Zone 9a — 20 to 25 degrees. That’s mostly accurate although the length of the freeze is what seems to determine what lives and what dies. I’ve had plants that are zoned for 10a and should wither in 20 degree temps but survived because the coldest cold only lasted an hour. I’ve also had plants that should survive in 20 to 25 degree temps but died when the low was only 30 because it stayed that low for 8, 9, 10 hours or the day time and nighttime temperaturea stayed below freezing for 2 or 3 days.
Of course, sunlight has a lot to do with it too. A plant that was warmed by the sun all day is better able to survive a plunge in temperature than one that spent the day in shade.
I plant my peppers in the sun and as close to the house as I can to provide some protection in the winter. When temps drop into the 30’s, I cover them. But, they won’t survive the freezes we had in January no matter how well they are covered!
My almost 6-foot hibiscus, which is zoned for this area, was killed back to the root this winter even though it was very well covered. I chopped it down to the ground and now there is growth rising from the root. I was worried it wouldn’t revive. My neighbor has two of the same type of hibiscus and did not cover hers at all. They turned black but are now also returning from the root, but their regrowth started before mine and are much fuller and taller than mine. Not fair!!!