Since I'm in the city with no garage, I have taken my very small potted trees (1 gal pots) inside already for the winter, but the medium potted ones (5 gal pots) I have  left on my balcony, although I admit with some hesitation. Does anyone know of some kind of cover I can buy/make to provide insulation so they have a better chance of making it through alive? Thanks!

Insulation will help a big pot a little but there really isn't enough heat mass inside the insulation to have a large effect. Said another way there isn't a lot of heat in a pot to retain. If your balcony has a concrete floor that would increase heat mass. Insulate over the pots down to the floor with anything having a high R value. Put a light bulb inside and now you have a good setup and some heat to retain. If 2nd floor you'd need to insulate under the pots as well.

something you may consider, get a string of incandescent rope lights. Wrap the pots with the rope lights then wrap over that with insulation and a waterproofing barrier to protect the electrical connections. Do not use LED rope light, they don't produce enough heat. there is no need for the rope light to be on all of the time, I would use a thermostatic control for it. The rope lights I have used in the past have a tendency to run around 80 degrees which would be too warm all the time. The thermostat would only bring the lights on when it drops below your preset temperature, I would put the temperature gauge into the soil of the pots when the soil reaches the ideal temperature the thermostat will obviously turn off, then back on as the soil cools.

Haven't used them myself but the gutter heating cables should be a good heat source. 5 watts per ft and available 20 to 200ft or more. I think they have a built in thermostat and are intended to melt ice out of home rain gutters. We want to keep ice out of fig pots, same deal. They are left in place yr around so should be fully weather proof.

There is also pipe heat tape that comes with a thermostat. 

Great idea fignutty!!