The Tropical Fruit Forum

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.

The true lemon flavor comes from the original citrus species citron. But I think a little bit of the fragrance of lemon also comes from pomelo as well. The juiciness of lemon does not come (or barely comes) from citron, but rather pomelo and/or mandarin. The proportion of mandarin cannot be too high otherwise the flavor will be too "orange" rather than distinct lemon.

Ichang papeda has a flavor that is half similar to citron, it does not have the "orange" component. (Actually I would describe it as half citron and half Kaffir lime) But the flavor of Ichang papeda is still a little bit inferior to citron, I would say. So to develop a high quality hardy lemon, I think perhaps citron should be bred into there. Unfortunately citron has very little hardiness.

I have had the opportunity to taste citron, both Ichang papeda and Yuzu picked fresh off the tree. And of course I have tasted pomelo. (I have not actually had the opportunity to taste Ichang lemon though)

You could of course try crossing a regular lemon with something else, but my thought was that if you began with a citron rather than a lemon, the resulting hybrid would retain a more distinctive citron/lemon flavor, since the citron ancestry has not been diluted as much.

A direct cross between a lemon and anything else that is really lemon-like, probably is not going to be cold tolerant enough. I think at least two subsequent hybridizations will be required.

I am not sure about Sunquat. However, none of the kumquats come true from seed except Nagami.  Therefore, I would surmise the hybrid Sunquat does not come true.

Except that Sunquat is a kumquat hybrid with Satsuma mandarin. Satsuma mandarins have about 90% nucellar seed (exact genetic clones of fruit parent).
Furthermore, hybrids between different original citrus species which each produce zygotic seed tend to then produce nucellar seed.

I cannot say for sure, but there is good reason to suspect Sunquat might not produce a very big fraction of zygotic seeds. (zygotic meaning coming about as a result of sexual recombination)

(a seed being monoembryonic is usually a strong indicator that the seed is probably zygotic, but this is not always so)

Citron does have a more "pure lemon" fragrance than lemon, in my opinion.
What I mean is that if there was a spectrum between lemon and mandarin, citron would be even further away, slightly past lemon, from mandarin.
The fragrance of citron is nearly the same as lemon, but just a little more "pure". When you smell citron, you realize that lemon has maybe a hint of mandarin/orange smell in it after all.

I would also say that citrons may be somewhat slightly more fragrant than lemon as well, though I could be wrong about this.
(It is a cleaner, more ethereal and less substantial smell in quality though, so actual intensity may not be quite the same thing as perception)

Those who are less discerning and do not pay close attention might say the two smell exactly the same.

I have been growing the rare Passe Crassane pear. It produced two fruits, two different years. The fruits grew to a decent size, but unfortunately both times the fruits failed to fully mature to ripeness, even after being picked and sitting in storage for 2 months. (The fruits eventually dropped off if not picked) The tree might still be a little immature. Chill hours do not seem to be too much of an issue, even though this is in climate zone 10 (on the border between 10b/10a).
I have speculated that since the variety Passe Crassane is parthenocarpic, chill hours are not as much of an issue.

Satsuma mandarins are another one, but it is harder to breed, since about 90 percent of the seeds are nucellar (genetic clones of fruit parent).

There are three different types of seedlessness in citrus varieties.
Some mandarins and tangerines are parthenocarpic, which means they produce seedless fruits so long as the flowers were not pollinated by some different variety of citrus.
This includes Eureka and Villafranca lemons. Most varieties of navel oranges are at least somewhat parthenocarpic, while Marsh grapefruit and Hamlin orange are more so.
Bear in mind that, to be more precise, the trait of parthenocarpy has to actually be combined with self-incompatibility or male sterility to get seedless fruits.
(self-incompatibility means no seeds are formed, and usually no fruits in non-parthenocarpic varieties, if the flowers are pollinated from the same variety with identical genetic markers. whereas male sterility means the flowers produce little or no pollen)

So as you can see, it is a little complicated.

Fresh Yuzu seeds are now available again. Ask within the next 14 days (starting January 17, 2022).