Here is the answer you were dreading. It all depends.

There are hardy azaleas that take temperatures below -25F without protection. There are hardy hibiscus that take temperatures down to -20F without protection.

So without knowing the varieties you have we can't be much help. Florist azaleas are generally house plants and many will not survive freezing temperatures. Some that do survive will have all their flower buds frozen and hence destroyed.

When you say you cover the plants, I wonder what you mean. A cover can cook a plant if he sun hits it. I can't imagine any cover should be left on. Normally covering a plant is just done to survive a light frost and nothing more. It is removed in the morning after the frost.

Thanks for your reply. I dont know the variety of plants we have cuz mom bought them when I was out of town.
She used to cover them with white bed sheets and since she's 84 she used to leave the sheets on for days during a extended cold spell BUT it didnt seeem to cook the plants, they still bloom, lucky I guess.
Now we are covering them with heavier bed blanket, but not heavy enought to bend the plants and also bath towels. Since we live together now I take the covering off dailey, or whenever we cover.
I guess the safest thing to do would be to cover the plants whenever it drops below freezing and remove the cover at sunrise, do you agree?

White cloth is good. It reflects the hot sun and breathes. The worst are clear or black plastic.

Yes, covering at night and removing during the day is best.

Good Luck!

Thanks for the info rhodyman.