I’m growing in ground with protection 4 mil plastic Ventable enclosures with small space heater on thermostat from late October to early April: owari satsuma, kimbrough satsuma, Changsha mandarin, Rio red grapefruit, Meiwa kumquat, and just planted an Ichang lemon On flying dragon in a frame enclosure. I also have Poncirus hybrids In ground that I cover with frost cloth or plastic and heat with space heater if temperature gets into the teens or single digits or if it’s going to be below freezing for several days: Thomasville citrangequat, citrumelo 80-5, Dunstan citrumelo, swingle citrumelo, ichangensis seedlings. I have lost over the years to week or month-long freezes all of the above varieties if not protected by electric space heaters in plastic enclosures. I’ve also lost these poncirus varieties and others ( nansho dai dai, bloom sweet, Changsha, Morton, Ichang lemon, swingle) when the trees started to show quarter inch spring growth and experienced a 14 degree freeze in mid March. In my experience, the only citrus that is truly hardy consistently without severe damage in my area every year is poncirus and flying dragon, and some years they bloom too early and their blossoms are not able to withstand a freeze. My Thomasville and Citradia and Dunstan have come back from roots several times over the years and Im protecting them some now to preserve them for grafting onto poncirus to make dwarf fruiting specimens, especially the citradia since I can no longer find this variety anywhere.