Frost hardy citrus in Poland, zone 6

[Marcin]

The spring is already here, so the extent of frost damage is becoming visible.

This winter went as follows:
-quite mild weather with a short-term drop to -10°C,
-warming in January with temperatures up to +15°C,
-then a prolonged cool period with ground frozen for at least 2-3 weeks and measured low -13°C.

Such conditions proved easy for poncirus and plants similar to it, but difficult for most other varieties. Most plants, except for poncirus, were protected with mounds made of peat, leaves, soil etc., of varying width and height. This for sure helped them survive, but on the other hand helped less than during the previous winter (with lows approx. -19/-20°C), because of the longer duration of frost.

What survived well: All poncirus (Flying dragon, Kinga1, and others), most Poncirus-like seedlings of citrandarin HRS899O/Q
Survived quite well: Poncirus hybrid from Schleipfer, Twisted Flying dragon hybrid (x yuzu/c4s/Morton)
Considerable damage: Yuzu N4, yuzu Cologne small leaf, citrandarin Forner-Alcaide 5, Ichang papeda IVIA F2, some citrandarin HRS899O/Q hybrid seedlings (Sorfo, Mazurek).
Badly damaged – survived or not?: Ichangstar60, citrumquat Maroon, other citrandarin HRS899O/Q hybrid seedlings (Korela, Lotka).

A positive suprise is the good survival of Schleipfer hybrid. While it's clearly of hybrid character, not a Poncirus-like plant, it survived even above the protecting mound.
The poor performanace of Maroon hybrid is a negative suprise. This is probably due to its poor root condition, and because I replanted it last autumn.

D3, one of the Poncirus-like seedlings of citrandarin HRS899O/Q. It survived well above the mound:


Schleipfer twig surviving above the mound:


Yuzu 'Cologne small leaf' surviving only below the mound:


Nagami x Poncirus 'Maroon' badly damaged this time:


I wrote a longer report on my new website, there you can also find more photos:
https://citr.info/reports/winter-2024-25-results/

[mikkel]

Hi Marcin, what variety of Schleipfer do you have? did you get a name or description? (I know, his names are a bit mysterious... )

[Marcin]

I got its budwood as Citroncirus English Large or Citrandarin English Large / US852. I think the leaves are different from the US852 variants I've seen.

[Ilya11]

Hello Marcin,
And what about your PTxYuzu Yolar hybrid? Is it still alive?

[Marcin]

Hi Ilya,
Yolar is alive near the ground. It suffered more than its twisted sibling, but I think it will regrow.

[Marcin]

This year I'm observing pure Poncirus producing flowers on new growth. On Flying dragon I found two twigs with terminal flower buds, and additionally having small flower buds in leaf axils. Another poncirus made one terminal flower. It's suprising for me, I always thought such mode of flowering was the domain of hybrids. Have you observed such flowers on Poncirus?



Yolar hybrid is regrowing, and its twisted sibling (Orla) survived even better. Maybe it's more hardy, or perhaps Yolar was damaged more because I cut it during winter. It should become known after the next winter.



A positive suprise - limequat × procimequat hybrid seedling (Prores4) survived in ground in the greenhouse. It wasn't protected in any other way, I didn't make a mound, it just stayed there as it was.



Longer report: https://citr.info/reports/frost-hardy-citrus-spring-update/

[Ilya11]

In my climate pure PT is very often flowers a second time in summer on the new growth.
Do Yolar and Orla come from FD fruits?

[Marcin]

In my climate pure PT is very often flowers a second time in summer on the new growth.

In my climate it also happens, blooming in July or August in leaf axils of already hardened twigs. But I've never noticed, or never payed attention to flowers occuring directly on new growth of Poncirus.

Do Yolar and Orla come from FD fruits?

Yes, both come from Flying dragon seed.

[Marcin]

This year, many of the twigs on Flying dragon are essentially thornless or have rather short thorns. It seems its thorniness decreases with age/node count.


Another poncirus (not fruiting yet) has produced some monstruous thorns:


Longer report: https://citr.info/reports/flying-dragon-less-thorny-polyploid-vs-diploid-leaf-differences-june-2025-update/

[BorisR]

Hello, Marcin!
A flying Dragon must have thorns. They decorate it. 

