I go by the depth required by local building department for foundation footers. The idea is they've already done the leg work in determining the deepest expected freeze depth. You can call your local building department or one in a nearby city. You want the depth for stem wall type footing, not slab. I'd expect something like 4', maybe a little less.

That's for like the expected worst freeze for the next 100 years. Maybe that freeze depth will happen next year, or in 50 years, or never. So you could reduce that amount and just deal with losing plants some years.

I think it's easier to remove the pot, remove the tuber from the soil and store in a 32-50F part of the house. Then replant in the spring. To me they just do a lot better when divided and repotted every year and fertilizer added. Moving the pots to a deep spot in the pond sounds easy but pots tip over, you need to remove the dead leaves and they don't do as well when not divided and fertilized. So I just think it's less work to remove completely.

waterbug guy, do you keep them in water when you store them? or let them dry out?

You can put the tubers into zip lock plastic bags with some damp mulch (wood, coconut husk, straw, dry leaves, grass from the yard. The idea is to keep the tubers from touching each other or sitting against the plastic or in water at the bottom of the bag. You can check them every month or so for mold and make sure it's still damp in the bag. Then put the bags into your fridge.

You can dig a root cellar, basically just a hole big enough for a bucket of tubers. Put some damp sand in the bucket along with the tubers. Cover with soil until spring. But in your area that would be at lease 4' deep I'd guess...maybe 3' to the top of the bucket depending on how lucky you feel.

Hopefully you'll have more tubers than you want to keep so you could keep a few in the fridge and try the rest outside. Great way to learn.

I have mine in three large tubs about 2 - 2 1/2 feet down. I am in Southern Ontario and my plants over winter in the pond extremely well. They do tend to grow outside the pots but we do cut them back and pass them on to other ponders yearly. I have many flowers daily throughout the summer. I am lazy at fertilizing but that doesn't seem to matter. I do have a heater and large air stone running all winter.