Species matter. Lots of roots aren't supposed to freeze solid. It's hard to generalize here because the list of species we bonsai is a mile long, and each variety has its own set of parameters, but generally speaking, fleshy roots don't freeze well, but tough, fibrous roots like Pines do fine. The parts above ground dry out for winter and can stand freezing just fine while the root zone just gets down to 0° C, but when they get down to ~6°C that's a different story. The ground here in 6b in the city where the wind doesn't affect the ground the way it does on open farmland, rarely gets much below ~-1°C below the top ~3" crust. Your pots will be whatever the air temp is and that varies all over the map. And that's not the same kind of condition as the upper roots getting down to ~0°C and staying there for a few months. In my garden over winter, pots sitting on a lightly mulched surface and mulched to the base of the tree (after it settles down from ~3" higher) probably doesn't get much lower than ~1 or 2°C. And they stay there without very much change for 4 or 5 months. They don't get any water other than what seeps in from rain or snow from late October to April something and they're never too dry when I fetch them. Too dry over winter for deciduous is not any better than too wet, except maybe dry pots don't break. The pots do become available for some other, living tree next spring... I'm going to go out on a limb and say that any tree that isn't hardy in zone 3, other than Pines, shouldn't allow the pot to freeze. Or, any tree that doesn't have heavy resin-type, pine-like sap shouldn't have frozen roots. And, probably more important: the number of freeze-thaw cycles a tree can take over winter probably counts just as much, with a high number being stressful for the tree.
Not only that, but I leave all my trees out in full sun over winter, including everything that needs partial shade in summer. When trees start to leaf out in shade it is vary hard to introduce them to the sun without frying the leaf edges. I never got it right when the trees were over-wintered in a garage, or behind a building, or anywhere except in full sun. It never seems to get too hot until I've had plenty of time to take the Japanese maples, et al to where they will spend the hot summer months, but could never get it right the other way. Another guess/generality: any tree that isn't hardy in 2 zones north of yours (whomever) shouldn't have roots subjected to the lowest air temps of that nothern zone.
. I sprayed with dormant oil on December 22nd and had to pull the remaining leaves off the trees to do so. Three weeks later and several trees are already budding out - I usually wait four weeks before spraying with lime sulfur or Bordeaux mix at dormant concentrations. So a few trees aren’t going to get it this year and I think that I’ll spray the rest this weekend in case more start moving over the next couple of weeks. In a climate like Houston’s, nothing is every TRULY dormant and you have to be on top of the dormant spray application by getting right on the horticultural oil as soon as the leave are gone to make sure that you have enough time for the fungal spray on the back end. Sometimes you don’t get four weeks. I’ll spray the maples that are budding out with Cleary’s and forgo the lime sulfur spray. Cleary’s is effective against anthracnose. I’ve never had fungal issues with bald cypress - just rust mites.
