As no-one has followed up this interesting post, I'll give my thoughts. First of all, the quoted hardiness temperatures generally refer to survival temperature, not the temperature when very slight damage occurs. Secondly, this temperature is very dependent on exact conditions and can only be considered a very rough guide. Length of exposure to damaging temperatures, humidity, wind speed,age, stem thickness, protective leaf canopy -- all will alter the result, and can vary over a few yards. Poncirus has developed hardiness by becoming deciduous, by generally delaying its springtime growth even if there is a short warmer period, and by developing bud scales which protect the flower buds as they form. Poncirus hybrids maintain some of these features but to different degrees. I also have citranges which show leaf curl after hard frosts, but I don't think the plant is damaged by this. Some leaves drop later, but are soon replaced by new ones. What is more damaging is a lighter frost after spring growth has started. Even Poncirus new growth can be damaged. I don't think your query about whether 'these plants haven't stayed cold enough overall' is relevant unless spring growth has begun.

John: US-852 was discontinued as a propagated rootstock for numerous reasons. One was that it did not produce enough nucellar seedlings...there were enough 'off types' that the seedlings were too varied, so this made it hard to propagate reliably on a large scale for the citrus industry. But someone told me it apparently has a very good long-term compatibility with satsumas. Apparently the hybrid has been remade by Univ. of Florida and called UF6. It supposedly grows more true from seed. Steve: The only reason this variety was of interest to me was because it's very hardy, approaches edibility (LOL), and actually survived and bore fruit in the ground in my region. But it's heplful to know that it does make a good rootstock as well, I suppose.

Stan is a real swell guy. A short time ago he was selling US 852 fruit (full of seeds), sending off small boxes through the mail. Apparently the trees at his farm were loaded with them and there were a lot of extras. I don't think he really does it for the money, he just has a love for hardy citrus and wants to spread the joy. Sorry to say, blkyn citrus, but I'm not sure Morton is going to be able to survive outside completely unprotected in 7b. US 852 has a much better chance.

Hard to say, but I can tell you that Kumquats and Limes are the only citrus that Saxon Becnel does not graft onto Kuharski they graft these onto C-22. This is according to an email from Ricky Becnel a year or two ago.