wmcorcor

that's a tough one. I think you have few options given your temperatures and circumstance. I can't tell you it's safe to keep it outside and not knowing it's exact species I can't say if it will live or die indoors for winter.

Most of my trees did survive a brutal winter last year with temps in the 10 to -10 Farenheit...but I have larger trees and a protected garage location.

I use an unheated garage for wintering. Some people use a cold-frame, some people simply put the tree in the ground for winter (pot and all) with mulch. If you have no access to anything like that being on the 2nd floor then I'd say you'll have to experiment with the outdoor trees. I have heard of people using refridgeration but I have no experience with that.

I worked with only indoor trees until I had a wintering location. Maybe some other apartment dwellers can jump in here and help or perhaps you can start a thread asking that specific question.

I wasn't trying to confuse your or worry you about your tree but seeing what is typically an outdoor tree being kept inside I was worried you didn't know better...and believe me this is the voice of experience. I've lost several outdoor trees by keeping them inside before I knew better and I lost a few more because they didn't get that dormancy they needed.

I hope this helps in some way