Jeremy;
Sorry I missed your post originally, Acer Palmatum in Calgary are a unique challenge and you're trying to grow one in an apartment to boot. I salute your enthusiasm and optimism.
Sounds like you have quite the set up with your growbox but my concern would be will it really support the tree through the worst of winter (can the heater you are using keep up to the -30's and wind chill like we had earlier this season) and depending on the exposure, will it now be turning into an oven when the sun shines? You had a pretty good solution with the fridge (provided no one objected to the tree sitting next to the brocolli and left over roast)
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Now that the tree is out of dormancy I would leave it indoors or in your growbox at night and when winter takes another swipe at us (which I'm sure it will, hell we've had snow in every month of the year since we moved here 20+ years ago) and outside in the fresh air whenever the weather is nice. A bit of a nuisance moving it back and forth, but then you're the one who decided to try a Japanese Maple out here.
BTW where did you get it from? I've seen some landscape size for sale at the Dalhousie Canadian Tire but never been tempted due to the ugly grafting jobs that were done to them (Obviously grafted to some hardier species root stock to try to make them able to survive out here). The lady in the garden centre claimed they did fine in sheltered locations but I can't say I've ever seen one ...?
Well I hope things go well for you with this one, post some pics when you get the chance (I'd be interested in seeing the tree and your growbox set up)