Frozen roots do not necessarily mean a dead plant, or even an injured plant. Plants hardy to zone 7 should be fine if you mulch them well above ground and zone 8 plants should be fine if you over-winter under cover or with the containers buried in the garden or beds. Even zone 9 plants would probably be fine over-wintered in your garage or buried against a heated building.

Commonly, each species of plant has a general range of cold-hardiness, but 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 cell walls.
There are actually three kinds of water to consider when we discuss "freezing" or cold-hardiness. 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 bound water, the frozen cells and the tissues they form die. 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.

Any/all parts of your plants 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 plants are still using energy from their reserves (like a drain on a battery). If those reserves are depleted before the plant can produce photosynthesizing mass, the organism dies.

There are a number of cultural factors that also 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.

I bet that was more than you bargained for, but I copy/pasted most of it from a reply I'd left somewhere previously.

Al

My winter weather is not much different from yours. I have a 'carport', that is what it is sold as in many mass merchandisers. It is ten feet wide by twenty feet long, came with a canvas cover, to cover a steel tubular frame. I discarded the canvas and covered it with six mill clear plastic. Both ends are open as are most carports. I over winter many potted plants there with no problems. I have many bromeliads happily multiplying there. Al

Our winters here in Michigan are quite different that in WA for sure, but here's what I do for my potted conifers (pines, spruces and firs).

In mid-November, before the ground freezes, I water the plants heavily several days in a row to make sure the soil in the containers is completely saturated. Then I bury the platic pots in the ground behind our barn, using the structure as a windbreak. I then mulch around the pots with grass clippings, leaves, etc.

During winter the trees receive rain and snow all season long. Sometimes the trees are completely covered in snow drifts.

In late March, when the snow melts and the soil thaws, I dig up the pots and remove the mulch.

Beginning in April, I repot and root trim the trees.

This cycle has worked well for about 3 years now and I've not lost a single tree due to severe winter weather.

Hope this helps.

Dave

I bank in all the potted trees, shrubs and perennials by grouping them together in an empty raised bed frame and covering everything with loads of mulch. Some perennials, the smaller pots, will be placed in a baby pool in an unheated garage. In three years, I've only lost the occasional perennial, and they were the weak ones. When everything thaws and Spring rolls around, I just uncover the pots buried in mulch, and pull the baby pool out of the garage. I water everything, and watch it grow!

For tender bulbs and plants that remain potted, I take them all into the basement where they can rest for the winter, but not freeze. I don't have too many pots of tender items.

Most everything survives for me using these methods... except the occasional weak plant that probably wouldn't have made it even if winter weren't an issue.