Q.I Have a Lemon Tree in a Pot
I have a lemon tree in a pot. Will it survive an Ontario winter if I leave it in my sunroom? The sunroom is warmer on a sunny day, but just as cold as outside on a cloudy day. The sunroom has no insulation, no windows, just plastic covering the screens.

Certified GKH Gardening Expert
Citrus trees cannot survive temps below 30F (and really, I would not push it to below 40F as below this temp leaves, flowers and fruit may be damaged). If this sunroom will be below these temps, the citrus tree will not survive in there.
That being said, citrus trees do well near a sunny window in the house. I have had one for 12 years that lives half the year in the house and half outside and is a beautiful plant that produces fruit.