I have a greenhouse in Western Washington State that I keep heated to just above freezing over the winter. I bought a temperature sensitive power socket that turns the space heater off and on, so that it doesn't either freeze or over heat. I have successfully wintered over geraniums. I figured what I spent on heat I saved on plants. Of course the plants are now larger than what I would normally buy. We had a very cold couple of weeks in December and I covered the green house with blankets for a couple of weeks to help insulate a little more. We normally have very mild winters here, so I don't know that anything I could add that would help you with a greenhouse.
What about hydroponics? That you could do that in the house with grow lights, and the temperature at what ever level is normal for you. I have a garden window in my kitchen that has a grow light on all the time and I am able to grow a lot of different plants.