Hybridizing cold hardy citrus to grow in the Pacific Northwest

Hello. I am breeding cold hardy citrus (oranges, grapefruits, etc) with a focus on developing new hybrid varieties that can easily survive in the Pacific Northwest climate (zone 8a, Seattle area) and that are better eating quality than current cold hardy citrus varieties.

I'm hoping to make outdoor citrus a new trend in this region.

Because of the extended cool temperatures and short growing season in this climate, I'm using indoor grow chambers for the plants that will be used in hybridization. With this approach I believe it will be possible to go from seed to fruit in as little as 3 to 4 years (maintaining optimal warm temperatures and continual light).


For any of you who may already be familiar with obscure field of cold hardy citrus, I will be hybridizing from the following varieties in my collection:

Citrumelo (trifoliate x grapefruit)
'Dunstan' citrumelo
(I've read reports of citrumelo being able to grow in England. At their best they are basically like a sour grapefruit and kind of lemon-like)

Changsha mandarin (not so different in flavor from a common mandarin, but flavor more watery and insepid, some say a slightly skunky tinge to the aroma, and fruits are very full of seeds. I obtained seeds from someone who was growing them outside in Tennessee)
changsha x satsuma mandarin hybrid

Yuzu (cold hardy down to 10 F, traditionally used to add flavor to things in Japan, one of the few varieties that can survive outside in the Seattle area)
Yuzu x Clementine hybrid (supposedly hardy to 10 F but in reality might be closer to 12 F, not as vigorous growing or resilent as Yuzu)
Ichang lemon (believed to be closely related to Ichang papeda but a pomelo hybrid, Chinese referred to it as "Fragrant ball", the same name they also used for citron)

Satsuma mandarin
Bloomsweet (believed to be the same variety as Japanese kinkoji, this variety is reminiscent of a grapefruit)
Keraji (supposedly hardy down to 12 F, mandarin-like small fruits but sour, said to taste like lemonade, although when grown from seed some selections may be watery and flavorless)
(from the research I've done from published DNA marker analysis published in Japan, it looks like all 3 above varieties were descended from a variety known as "Kunenbo". Kunenbo translates as "nine year mother" in Japanese, and appears to be some sort of tangor, with probable parentage from pomelo. It is said to possess a distinctive aromatic fragrance. Kinkoji is a backcross between a sour citrus and kunenbo. This sour citrus appears similar to Sudachi. I don't have kunenbo and it appears to be a difficult variety to source outside of Japan)

a seedling from
(edible citrange x Ichang papeda) x Minneola Tangelo

Red Shaddock
Chandler pomelo
pomelo (I believe 'Reinking')
grapefruit (common supermarket)
Oroblanco grapefruit (grown from seeds, Orablanco is triploid hybrid between grapefruit and pomelo, when a triploid variety is grown from seed I believe most of them will turn out to be normal diploids, though some will be genetically exactly identical to the parent, and a very small fraction may be tetrapoloid)
Duncan grapefruit (white grapefruit variety, unparalleled grapefruit flavor, used to be one of the most popular varieties)
Minneola Tangelo
Page Mandarin
Valencia Orange

Lemon (probably 'Lisbon')
Giant Yemenite citron (these can get bigger than a grapefruit, but almost don't have any flesh inside, all pith)
citron (probably 'Diamante')


I currently have over 100 seedling plants in large cups grown from seed.

As I'm sure many of you know, common citrus varieties have difficulty growing in this climate, and few of the common varieties can survive planted outside in the ground.

Out of the common citrus varieties, the ones with more cold-hardiness are: kumquat, Calamondin, Satsuma mandarin, Gold Nugget, Kara mandarin, Meyer lemon

I believe Satsuma mandarins are pretty borderline in the Pacific Northwest climate and they will need some light protection some years. I've heard reports of Meyer lemons in large containers surviving being left outside over the Winter in a sheltered sunny spot facing the South.
Young plants less than 2 feet tall are more vulnerable to cold and need to be brought inside.

One thing that's interesting to think about, if you look on a map, Seattle is actually a little bit farther North in latitude than Duluth, Minnesota.

Super cool plan, I hope it works out and look forward to results. The one issue I see is, if you are using heated grow rooms for the grow outs how will you be able to select for cold hardiness?

