Answer: The standard guideline for tropical hibiscus is to remember it is a TROPICAL. They will not tolerate more than a night or two of light freezes. Even one hard freeze (below 25) could kill the plant. These plants are native to sunny, warm and usually humid tropical places. They detest cold, rainy weather and cold, wet soil. They will not reliably survive outdoors north of zone 9.
Since taking them indoors is not an option (BEFORE temps regularly drop below 40-45 F at night to avoid any damage), you can try to overwinter them on a protected patio. Erecting a temporary structure (teepee style) to place over the entire plant and draping either Reemay (floating row cover) or an old blanket over the structure when the weather threatens to freeze may be all the protection your plants may need. Be sure to remove the blanket during the day so natural light is available to your plants. Hope this does the trick for you!
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