily, or sometimes multiple times a day. I have even left the soil in them after removing the cannas, and they sit directly on the ground. Any snow melts into them and freezes. I want to see how much they can take. So far they are showing no signs of cracking.
The mix I used was 1 portland 1 peat 3 vermiculite.

What happens to concrete planters? Do they winter well in colder zones? Leave soil in or remove it? So many thought, so little time.

Clairdo2, I like the shape of your planter. Did you do a freeform on that or did you use a combination of mold forms?