Cool! Exciting stuff! We've certainly talked about that kind of stuff before! You might find this thread interesting. I know there is another one somewhere but can't find it.

http://alanbishop.proboards.com/thread/6531/cold-hardy-citrus

http://alanbishop.proboards.com/thread/7870/cold-hard-citrus-hybrids

I'm in Victoria on Southern Vancouver Island in Canada. Someone is growing lemon about 2 blocks from me. Not sure what kind they have, but I would be willing to bang on the door to find out, and see if seeds or cuttings are available. Chances are, it's a variety you're familiar with. I think they just have the one specimen, so probably not as into it as you are.

There's a guy on Denman Island who sells Russian pomegranate. Not citrus, but certainly not a common Canadian crop. Denman Island is about 49 1/2 degrees north.

Another non citrus crop that you might be interested in, are the paw paw being developed by the University of British Columbia. My former home in St Catharines Ontario, used to be the northernmost range of the paw paw. Coastal British Columbia has many areas that see less frost.

stephen lowe wrote:Super cool plan, I hope it works out and look forward to results. The one issue I see is, if you are using heated grow rooms for the grow outs how will you be able to select for cold hardiness?

I'm curious about this too.  I'm in Central Oklahoma where the Trifoliate Orange (poncirus trifoliata) grows really well (if you can get it through the fragile seedling stage) but none of the other cold-hardy citrus and near-citrus as far as I know; we get temps down in the 12F range pretty much every year with single-digit temps ever few years and the low-temp extreme in 25 years being -8.  I figure there ought to be something more useful than the poncirus that would grow here, but the only way I'm going to find it is to build a big cold frame for my seedling nursury and some semi-protected outdoor environments (lots and lots of south-facing masonry walls) and then start slaughtering four-year-old trees on an epic scale.  I've been saving kumquat and meyer lemon seeds, but that's as far as I've gotten...

The Tennessee citrus is very interesting. Are they producing in TN? Are seeds available anywhere?

Dan, have you found any uses for the Poncirus?

Very cool.  Don't forget that most seedlings of Citrus are clones of the mother plant (nucellar seedlings).  When choosing a mother plant try to use those that set few seeds that are clones.  One of the better choices for those of us trying to breed hardy citrus is US-852, which is a Poncirus x Changsha selection that sets 50% zygotic seedlings, thus greatly improving the odds of getting genetically new combinations.  That and US-852 is quite hardy to start with having excellent parents for that.  Flavor is also quite good for a Poncirus F1.

In general look for variation in leaf shape for finding zygotic seedlings.  Trifoliate foliage is a dominant trait so F2s with single blade leaves will be zygotic seedlings.  Here in Maine I just grow them all out and let winter take the non-hardy (basically everything, so I need to give a little protection next time I try this).  So far I've had seedlings that die back to their roots and then regrow, but nothing fully hardy in zone 5.  I keep hoping :)

Very good luck to you!  I'm sure you'll create some great stuff.

Ken W Wilson wrote:
Dan, have you found any uses for the Poncirus?

Nope, but I don’t have my own trees of fruiting age yet, my oldest are about 4. There’s a heavily fruiting tree about 2 miles from my house in a yard full of rednecks and pitbulls, and another one that was in the next county over until the new owners brushogged the field it was in.

I got all my seed from a tree in a public park a few counties south of here.

There’s a bit of a trend in fancy bitters for cocktail mixing; I think the poncirus fruit would be good for that.  But really I am just growing it because it’s exotic and different and pretty and smells good.

Greg Martin wrote:Very cool.  Don't forget that most seedlings of Citrus are clones of the mother plant (nucellar seedlings).  When choosing a mother plant try to use those that set few seeds that are clones.

Some citrus varieties produce more zygotic seeds than others.

Generally most oranges (and citranges) produce almost entirely nucellar seed, so are difficult to breed unless they are the pollen parent. (One notable exception is Temple Orange, which is actually more of a tangor, to be more technically accurate)
Grapefruits produce about 30% zygotic seed, depending on the variety.
Lemon is only about one third nucellar.
Varieties like mandarins, pomelos, and citrons tend to be all zygotic.
Kumquat is zygotic but Calamondin in nucellar. Tangelo is 83-97% nucellar.
Flying Dragon trifoliate is roughly 25-30% zygotic.
Ichang papeda is zygotic, while yuzu is mostly nucellar

(For those who do not know, nucellar = genetic clone, zygotic = formed by sexual recombination of genes)

Dan Boone wrote:Location: Central Oklahoma (zone 7a)
I figure there ought to be something more useful than the poncirus that would grow here, but the only way I'm going to find it is to build a big cold frame for my seedling nursury and some semi-protected outdoor environments (lots and lots of south-facing masonry walls) and then start slaughtering four-year-old trees on an epic scale.  I've been saving kumquat and meyer lemon seeds, but that's as far as I've gotten...

