You may find this copy/paste job of some interest. If you still have questions, please ask. I've seen general cold hardiness limits published for various perennials (trees & shrubs) in some of the texts I have on plant production methodology, but nothing I recall on the net.

Commonly, each species of plant has a general range of cold-hardiness. Within species and cultivar, cold-hardiness is genetically determined. That is to say that a plant that is propagated from cuttings or tissue culture will have the same ability to resist cold as the parent plant. Plants cannot "develop" a greater degree of cold-hardiness by repeated or prolonged exposure to cold, even after 100 years (trees).

If we pick any plant at random, it may or may not be able to withstand freezing temperatures. The determining factor is the plants ability to prevent freezing of bound water. Bound water is the water inside of cells.

There are actually three kinds of water to consider when we discuss "freezing". The water held in soil - When this water freezes, and it can freeze the soil mass solid, it doesn't necessarily kill the plant or tissues. Then there is free or unbound water, also called inter-cellular water. This is water that is found in plant tissues, but is outside of living cells cells. This water can also freeze solid and not kill the plant. The final type of water is bound water or intra-cellular water. If temperatures drop low enough to freeze this water, the cell/tissue/plant dies. This is the freeze damage that kills plants.

Fortunately, nature has an antifreeze. Even though temperatures drop well below freezing, all plants don't die. In hardy plants, physiological changes occur as temperatures drop. The plant moves solutes (sugars, salts, starches) into cells and moves water out of cells to inter-cellular spaces in tissues. These solutes act as antifreeze, allowing water in cells to remain liquid to sometimes extremely low temperatures. The above is a description of super-cooling in plants. Some plants even take advantage of another process to withstand very low temps called intra-cellular dehydration.

The roots of your trees can stay frozen for extended periods or go through multiple freeze/thaw cycles w/o damage, so long as the temperature does not fall below that required to freeze intra-cellular water. If roots remain frozen, but temperatures remain above killing lows, dessication is the primary concern. If the tree is able to take up water, but temperatures are too low for the tree to grow and make food, stored energy becomes the critical issue. Dormant and quiescent trees are still using energy from their reserves (like a drain on a battery). If those reserves are depleted before the tree can produce photosynthesizing mass, the organism dies.

There are a number of factors that have some affect on the cold-hardiness of individual plants, some of which are length of exposure to seasonal cold, water availability (drought stressed plants are more cold tolerant), how recently planted/repotted, etc

No one can give a definitive answer that even comes close to accurately assessing the temperature at which bound water will freeze that covers the whole species. Unbound water is of little concern & will usually freeze somewhere around 28*.
Some material will be able to withstand little cold & roots could freeze/die at (actual) root temperatures as warm as 25-27*. Other plants may tolerate much colder actual root temperatures - as low as 10*. There's just no way of knowing unless you have a feeling for how cold-tolerant the genetic material the plant was derived from might be, and finding out is expensive (from the plant's perspective). ;o) Another example of this genetic variance is that trees found growing and fruiting well closer to the equator need no chill time, while other trees, derived of genetic stock from a more northerly provenance may need a period of chill to grow with optimum vitality in the subsequent growth period/cycle.

It's wise to remember that root death isn't instantaneous at one particular temperature. Roots succumb to cold over a range of chill with cultural conditions affecting the process. The finest roots will die first, and the slightly thicker and more lignified roots will follow, with the last of the roots to succumb being the more perennial and thickest roots.

Since any root death is a setback from an energy allocation perspective, and root regeneration takes valuable time, it's probably best to keep actual root temperatures in the 25-40* range as long as we can when the tree is resting, even though the organism as a whole could tolerate much lower temperatures. Even well established trees become very much like cuttings if all but the roots essential to keep the tree viable are lost to cold. Regeneration of roots is an expensive energy outlay and causes the trees to leaf out later than they normally would and shortens the natural growth period and reduces the potential increase in biomass for the next growth cycle and perhaps beyond.

Al

Thanks - this is good info.
Is it safe to say that some plants will be more tolerant of the longer periods below 25-40? And the rule of thumb of 2 zones lower is somewhat factual?
I do a lot of Hypertufa container gardening and it is not practical in the winter to move the containers - ANd Some contain conifers. These conifers range from zone 3-5 - the zone 5 would be more risky of course. I aways thought the ability for the soil to drain well, helped with viability through the winter, but maybe that is not as big a factor as I thought?
Maples have worked in these hypertufa troughs when protected from the wind, next to the house. BUT others maples still in protected spots , in ceramic pots have died. ( This one was next to my front door, in a very protected setting.)
I am reading that that drought stressed are move tollerant. So ocassionaly warm weather winter watering may not be helping?
I want to be able to use trees and shrubs in containers (some plastic) and it seems, after reading this that it made me think that it may be more challenging that I tought.

There is much to re read and digest. Thanks Mike

Mike to deal with the problem your having. My recommendation, as I deal with Bonsai, one of which is a large bonsai Japanese Maple now coming out of its first winter with me. And already has new growth on it. This is what I would recommend, and that you provide some form of protection for the roots. Which can be done, by covering the pots in peat moss, or bark mulch if your unable to bring the tree's in to a green house or something.

Another option is, to cover the trees with some clear plastic to form a make shift green house. The main trick is keeping the roots warm, through the winter. I've even used old leaves, and pecan husk on freshly planted trees to provide that protection from extreme cold. And yes, still watered the plant during the winter. Potted trees still need water and yes fertilizer even during the winter. Hense why I use old tree leaves, and pecan husk as they break down over time to provide nutrition the trees need. That decay also provides some hear, to protect the tree roots from a deep freeze.

If you use, clear plastic or even burlap to cover the plants. Make sure that, you ancor it some how, so it does not blow away. And I'd provide, some form of support for your makeshift green box, so you don't cook the tree. As too much heat, and rob a tree of its moisture during the dormant stages.