I live in Utah zone 6b - 7a fluctuations. Last winter I grew a 6 inch tall tiny 'Prague' chimera citrus outdoors in-ground in a micro climate next to the south wall of my house. Temperatures dropped as low as 15 F. This tree was heavily mulched with wood chips and caged in poultry wire with a plastic tarp wrapped around it. From December 20 to Jan 15 I surrounded the cage with old milk jugs that were repeatedly warmed in the micro-wave around midnight any-time temperatures dropped below 20F. Not only did the 'Prague' tree survive the winter without defoliation but the citrus has tripled in size this year of 2022. Additionally I grew a 2 foot tall 'Nippon' orange-quat with the same procedure and the Nippon pushed out beautiful new growth last spring and is fruiting for me this year although it suffered some defoliation and 1/3 of the old leaves look damaged. I was able to harvest some fruit from 'Nippon' last December and successfully germinated a dozen seeds. Additionally both of these trees have survived summer temperatures up to 107F. The Nippon suffered some scorch in the most exposed leaves. I intend to continue growing 'Prague' and 'Nippon' citrus tree in the ground and espalier them against my home each winter as they mature. Due to my recent success I intend to also try a very cold hardy tangerine hybrid the same way. All of these citrus are grafted to Poncirus 'Flying Dragon' root stock. Good Luck Howard! Most of the folks bragging about growing a citrus tree in a cold zone are from zone 8 so I think growing citrus in the ground outdoors in zone 6or 7 takes effort and is something to be proud of. Keep trying! You will figure something out.

I believe all the quoted temperatures are far too low for any practical purpose of attempting to grow citrus. Yes, you may find the very occasional mature plant that has survived such temperatures. But, in general, if your minimum temperatures approach these figures even, say, once in five years, then your citrus will not survive and certainly not thrive. I live in an area of England where temperatures rarely go below -8C (18F). Even though gardeners here love their 'mediterranean gardens' you don't see any citrus outside. They don't live here for long. I have been growing citrus for 30 years. Poncirus, citrange and citrumelo survive outside, but suffer some damage in some years. Ichang papeda tries hard but needs protection. Everything else dies.

It will be very difficult, and likely impractical, to grow any intergeneric hybrids in zone 6. US852 and TaiTri might be the only ones that would have any chance of making it, and even then I'd strongly recommend planting in a protected spot, somewhere that won't get too much wind, and putting a pile of mulch and a cover on it over the winter. Probably close to the wall of a house would be good, surrounded by other nearby bushy plants. Bear in mind these two are not exactly "delicious" tasting. TaiTri is not very much better in taste than poncirus, while US852 has a bit of off taste and is nowhere near as good fruit quality as the crappiest mandarin you might ever find in a supermarket. Though I did see a video where one very adventurous enthusiast claimed he was able to "enjoy" eating them. (full of seeds of course)

Jenny, that’s very useful, understand how the hardiness zones are calculated. Since my area has hot humid summers, I’m gonna guess it’s probably a lot like Virginia or the Carolinas? Crazy pest and mold problems here, but when things grow they grow like crazy. The weeds in the summer are like nothing I’ve ever seen. Yuzu was the first citrus people recommended for this area. Mikan (mandarins) were recommended too. The resprouting is kind of neat, and at least before the moles showed up, they had very extensive root systems, which should help with regrowth. I’m not expecting much or any fruit though, as there’s no leaves to support them, and all has to be regrown in one season. Not sure how useful that is. Thanks Cory. The last time I tried grafting I cut my thumb open in the grafts never made it anyway! Pon, I agree with you, it’s nice to use the passive heat systems. I’m still interested to experiment with some ideas. The typhoons really make it tricky here though, as things being blown away can be a bit dangerous. I don’t miss low temperatures like that anymore. I would have no problem moving to an even warmer area that I’m in now.