Where you are something like a Citrangequat may be able to survive, or a Citrange cross with something else that has a high degree of cold hardiness but is more edible than citrange. Maybe Citrange x (Satsuma x Yuzu) or (Citrange x Ichang papeda) x Citrumelo
Obviously the colder climate you're in the harder it's going to be to get something very edible.

For zone 6 the only thing that might have any chance of surviving, besides Trifoliate, would be something like Citrange x Trifoliate

I've also seen simple plastic enclosures lined with large water containers (painted black) that can effectively knock the climate zone inside the enclosure up a notch. The enclosures are just big enough to fit one small tree. Up against a South-facing brick wall and a pond in front of it (with large boulders in the pond to help absorb and retain heat) would also help, possibly a terraced mound to provide shelter from the wind. I saw a video where they were able to create a zone 7 to 8 microclimate on a Montana ranch by carefully engineering the modication of a huge ditch that had already been excavated to take out gravel for a the construction of a road, and turning the bottom into a small lake.

I am just south of Seattle between Carr and Case inlets.  I have not tried citrus but I have had success with peaches by putting a portable garage over them to prevent peach leaf curl. I move many of my barrels in with them to overwinter which gives additional mass.  All of my frames are used ones that I have recovered with scaffolding fabric.
 

Would love to have more cold hardy citrus options!

Where can I buy a cold hardy citrus hybrid or seeds? I’m zone 6.  I like to push the limits.

Definitely interesting work.  I had not seen mention of citrofortunella hybrids like Indio Mandarinquat, the rind is amazing and if you let them hang long enough they are sweet.  I think the smaller leaves also helps them when there is less water.
I found it interesting that mandarins, pomelos, and citrons are all zygotic, since all commercial citrus traces back to these three "parents".

Microcitrus are very interesting too, especially since they have resistance to citrus greening.  Apparently Microcitrus spp. can cross with Citrus ichangensis, see the link below.
http://hortsci.ashspublications.org/content/32/3/440.6.short?related-urls=yes&legid=hortsci;32/3/440-e
The hybrids of the two should be really interesting and cold hardy.

I've mentioned this before, but I've seen lemons survive Montana winters. We planted random seeds outdoors in Great Falls MT ca. 1970 (an era of hard winters) and tho they stayed small and didn't bloom, they survived several years that I know of (they were about 3 feet from the house, which lacked a basement so there was little ground-heat effect, and fairly exposed to the weather). And a friend swears she used to eat lemons off an alley tree in Bozeman MT, back in the early 1970s. So I'm thinkin' plant lots of seeds and see which ones survive the winter on their own might be the way to go.

Are there any improved selections of Poncirus? It sounds like the flavor is variable. Just better than most would be helpful to those of us in Zone 6

Meyer is apparently not the most cold hary lemon it is either lisbon or eureka (forgot).
I wonder how you distinguish between the seeds which are equal to the parent in genetic makeup and which are different other than growing them out??
We are at the border for citrus here. And kumuats are definitively cold hardy. But the problem with our citrus generally is not the cold hardiness but the lack of summer heat, which means that the sugars don't develop and the taste is so so only. I often buy the cheap local oranges as a replacement for lemons. Cold hardiness is not the only problem the other one is getting the fruit ripe.

Ken W Wilson wrote:Are there any improved selections of Poncirus? It sounds like the flavor is variable. Just better than most would be helpful to those of us in Zone 6

There's a variety called the swamp lemon that is supposed to taste better.  There's also a variety that flowers after only one year when grown from seed, which might be a great trait for breeding purposes.

