You should prevent white build up.
Folks call this patina. It isn't.
Using organic ferts is a good prevention.
Chemical ferts, especially excessively, is a sure fire way to ruin a good pot.
If that part is not adhered to, the rest is moot. So do take preventative measures first.
Once you have prevented white build up, be it calcium, salt, whatever your preferred argument, using oils is not only to shine them for a show, but to lock in handling grime, this is patina.
So don't be sold on white nasty use as patina, chances are that pot is also not Frost proof, due to the seller's ignorance.
People say you can't speed up patina.
Lie.
It's a slow process, but there is a difference between a pot in a glass case that will never build patina, and a used pot, always handled with garden hands.
That said, speed up your patina by picking up your pots regularly. This is also another good reason to display trees where they can be easily picked up and inspected for pests.
Daily handling for pest inspection, is daily adding patina.
A tree in a pot won't cause patina, it is the act of rubbing dirt in with the oils on your fingers that causes patina. So when you take into account how often we actually need to, or can pick up a pot, we see why it is a long process.
Deep freeze.
First. Definitions of Frost-proof, and vitreous, must be fully understood, to understand what pots won't be destroyed over winter.
Vitreous means Non-porous. The Latin "like Glass".
Frost-Proof, to industry standards, doesn't necessarily need to be vitreous. Rather, the ceramics must have an appropriate percentage of open and closed porosity in order to accommodate the expansion of freezing water within the ceramic.
You can lick test a pot to see if it is vitreous, if it licks like glass, it is likely vitreous. (Remember the only applies to non glazed parts, glazes are vitreous themselves almost always.GLAZES NOT GLAZED POTS)
If water on the surface doesn't visibly absorb in some time, it also is a good indication of vitreousness.
The higher the ring when blipped, best done while holding the pot upside down with one finger as a cymbal, the tighter the molecules, which is also a good indication of vitreousness.
Vitreousness isn't going to keep a bag shaped pot from breaking in the freeze.
However, choice of soil plays a big part in this scenario, more on soil later.
So just because a pot is vitreous, doesn't mean it can withstand winter freeze. This is purely shape dependent.
Tests for non vitreous Frost proof pots can be found
Here. These tests can be done with entire pots without ruining them for future use, and if it is a great concern, I highly recommend performing them.
An argument can be had, that it is likely safer for a bag shaped pot to be non vitreous yet Frost proof, as water in the bag will have somewhere to go.
Soil.
I can only speak about what I have observed, which happens to be the 2 extremes we need to understand about soil and freeze anyway.
When DE, over 2mm sifted Napa 8822 in this case, freezes, the way the ice crystals form actually cause the DE to "Fluff" for lack of a better term. The resistance needed to stop this fluffing heaving action is so little, it is by far the best soil choice for any bag shaped pot IMO. The Ice is essentially seperate from the soil, leaving space for movement, not acting as a single unit.
Regular Soil, standard dirt, even tighter nursery Soil, acts more in a fashion of a single unit, like an ice cube. The entire soil and water acts as one mass, expanding together, with no spaces. This is what causes it to break pots easier than DE.
Resorce.
Sorce