I now that sounds ridiculous but if someone has that one special tree that is subject to freezing why not embed a heat tape in the pot to warm it to what ever temperature that you set the thermostat to. I have never done it because I am in heat zone 7 and don't need it, but it is just an idea. So how did I get such an idea, well that come from my experience with aquariums, I alway used a heat tape or pad inside the aquarium to maintain the temperature of the gravel bed. We embed heat tapes bird baths to keep them form freezing so why not in a pot for a maple? I have alway wondered about that since I was a boy because this guy out on Rt. 33 installed all sorts of cactus that don't live here, or was thought not to live here, it was rumored that he ran hot water pipes under the ground to keep their roots from freezing all winter. I have no idea if it true or not, but his place is still there and it is truly a weird sight to see in Virginia. It will not hurt my feelings if no one replies to my post it was just a thought.