Hey there Parker Turtle,

Interesting stuff! I too live in Seattle and have a cold hardy citrus collection of my own. In the ground i have:

Indio Mandarinquat
Owari Satsuma
Kuno Wase Satsuma
Nagami Kumquat
Fukushu Kumquat
Marumi Kumquat
Calamondin
Chinotto Sour Orange
Bloomsweet Grapefruit (kinkoji)
Yuzu
& Poncirus trifoliata

To date, these have all seen 18 degrees unprotected with no damage across the last 3 winters.

If you're ever looking for rootstock, I've got a couple hundred poncirus trifoliata that are just about grafting size! And I'm always on the lookout for new varieties.

You really want to obtain the Honey Changsha citrus. I had a young one take 15F this year with no leaf damage and I had one take around 11F with just leaf damage. The difference between the standard and Honey is all taste. The standard Changsha is almost inedible in my opinion. There is not flavor and seems a bit pithy to me. The Honey is just as good as those Little Cutie citrus you buy in the store. You still have the issue with seeds. I have about 14 trees and one only produces about 8 seeds per fruit where 20 is about normal. You get used to just eating the seeds with the citrus after a while.
There are seedless varieties of Changsha on the market as Artic Frost and Orange Frost. They used radiation on the seeds to create this effect. However, they are over-advertised in cold tolerance and only hardy around to 15F. Since they are on their own rootstock, it isn't a problem if you freeze to the ground because the same tree will come right back.

Good luck!

I would love to find a lemon tree I could put in the ground here in the PNW.  Is there a lemon that can handle zone 8b?

Would it be better to have it facing south with my other fruiting trees, or close to the stone patio (facing east) for heat transference from the house?  I don't have a greenhouse, yet.

My Yuzu limes have survived three days of 10 degree weather with just some minor stem damage.  It is producing lots of good fruit with no freeze damage the last two years.
I use my trifoliate orange to make a household cleaning solution by soaking cut up and squeezed oranges in white vinegar.
I am in Southern Oregon.  Most years we hit 10 degrees in winter but sometimes a bit lower...

Frank Cordeiro wrote:My Yuzu limes have survived three days of 10 degree weather with just some minor stem damage.  It is producing lots of good fruit with no freeze damage the last two years.

Now that's an interesting prospect, both itself and for hybridizing... apparently they're native to Tibet, not exactly a mild climate.
https://en.wikipedia.org/wiki/Yuzu
see also its ancestor, https://en.wikipedia.org/wiki/Ichang_Lemon

Would it be possible to beg some seeds?

Send me your address.  I am just about to pick the old fruit.  The seeds should be ready to sprout.
ivpg@frontiernet.net

Here in Maine I just grow them all out and let winter take the non-hardy (basically everything,

That sounds depressing.

Is "Maine" a city in Oregon?  

Frank Cordeiro wrote:Send me your address.  I am just about to pick the old fruit.  The seeds should be ready to sprout.
ivpg@frontiernet.net

Thank you! Will do.

Mike Jay wrote:Is "Maine" a city in Oregon?  

Nope, Stephen was quoting me!  Surprisingly it's not depressing.  I just expect it to be a one in 5000 goose chase and stay optimistic.

Ok, I'm caught up now.  I'd love a zone 4 citrus but I'm pretty sure that isn't happening.  Once I get my sun traps and microclimates set up, maybe I could get some zone 6 spots around my property.

I always wonder what dictates cold hardiness?  Is it the cold air killing the branches, cold enough frost killing the roots, a combination or something else all together?

I could make a microclimate that keeps the branches a bit warmer but in doing so it would melt the snow on the ground and possibly allow frost to penetrate deeper.  Alternately, I could set up snow traps to keep snow cover and keep the ground above freezing but unless the branches are also under the snow, they would be experiencing full on WI winter.

i live near Seattle, I have several citrus growing, its such a fun hobby. Ive just started to grow some Flying Dragon and soon some yuzu, I hopefully can plant some outdoors for seed stalk. any advice for growing em outside will be duely appreciated. I would love one to be big enough now to plant.

hey parker turtle. i have the same mission with citrus and have some projects going. may i come and visit you and talk shop. i have lots of people that would be willing to buy trees from you.

lets chat

steve
408-930-8810

Matt Hedlund wrote:Hey there Parker Turtle,

Interesting stuff! I too live in Seattle and have a cold hardy citrus collection of my own. In the ground i have:

Indio Mandarinquat
Owari Satsuma
Kuno Wase Satsuma
Nagami Kumquat
Fukushu Kumquat
Marumi Kumquat
Calamondin
Chinotto Sour Orange
Bloomsweet Grapefruit (kinkoji)
Yuzu
& Poncirus trifoliata

To date, these have all seen 18 degrees unprotected with no damage across the last 3 winters.

