Barbie, first, I wasn't referring to your question. However, living on the gulf coast is much, different than living in an inland zone 8 location. The proximity to the Gulf actually makes the air much warmer, thus the strengthening of hurricanes once they hit the Gulf. Zone 9 on the coast is considered sub-tropical, however, the zone 9 we have in the central part of Florida can be inhospitable to tropicals. And I wasn't talking about people in zone 8 or 9, I was talking about people in zone 5 and 6, where it freezes regularly. If you get temps that stay below freezing (especially below 28 degrees) for over 5 hours at a time, more than likely, a tropical hibiscus will not survive, and if it does, it will not thrive. Rose of Sharon is much, much hardier than tropical hibiscus. Where I was raised, they can withstand temps of down to 0 for short periods of time, and often live through ice storms with nary a scratch. Put a tropical in those conditions, and they're toast. Like you said, you asked for people's experience, and this is my experience.

Not all of my plants have been moved into the greenhouse and the only one that suffered any damage from temps below 40 F was my Cryptostegia grandiflora. My mandevillas are still in the garden and doing well even though we've had a couple nights with frost. I'd better not push my luck and get them dug up but I wouldn't worry about your plants with those temps, especially if it's only for a short time.

Not having a green house but having had to protect tender plants over winter more times than I can count, I'll offer the following suggestions: Add another layer of clear plastic over and around your greenhouse (if it's not too big to do so). The small but effective layer of air you'll create will lessen/slow the escape of heat from the glass panes. Big box stores carry rolls of heavy gauge plastic which open out to large widths and work perfectly for the purpose. Use duct tape or similar to attach as necessary. Remember that heat radiates easily through single pane glass (both in and out) and any efforts to minimize that heat loss will pay off over both the short and long term. In summer, just remove the plastic and fold for use in future seasons... The size of the extension cord you use may not necessarily provide more heat but may/will reduce your electric bill if you have a long run (distance) from the power source to the heater. Heavier is always better once you get past the "just light bulbs only" stage... Although it may not be possible on short notice, add dark colored bricks or masonry blocks (used or broken works fine) to your greenhouse floor and shelves to trap ambient heat (both sun and other sources) during the day and which will be released slowly during the night time hours. I doubt that the Christmas lights will add even a degree to the overall temp although I applaud you for thinking of possible remedies. Good luck! Steve

I've covered with plastic on the 3 or so nights where it reached into the 30's in my neck of DFW, and the plants have all survived. The beefsteak's I've grown are ripening, but quite mealy. The cherry varieties are doing great at least. I guess the lack of hot weather has a bigger impact on the larger fruits?