Thomasville is beside keraji my favorite citrus in my garden. If you can find a way to keep the fruits alive through winter, they turn out to very juicy and sweet orange like fruits with a good amount of kumquat in it. So don’t waste the peel eat it as a whole. Delicious! Without any poncirus off flavors.
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Really good points, the dose makes the poison. Nothing is exact, I’m sure a mango wouldn’t die to 9* if it was only for a few seconds and the adjacent seconds were well above freezing (I know that isn’t possible just saying). It’s hard to have a scale that shows what temp starts damage and at what temp things are safe. I’ve had spring storms kill all of my new tender growth when the temps were above 50. That cold constant wind with chills into the 40s on new growth for an extended amount of time definitely singes it back. Now I don’t remove my incandescent lights off of my citrus all year so if need be I can simply plug them in rather than trying to scramble to string them all up on 14 trees (3 I don’t ever protect).
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What kinds of circumstances would 30 kill a trifoliate? And when you say that, do you mean fully dead, even the roots, of a mature tree?
Not under natural circumstances, but like if a trifoliate in a state of no dormancy, say in a heated greenhouse, were then exposed to a prolonged light freeze, I doubt it would survive.
So for example, last winter Dunstan, which can take pretty close to 0 F, was severely damaged and lost several inches of branches despite being covered with cardboard and a tarp, during an upper twenties frost. It was one of the first frosts of the year, and that particular plant had not gone into any dormancy at all, even pushing new growth a few weeks prior. That same frost split the bark on several of my figs and knocked them back to the ground, and while I don’t know if it was that frost or another one, but one of my mulberries and one of my jujubes were both killed this winter down to a few inches above the graft line.
I don’t remember which talk it was, but Mark Weathington, who directs the JC Raulston Arboretum in Raleigh, was one time talking about how for some sub-tropical plants, they have better winter survival than his colleges along the Gulf Coast who are a full zone warmer. Dormancy, or lack thereof, being the issue.
I suspect this is way more of a problem down here than on the West Coast, though. Our winters are just a lot warmer and sunnier for a given zone than compared to along the Pacific. And we get a lot more sudden changes in temperature. I personally have seen a 90 F high followed by snow flurries a week a later…
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The only thing I can think of is if it’s actively growing with new growth.
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Interesting, my Dunstan was severely injured, perhaps killed, by 15°F during our 6 day freeze in January. It seemed sufficiently dormant, but was pretty small, about 18 months since rooting on its own roots. It still looks a little green at the base, but no sign of new growth yet this spring. My nearby yuzu seedling, similarly exposed and in a similar level of dormancy, defoliated without any stem damage and is already pushing a strong flush now. It made me wonder if Dunstan is maybe less hardy than advertised. Here’s what mine looks like now:
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If I remember correctly, Ilya on TTF doesn’t think Dunstan is very hardy. But he’s in France, which is more maritime and cool summer.
That being said, the one in Raleigh has low single digit temperatures at least twice. I have two other Dunstan plants in a fully exposed spot that I didn’t protect at all and they made it through the winter without losing a single leaf–but they had not put on any growth in months and so were much less vulnerable.
My guess would be the grapefruit ancestry of Dunstan making it need really good summer heat for full hardiness.
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‘Dunstan’ is definitely more hardy than yuzu if you’re comparing trees of similar size. My three ‘Dunstan’ trees get a bit of twig die back each year (was particularly bad this year), but my yuzus already gave up the prior winter which wasn’t as bad as this one.
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That’s very interesting. I was comparing trees of a similar age/size, planted in a similar location, but the yuzu was slightly larger and was a seedling rather than a rooted cutting, so had grown with greater vigor and likely has a much larger root mass.
The yuzu seedling was a bit over 2 years old from seed vs 18 months from rooting for the Dunstan. The caliper of the yuzu is maybe 50% larger, and it was about 3x taller (3 ft vs 1 ft). No stem damage at all on the yuzu, even the tips of the thinnest twigs, while Dunstan showed significant blistering on all green stems immediately following the freeze, and died back to near ground level, as you can see above.
Here’s a terminal bud on the yuzu today, much thinner than the thinnest Dunstan terminal bud was, and the stem doesn’t seem to have any cracking or blistering from the freeze:
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I notice ‘Dunstan’ tends to put on a lot of growth late in the season. That’s what usually gets damaged over winter. The more mature hardened off portions seem to be hardier though. I’ve lost a total of four out of four yuzus over the years, but have not lost any of the three ‘Dunstans’ I’ve planted.
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I guess I’ll give it another try if this one fails to wake up, but next time I’ll wait another year or two before planting it out. Are yours grafted on trifoliate, something else, or on their own roots? Mine stopped growing back in September and the leaves seemed fully hardened, but obviously the stems were still green and barely lignified.
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My ‘Dunstan’ are all own root seedlings. I had planted three so that statistically I’d be likely to have true to type ‘Dunstan’ clones even if one of them ends up being a slightly different version from a zygotic seed. So far all three appear identical, but have not fruited yet.
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I just got yuzu and so far it’s my only citrus vigorously growing right now in our cool weather hovering around the low to mid 50s.
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One of my Prague grafts from almost 18 months ago is showing a hint of waking up finally? Patience is a virtue, they say. The tree this is on has been plagued by spider mites, so I pruned off everything except this graft, sprayed it all down with a strong mist, and then put some duct tape around the stem just below the graft, sticky side out, hoping to keep the mites off this graft long enough for it to grow a little at least.
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I decided to make a kumquat hybrid Frankentree, and had some success. Nippon orangequat and Excalibur red lime grafts took. Also later tried a few clippings from my UGA Ichang. We’ll see how they do…
Also did some sweet grafts on my UGA changsha. Bloomsweet took and is pushing. Miho failed 6 times, so must not be compatible. 
Added a browns select to try and get an earlier variety on there as well…
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Just got a bundle of Prague scions from someone on TFF and grafted over this trifoliate seedling that’s been in the ground here for a couple years:
One of the grafts is obscured, but I did two clefts and two bark grafts. I tried to prune off all the thorns first, but missed a few so now my hands have some fresh puncture wounds.
I also got a bundle of Taitri cuttings that I’m going to try to root, but first I’m giving them a thorough rehydration:
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I just got a Thomasville from Madison citrus. They still have some stock left if anyone is interested.
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Thomasville is a crazy little one for sure. Not a whole lot of vigor–eveything goes into flowers and fruits. I can certainly see the kumquat parentage.
Dunstan continues to impress. Some of the new growth in the current flush is a foot and a half long. Three year old tree on its own roots.
I grafted some mature Dunstan to my young Dunstan, we’ll see if it flowers next year.
Six month old Citrus taiwanica. It seems to be another vigorous one.
My ichang papeda grafts had good takes, two of three.
US-802 is insane. There was a discussion on the TFF and the consensus was no one knew what it tasted like, so I decided to let the rootstock on my Satsuma (planted last spring) send up a waterspout. Holy smokes! Six feet of new grown already, and that despite no rain since May.
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I got my Thomasville Citrangequat from Stan SC and I planted it right away. It went straight into making fruit. I decided to remove 5 fruits from my baby tree today. Grow up before you start having kids! 
John S
PDX OR
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