If you're ever looking for rootstock, I've got a couple hundred poncirus trifoliata that are just about grafting size! And I'm always on the lookout for new varieties.

Hi matt,

Owari Satsuma
Kuno Wase Satsuma
how do their taste differ? where in PNW are you? did they do well in ground last winter? do you protect them with cover or anything?
thank you

My father grew a cold hardy lemon tree from seed many years ago.  He passed away in '98.  This tree looks and smells like a lemon, very thorny, but the large yellow fruits resemble more a grapefruit.  They don't taste like either a grapefruit or lemon however and the longer they stay on the tree, the less bitter they become until they are perfectly palatable right off the tree.  My mother had this tree for many years and it never bore fruit until I started managing it and realized that it must get adequate water when the flowers are blooming.  It has died to the ground twice, once many years ago and again during the last deep freeze here in El Paso.  Both of these freezes were in the neighborhood of 0 degrees F.  Otherwise it endures temps down into the teens, occasionally with minor frost damage.  The tree is approximately 20' tall and presently has about 75 unripe "lemons".  My research indicates this thing is called a Pasadena lemon, a tree I occasionally see for sale here at about twice the price of the next tree.  But I have no idea really.  My wife's grandmother had a cold hardy grapefruit that behaved similarly to this lemon, but the fruit never survived long enough to get ripe until the advent of global warming.  Now they do, or did, as she passed away a few years ago and the house was sold, so don't know how the tree is doing.

Jerry Davis wrote:My father grew a cold hardy lemon tree from seed many years ago.  He passed away in '98.  This tree looks and smells like a lemon, very thorny, but the large yellow fruits resemble more a grapefruit.  They don't taste like either a grapefruit or lemon however and the longer they stay on the tree, the less bitter they become until they are perfectly palatable right off the tree.  My mother had this tree for many years and it never bore fruit until I started managing it and realized that it must get adequate water when the flowers are blooming.  It has died to the ground twice, once many years ago and again during the last deep freeze here in El Paso.  Both of these freezes were in the neighborhood of 0 degrees F.  Otherwise it endures temps down into the teens, occasionally with minor frost damage.  The tree is approximately 20' tall and presently has about 75 unripe "lemons".  My research indicates this thing is called a Pasadena lemon, a tree I occasionally see for sale here at about twice the price of the next tree.  But I have no idea really.  My wife's grandmother had a cold hardy grapefruit that behaved similarly to this lemon, but the fruit never survived long enough to get ripe until the advent of global warming.  Now they do, or did, as she passed away a few years ago and the house was sold, so don't know how the tree is doing.

By El Paso do you mean El Paso, Texas, or a different El Paso? I think Texas has a different climate from the PNW.

Yep, El Paso, Texas.  People are quick to lump us in with Texas.  Stores like Lowe's sell plants specially designated for our area, meaning Texas.  But we are not like the rest of Texas.  We are in the Chihuahuan Desert at an elevation of about 4000'.  Over 100 F in the summer, 10 - 20% humidity year around for the most part, freezing for a month or 2 at night in the winter.  At the moment we are at 4" of rain for the year.  And the soil is very diverse depending on what area you live in.  My present home was previously hardpan desert, most creosote, caliche and dry below 12" or so of soil.  Very difficult growing conditions so I've had a lot of fun, and blown a lot of money, trying to figure out what will make it here and how.  Anyway, part of it is about microhabitats.  The lemon tree is in a northeast nook of the house.  I water it religiously every other day for 1 minute.  That's it.  So, yes, the basic climate is very different, but I am presently also trying out a gingko, pawpaw trees, and a monkey puzzle tree among others.  So far so good.  Also recently got some trifoliate orange.  I've learned a whole lot about various types of plants and still learning day by day.  There's no end to it.

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.

first picture - Kabosu tree, Vancouver, WA, November 3, 2020

second picture - Yuzu seedling, growing on own roots, just over 2 feet tall now, Yashiro Japanese garden, Olympia, WA, January 5, 2021