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.

Here is what is said by the Southeastern Palm Society (which also does a LOT of work with cold hardy citrus varieties) about the Nippon Orangequat . Nippon Orangequat - (Fortunella crassifolia X Citrus reticulate) is a cross between the Meiwa kumquat and the Satsuma Mandarin orange. The resulting tree is very cold hardy and specimens in protected sites have easily withstood upper single digits with no protection. Characteristics are intermediate between the two parents, the egg shaped fruits are bright orange inside and out with an edible peel. They're sour and juicy making an excellent marmalade and drink. Nippon orangequat bears reliably and abundantly under southeastern conditions and rarely fails to make a crop. The plant is fairly low and spreading, reminiscent of Satsuma, with large pointed kumquat like leaves. It makes a very handsome ornamental plant and is hardy to at least 10F. Flavor: Sour tangerine, no off flavors, good quality. Use: Juice, preserves Note: Becomes sweeter as season progresses, but fruit must be over wintered to become sufficiently sweet to enjoy out of hand.

Here's my precious seed that came from a hybrid between Ichang papeda x trifoliate. It's just beginning to germinate. (It originated from the same person) Trifoliate and Ichang papeda are the two most cold-hardy citrus species, so this thing should easily be able to survive zone 7 when it becomes a bigger plant. He tells me this hybrid barely has any of the characteristic bitterness of trifoliate, and the fruits are bigger and better quality than one might expect from such a hybrid. Being a second generation seed, it's possible the genes got mixed around again, so this seed could possibly be a bit different from its parent. (If there are desirable traits that have recessive genes, or undesirable traits with dominant genes, it's potentially possible for a self-cross from a hybrid to be an improvement) The hybrid shows intermediate leaf characteristics between trifoliate and monofoliate, but mostly monofoliate.

Did you know the Chinese city of Yichang, in the region where the Ichang papeda is believed to have originated, is just a tiny bit further south in latitude than San Diego? That area of China gets most of its rain during the Summer. It doesn't have much elevation, but it is pretty far inland so can still get cold in the Winter, equivalent USDA climate zone 8. So warm most of the year, not too extremely hot and dry during the Summer, Summers can still be fairly humid, this is exactly the sort of weather citrus likes to grow in (except for the colder Winters). While they are separate species, the papedas are believed to be more closely related to each other than to other citruses. C. micrantha (ancestor of lime) and Khasi papeda (C. latipes, the ancestor of Kaffir lime) both have symmetrically large winged petiole leaves like Ichang papeda (C. ichangensis). Ichang papeda is also believed to be closer in genetic relation to citron and pomelo than to mandarin. However, unlike C. ichangensis, C. micrantha and citron are both very sensitive to cold, more so than orange. Since Kaffir lime descends from a different species papeda than real limes, it can grow in zone 9 (even saw one source that says it can survive down to 15F, not sure if that's true). (People most often don't eat the fruits of Kaffir lime, but rather the leaves of this citrus are really good for adding flavor to Thai cooking recipes) Changsha mandarin is either an indigenous species in China extremely closely related to mandarin, or became naturalized in the wild a very long time ago. There are several different cultivars of Changsha that evolved to be suitable to different growing conditions in different areas. There was one study, looking at genetic markers, done on an accession of Changsha mandarin which indicated it was mandarin but may have had up to 15% pomelo in its ancestry. (Genomics of the origin and evolution of Citrus, Guohong Albert Wu, Javier Terol) This could possibly be from genetic introgression over time from surrounding cultivated hybrid mandarin varieties. Stan McKenzie sent us a picture of a large Changsha tree on his citrus farm in Scranton, South Carolina, that he said had survived a drop all the way down to 8 F degrees on January 18 this year, and it was full of fruits by November 5. (I think normally Changsha trees can only survive down to about 10-12F) Here's the picture: The fruits are filled with seeds though. Changsha mandarin is also notable for its drought tolerance (since it comes from a drier farther North area of China).