The Tropical Fruit Forum

Let me first say I am not expert in this specific area, but from what information I have found, I believe that Citrus glauca is only about equal in cold tolerance to Satsuma mandarin. (Even though Satsuma mandarin can be rather cold tolerant in some situations)
This makes me skeptical that Eremo- hybrids with ordinary citrus fruit species could be as cold tolerant as claimed.

I do know that Citrus glauca is very drought tolerant, so that might potentially resist desiccating cold winter winds, and perhaps it enables the root systems to sprout back.

Of course, the person who made the opening post lives in climate zone 8b, so I am sure all sorts of only marginal hardy citrus varieties could survive there. Even a Satsuma mandarin might perhaps survive for him if planted in a protected spot. If he lived in 8a, it would be very much more difficult, and zone 7 nearly impossible.

It should also be noted that there is a big difference between zone 8 in the US South versus zone 8 in Europe. (Given the same climate zone number, I think the South is easier to grow in most years, due to the longer growing season and greater warmth)

I thought that sudachi is significantly less cold hardy than yuzu? Still better than most citrus, but closer to the level of a satsuma?

From my testing 2 hours north of jim VH, it seems like Sudachi has close to the same level of absolute cold tolerance. But Yuzu is a more vigorous grower and can more easily recover from damage. The Sudachi and most of the Yuzu plants finally died after being exposed to a brief temperature drop to 9 °F, even though the rest of the winter wasn't that cold. They had previously survived a quick drop down to between 12 and 14° a previous winter, with only slight to moderate damage.

Two small updates

One of the Yuzu bushes that I thought was killed seems to be sprouting a few tiny leaflets from its base, right above the graft line.

A seedling that I grew from US-852 citrandarin (poncirus x Changsha) seems to be sprouting up from the ground. I had presumed it was killed back to the ground by the winter, but it is possible my gardener carelessly wacked it down. It is planted in a shady colder part of the yard where hardy citrus has not seemed to do well.

It does appear that I see some green at the very base of one of the Ichang papeda plants, close to the ground. I don't know if it will be able to grow out.

If it's that tall, it should be producing blooms by now. My guess is it's a combination of both being a seedling tree (by that I mean growing on its own roots, no dwarfing effect from being on grafted rootstock) and the climate (recovering after the winter making it more reluctant to bloom).

It did not do well for me in the US Pacific Northwest, zone 8a. It was obviously less cold tolerant than Yuzu. It was almost completely killed, but managed to hang on to life for three years, slowly declining until it finally died. After the first winter it lost all its leaves, and I could later see the next two Spring seasons that the plant was trying to bud out leaf growth in two areas, but the plant was just not successful, and so the plant began declining since it was not able to grow out any leaves.

I would guess this might be a variety fit for a protected spot in zone 8b, or might be useful for further breeding purposes.

It might be able to survive in a protected spot in zone 8a in the US South. I don't know.

Why would my 5 ft tall Seville sour orange tree have fruit that won’t ripen after a year and a half?

This is not unusual if the tree is still small, and especially if it is in a Northern climate with a shorter growing season.

I can provide an update. Not good news. Almost none of the hardy citrus seemed to do well this winter. The temperature dipped down to 9 degrees F on New Years Eve, despite the temperature not dropping too low for the rest of the winter. (Although it did dip down to between 16 to 18 in the early morning of February 23 and 25)

The only one that was completely undamaged and looks great was the Changsha mandarin, but that was planted in a more protected location and surrounded by a big bush that probably helped insulate it.

Keraji, with a gallon container of water right next to it and covered with a paper bag, died down to the rootstock.
Why did it survive fine through last winter but was killed this winter? I do not know. Maybe it ran out of energy?

Bloomsweet, which also had a water container next to it and was haphazardly covered with a plastic bag through the worst part of the winter, suffered severe die back. One two little branches down towards the bottom are still alive and green. I am not sure if it will be able to recover.

The Dunstan citrumelo lost nearly all of its leaves, except some towards the very bottom near a branch that was weighed down against the ground by the weight of snow. Those leaves will survive. Most of the branches look green and fine, but there were just a few segments of branches that have turned grey and look like they were very damaged. Fortunately it is a vigorous grower and I am sure it will recover and survive.

Sudachi survived, but only because almost the entire little plant was bent down against the ground and so was burried under the snow and insulated under the winds. It does not look great and looks somewhat tattered but the leaves are still alive.

The two Ichang papeda plants seem to have been very much killed back. Only the base branch very close to the ground is still alive.

Three Yuzu plants, all on grafted rootstock, appear to have finally died. They survived through several winters before, so maybe they gradually ran out of energy and then were not able to recover? It is interesting.

