I can provide another update now, from Olympia, WA.
I have planted several Yuzu trees, some on rootstock and some seedlings on their own roots, and they all look like they are doing well. They hold onto their leaves during a normal winter, the leaves turn a little yellowish-green but then recover most of their green color after the winter is over, and then in about June they start sending out additional leaf growth. Yuzu seems to be the most vigorous growing of the hardy citrus varieties I am growing.
During a colder than normal winter, the Yuzu did suffer some moderate bark damage and half the leaves fell off, but it easily recovered much later in the year. The Yuzu has been in the ground for about four years, and gone through 3 winters.
I have a Sudachi tree which has gone through one winter, not counting this winter we are already in. It has survived and has just been doing okay. It seems to be a slow grower. I even saw it began to put out a tiny little fruit druplet, which appeared on the tree too late into the season and never was able to further develop.
I also planted a small Keraji seedling. Unfortunately, its first winter was a cold winter. I put a little plastic covering over it but it was still killed to the ground. Amazingly it was later able to begin recovering, and I saw leaf growth by July. It survived through the next winter but was still not very big, maybe only 2 inches tall, even at the end of the next year. It has slowly been able to grow, but I can tell by the coloration of the leaf growth over the winter that it probably does not have the hardiness level of Yuzu or Sudachi. Still, it is remarkable that such a small seedling was even able to survive. I am experimenting with a larger grafted Keraji that was just planted this year. So far, as of January 20, it still has fairly deep dark green colored leaves and looks like it is going to do well so long as this winter does not have colder temperatures. I planted it in more protected optimal spot (still outside and not covered).
I also have a Bloomsweet outside, planted in an optimal spot, with a warm south-facing brick wall of the house only two and a half feet behind it. The Bloomsweet has now survived through 3 winters (including this one). I covered it with a large paper grocery bag with two gallon containers of water under it only on the coldest night during the unusually cold winter. Otherwise it has not been covered. It suffered some severe bark damage at the base that winter, but was later able to recover. It has been growing moderately slow.
In Vancouver, WA (right across the bridge from Portland, OR) there is a big Yuzu bush in someone's yard that I am told produced 50 pounds of fruit one year. But fruit production can be off and on. (which is not that uncommon for many fruit trees under non-optimal conditions) This year the big bush produced very little fruit. The typical pattern goes one year with lots of fruit and then the next year with very little or no fruit. With about half the trees producing fruit in any given year.
This person also has a Kabosu tree which grew vigorously this year and was full of lots of fruit. I got to try one of the fruits, the quality is somewhere in-between Satsuma mandarins and Yuzu. The inside is like a Meyer lemon, kind of bland and insipid but a little more fragrant, but surprisingly the peel is moderately edible and is where most of the flavor seems to be, kind of analogous to calamondin. This person's two Sudachi plants did not grow anywhere near as fast.
This is some additional information about the lineage origin of these unusual Japanese varieties I was able to find.
First of all, I need to describe one of the origin varieties, which I do not have, but seems to be an important ancestor. Kunenbo was, in Japan, a large sized very aromatic mandarin, almost similar to a tangor, but very seedy. Along with Kishu mandarin, Kunenbo was one of the original two parents of "unshiu-mikan", what we know as "Satsuma mandarin". So that would make Bloomsweet a close sibling related to Satsuma mandarin. Judging by many of its offspring, it seems apparent Bloomsweet is probably even slightly more cold hardy than Satsuma. The name "Kunenbo" translates as "nine year mother" or "ninth century mother" (which might make sense since a lot of citrus varieties were brought to Japan from China around the Ninth Century). A genetic analysis has shown Kunenbo to simply be a mandarin with a little bit of distant pomelo ancestry, which is not that unusual for many common mandarin varieties. (one of the parents of Kunenbo appears to have been Kishu mandarin)
Bloomsweet is believed to be the same as the Japanese variety which was called "Kinkoji", which came about from a cross between pomelo and Kunenbo. It was probably brought by Japanese immigrants who came to Texas to farm grapefruits, where it later was given the name "Bloomsweet grapefruit", although some might not really consider it a true grapefruit. The fruits are supposed to be somewhere in-between an orange and a grapefruit, but the inside is kind of watery and bland, and seedy, not really the best quality. Bloomsweet is said to be able to survive down to maybe 16 to 18 F.
Keraji, according to a genetic analysis, appears to have come about from Shikuwasa, which got backcrossed with Kunenbo three separate times (first cross was called kabuchi, second cross was called kikai-mikan, after the name of the island). It is not known exactly how Shikuwasa originated, but genetic analysis has shown it seems to be closely related to Tachibana, and both have some small amount of Yuzu or Ichang papeda genes. Tachibana seems to have been brought to the Southern Japanese islands very early on, before any other citrus. (Some even believe it might be the only citrus species actually indigenous to the Japan) It might have been used as a rootstock in China in ancient times, and then brought to Japan because it was more hardy and able to survive, and could have had uses for flavoring in cooking, but this is just speculation. It is said that Keraji can survive down to maybe 12 or 14 degrees F, making it significantly more cold hardy than Satsuma mandarin.
It is not known exactly how Yuzu originated, but Yuzu does seem to have originated in the same area of China where Ichang papeda originated, near and around the city of Yichang, on the upper reaches of the Yangtze river. There are two theories, Yuzu could have been a separate subspecies, which originated from genetic introgression of mandarin genes into a population pool of Ichang papeda. Or it could have resulted from human hybridization between mandarin (possibly sour mandarin) and Ichang papeda. (It would not have been a single hybridization event). Sour mandarin appears to have a little in common with Yuzu, but it is not clear which one of the two might be the ancestor of the other. (Yuzu is the Japanese name. In China they simply refer to it by another name that simply and vaguely translates as "fragrant orange").
I am one of the rare people who have had the opportunity to taste both Yuzu and Ichang papeda, picked fresh from the tree, and they are both very uniquely similar to each other in a way, yet substantially different in a different way. Comparing the two, Yuzu is much more sour orange-like whereas Ichang papeda is much more lemon/lime-like. Yet they both have a unique deep pungent fragrance, different from any other citrus.
Sudachi originated from a cross between Yuzu and another parent that was closely related to, or some hybrid of, Tachibana. Kabosu originated from a cross between Yuzu and Kunenbo.
Yuzu is said to be able to survive down to 10 F. However, it will suffer some fairly substantial damage at a temperature that low. It can however come through completely undamaged at 14 F. Sudachi appears to be nearly as hardy as Yuzu, but not quite as much.
Hopefully this hard-to-find and obscure information will be of interest to some of you. I hope this information might be useful to some of you. That is, if any of you are even able to get your hands on these varieties, which can be very difficult.
I actually have so much more that I would like to share and write on this topic, but this is enough for now.
