In pots I believe you lose 2 zones hardiness. Could be wrong there but think I've read that. Since the top isn't less hardy when in pots that's a root issue. Figure top hardy to -10F so roots hardy to +10F.

That looks like a good estimation for northern varieties fruitnut.

I skimmed through this Cornell paper from 1920. They used Elberta (Redhaven not around yet). They make a point that different tissues have different thresholds that vary with the time of year. This makes it difficult to conclude any generalizations. However, it seems that soil temps above 14F were safe. Below 14F is risky (for Elberta).

Obviously I want to keep the soil temp just above freezing if I can. I just wanted to know, for future reference, which soil temps should give me dreadful anxiety. LOL

Here is a link that might be useful: Resistance of the roots of some fruit species to low temperature

After going thru your link and converting C to F I'd feel safe at 20F which has kind of been by worry point around here for potted material. Holding freezing sounds even better but isn't feasible even here in small pots with no protection.

Can't say I've ever thought I had freeze injury to roots.

Yeah, you're right... 20F is more like it.

Last night was an interesting test. I hadn't yet setup any plunge protection soil heaters. Covering in the lean-to shed didn't buffer as much as I thought it would. It was no better then leaving them outside. I was a little worried when I saw the soil froze and temps around 10F. Luckily the soil 3" below surface was 40F, so the moisture trick worked well on the short overnight dip. However, trouble this early forced me to change plans and move them to subsurface storage area that I know will stay closer to 30s through most of the winter.