A trick I used in Denver in the 1990s to grow bananas and hibiscus and Old Roses indoors each winter was to keep the ROOTS warm:
1. In Denver I grew various Tea and China and Noisette roses in 3 gallon black plastic pots in my south and west windows.....that black plastic acted as a solar collector a rose can live in.
2. In my double-paned hotframes I made in the backyard from discarded, newly manufactured sliding glass patio doors, I had dumpster dived water bed warming pads connected to an extension cord from the garage. All that square footage of warmth let me germinate seeds and grow roses and veggies all winter. Once I insulated the soil mass by burying foot deep slabs of 3 inch thick styrofoam that came from the same door factory, all along the interior of the hot frame's length and width, the heat build up was dramatic.
3.In Denver I grew potted bananas and hibiscus indoors all winter in my livingroom atop dumpster-dived heating pads, like those people use on sore knees and elbows. Tropical plants struggle BIG time when the soil is cold.
4.Here in Tampa I use a heating pad my friend Allen gave me both to heat potted plants, and to keep baby chickens of all ages warm and snug inside a scavenged plastic dog Igloo-style dog house inside their pen when cold fronts pass through, like last night.
"Peek and You Shall Find!"
(a dumpster diver's motto).
p.s. I usually got heating pads and water bed warming pads from apartment complexes and condos, and sometimes from dumpstsers behind Good Will and St. Vincent de Paul stores in Denver.