How cold does it get, at worst in your winter, David? No matter how much you insulate the plants, if they are up out of the ground (which retains a lot of heat) they will eventually get down to the lowest temperature of the ambient air unless you supply a heat source. Just like the ice will always melt eventually, no matter how well insulated a cooler is.
Things like hibiscus will not tolerate having their roots get too cold. You'd be better - and further ahead next spring - to take cuttings and bring them indoors for the winter.
Btw, there is a whole forum on Tropicals and a great thread about "Growing Tropicals in the North" where there are a lot of people who do what we call "pushing the zone". Which is what you are doing - growing plants that are too tender for your climate zone.
The thread "Growing tropicals in the north" in
Tropicals forum
You need to branch out and make use of the specific threads for your plants! You will have contact with many more people on ATP that way.
