Thank you for posting the info / experiences... please do update up on this whenever you can. Temps in the teens will make or break certain citrus. Although its highly variable as to age of the tree as well as the yard's micro-climate too. The lowest my yard experienced is 16F for a few hours overnight....uncovered. The results were ugly, but most bounced back... even the Bearss lime.

Well the temps under the cloth were higher from the lights. Good that this setup works for 13 degrees below but not sure how much farther it could protect. hardier varieties would be a safer bet

I think we had a thread on this in the past.

Kumquat is just barely not going to be able survive in zone 7b, as far north as Atlanta. Yuzu might have a chance there, but I would advise you to give it some light protection and/or plant it in a very optimal spot, maybe up against the brick wall of a house.

Once you go from zone 8 to zone 7, the options for "hardy citrus" becomes much more limited, and what can grow is harder to find and won't taste as good.

You might look into citrumelo.

Ichang papeda on Poncirus rootstock might be able to make it.

Taiwanica lemon is pretty hardy and can easily recover from damage, so could be another option, although the fruit quality is really subpar. I mean it's "edible" but only fit for flavoring vinegar.

There's also Arctic Frost, but I think that's only going to grow right on the border of zone 8 to 7, once you move to zone 7b it's going to require at least some light protection.

To provide you with some additional information, I tried Arctic Frost in the Pacific Northwest zone 8a (a different climate than where you are) and the tree did not survive. But there's someone with one 2 hours south of me who has a tree that is growing outside, they give it a little protection during just the coldest winters, and they're in the middle of a suburban neighborhood (so I'm guessing that makes it solid zone 8, rather than 8a).

(The PNW can be a little harder to grow things in than the South, climate zone number being equal, that shouldn't be surprising)

Yes I was planning on protecting all with frost fabric/c9 lights. I believe that would be enough for those citrus in 7b.