One tiny little Yuzu seedling in the yard close against the ground survived, though it has no leaves.

Another Yuzu seedling, 3 feet tall, that is in another location that is in more of a developed area near the downtown has survived. It lost all its leaves but the branches are still all green and it looks like it will recover just fine. It is growing on its own roots.

The Ichangquat seedling (the one that I thought might be a complex hybrid and be more hardy) was killed down to 3 inches above the ground. The other Ichangquat seedling only has one narrow little branch that is surviving, seems like the other two were killed back, and even that one surviving branch was killed back to only less than 2 inches above the ground, no surviving leaves. I do not know if it will be able to recover.

That tiny little keraji seedling is still alive and has a tiny healthy colored green leaf, but it was completely buried under a layer of mulch and was very close to the ground.

I would recommend saving your time and grow something else. I have gotten them to fruit but they get to the size of a golf ball then dry up.  Fungal issues prevent full maturation.

What if you just put a frame over the tree with a transparent plastic cover during the warmer half of the year? That might prevent rain from falling on the fruit.

I know this is what they do for harvesting cherries in Japan and tomatoes in Korea.

Interesting, I wonder if like kabosu its only sour before ripening?

I have had one picked from the tree that was fully ripe (but grown in a cool short season climate).
They are about as sour as a Meyer lemon, but just a little bit watery, bland, and insipid inside. They might be a little less acidic than a Meyer lemon due to being watery. This was when it had reached a full yellowish-orange color.
You want to pick them when they are still mostly green.

The peel makes a good marmalade at this stage though, when they are orange.

I will say that the inside quality of Kabosu, fully ripe, is definitely better than Yuzu, if you are only focusing on eating the inside. It is a little like Yuzu, but also like a Meyer lemon and even maybe just a little bit like an orange. Not great flavor, but definitely edible inside. Though this is still not something you would buy from the store if you were only wanting it to eat the inside.

Lastly, I will say that the leaves on my Sudachi survived through a zone 8a Pacific Northwest winter, but I think that was only because the trunk was bent down by the weight of the snow causing the entire plant to be buried under the snow. If it had been exposed to the above air, the leaves would have been completely fried and there would have been severe stem damage and branch death.

I do have a small Yuzu tree growing in the same yard space as a large Eureka lemon in climate zone 10. I suppose I could try to do a cross pollination and share some seeds. I highly doubt the offspring could grow in zone 8 though.

I like swingle citrumelo as a lemon substitute too.

Many people say Dunstan citrumelo is not too far from the feel of a lemon, and at its best is almost edible without too much trifoliate taste.
Well, more like a sour grapefruit of course, but it might make a good starting point. Maybe cross it with a normal lemon. I doubt that resulting hybrid would have cold tolerance, so you'd probably have to take a large number of seeds from the resulting fruit of that new hybrid, and see if any of the subsequent offspring have cold tolerance. It would probably take one more generation to bring out all those recessive genes.

I think that may be why developing hardy citrus has not been so easy and many have not succeeded.
If you just take trifoliate and cross it with another normal citrus, and then take that and cross it once more with another normal citrus that has no cold tolerance, very little of the original trifoliate cold tolerance is going to manifest, even under the best chances. I think you would have to do one more additional self-cross after that, to have any chance of strengthening back the cold tolerance genes.

It's clearly obvious that most of the genes for cold tolerance do not appear to be dominant genes.

Another update, February 17, 2022

This is a time when not much is happening in the climate where most northern growers live, but the PNW is kind of a strange climate. Due to the very "mild" (but still cold) winters, all sorts of certain species of plants can be blooming surprisingly late into the early part of winter, or early in the late part of winter (just not during the very coldest part of the winter, which might only last 2 or 3 weeks).

In this picture you can see the Yuzu plant with a Daphne bush right next to it beginning to bloom. (I also saw a large red camellia bush in the neighborhood just beginning to bloom today). On some parts of the plant the leaves are completely fried, other parts nearly completely so, yet on other parts of the plant the leaves seem only moderately damaged. You can still see some leaves on the plant that look green and relatively healthy.

It seems inexplicable, but it does seem noticeable that the longest offshoot of the plant growing the furthest away got almost no leaf burn, when one might expect this to be the branch that would get the most leaf burn, since it is so exposed and furthest away from the canopy of the plant. Two factors may be at play here. First this branch is directly above a stone which may have stored heat, and second, even though it is on the north side it is on the side of the plant that would be less exposed to wind.
The stems of the plant are not damaged at all and still appear very green.