I have kept a Cryptomeria japonica 'Spiraliter Falcata' in a container for maybe 5 years by overwintering it in my sunroom, where a heater does not get let it below 48 degrees. More recently I've tried keeping Juniperis chinensis 'Shimpaku' as bonsai in that same room, but this is my first year for that. The trees stop growing during the darkest/coldest time of year, but resume in early February when the sunlight heats the room to over 70.
I don't know if this would work with most evergreens or only certain types. If you have any experience with keeping plants that aren't hardy in your area in containers, I'm curious about your methods for overwintering and success rate. I especially want to try a Cedrus deodora. Has anyone had experience with keeping that by this or similar method? Some sources say that conifers must have the cold for a certain period of time to be healthy over the long term. My heater can't be set lower than 48, so that is the coldest I can make it for them.
Overwintering non-hardy conifers in containers
C japonica and C deodora should over-winter ok in an unheated garage, which would satisfy their chill requirement. The japonica should be ok just setting on the floor, but you may need to cover the deodora with a cardboard box during periods of extreme cold, or if you're prone to leaving the door open for extended periods.
Most temperate trees need a cold rest period to grow with the vigor they're genetically programmed with (within the limitations of other cultural conditions) in the subsequent growth cycle. A friend of mine, Jack Wikle, has made considerable progress in being able to grow some temperate species indoors year round, so you might wish to take a look at what he has to say: http://www.fukubonsai.com/2b2a2a.html
Al
Any thoughts on Cupressus cashmeriana? It's such a lovely tree, but pretty much of a Z9-10 kinda plant. It shouldn't need much of a chill, eh?