Interesting information. I didn't know that a Flying Dragon could lose its thorns.

[Jibro]

Marcin it would be interesting to graft this thornless shoot onto a new rootstock to see if it retains its thornlessness, I haven't seen thornless branches on my oldest FD and trifoliates yet.
I have seen similarly large thorns on one of my trifoliates as well, on a vigorously growing shoot.
I also have one off-type FD seedling with straight, very small thorns. It would be nice to have a thornless trifoliate, although I agree with Boris that FD looks better with thorns.

[mikkel]

Thorniness is genetically determined, in experiments in China thornless plants were produced by inbreeding. This FD may also be suitable for producing thornless seedlings through self-pollination.

[Marcin]

I've seen a photo of Flying dragon with thornless branches on the web, but I don't know how common that is.

I agree with you Boris and Jibro, Flying dragon looks good with thorns - they're its inherent characteristic. Then without thorns it could be called Flying snake

Jibro, is that seedling with small thorns a dwarf or a normal growing one?

Mikkel, it gives variable seedlings so it may be worth trying. I don't remember any entirely thornless seedlings from it, but I didn't pay much attention to it either. There were for sure plants with different degree of crooking, straight ones, and dwarfs.

[Jibro]

The seedling looks like regular trifoliate with short thorns few biggest are 1cm but most of them is around 0,5cm long, it grows more like Flying dragon not vigorous as regular PT but not dwarf either.
When I used FD as a seed parent I found few hybrids with curved thorns looks like it can be inherited, this is photo of FD x Ichang papeda Ivia


Another seedling with curved thorns from cross FDxN1 Tri Voss

[Marcin]

Congrats on getting these twisted hybrids! I think they may also be deciduous, and should be very hardy. My experience has been the same, that hybrids can inherit the twisted growth habit when Flying dragon is used as the seed parent. But when used as the pollen parent, I've never found a twisted seedling - it probably can't be inherited that way.

[BorisR]

Yes, there is an article about it.

https://www.jstage.jst.go.jp/article/jjshs/74/3/74_3_189/_article

[mikkel]

yes that`s the paper I had in mind.  Japan though, not China

[usirius]

Hi Marcin,

Thanks for this detailed report!

It confirms some expectations, but some expectations were also not met.

This may be due in part to the long frost period and perhaps also to the freezing of the top layer of soil. DDid the seedlings have direct sunlight last winter, and was there also a mild period of weather lasting several weeks before the long frost period?

Winter shade as a location or sun protection definitely helps prevent the plants from emerging from dormancy too early. Standard grafting is also recommended, as the temperature higher up is not as cold as at ground level. And in general, the plants should not develop excessively strong shoots, for example, after heavy pruning or overfertilization.

I have one more question: How did the HRS899 O/Q seedling variant 'A3' perform this winter? Had it been exposed to frost unprotected in previous winters? If so, what temperatures did it experience and what kind of damage did it cause? Bythe way, did you give to the seedling 'A3' also a special name?

[Marcin]

Hi Uli,

I think that the most damaging factor this winter was not the lows at approx. -13C, but the long period of soil being frozen. Depending on the location it could be from 2-3 weeks to 4+ weeks of constantly frozen soil. The plants were covered with mounds which also froze, so the bottom stems were subjected to prolonged freezing. The HRS899O/Q seedlings grow in a place which is partially shaded in the winter, and this may be beneficial on the one hand, but on the other hand the soil and mounds may defrost later in that location. But it's not full shade, so above the mounds they were exposed to some sunlight. Additionally, this prolonged cool period occured after a significant warming in the middle of winter.

The seedling A3 (I called it Korela) survived two previous winters, including winter 23/24 with low of approx. -19/-20C. But it used to survive only below the protection level, and then was regrowing. Its positive qualities are pleasant leaf scent, and probably some tolerance of higher pH. This winter killed both inground A3 (Korela) and E4 (Lotka), while E2 (Sorfo) survived at ground level and is regrowing now.
Because they are hybrids of HRS899O/Q with some other citrus likely not related to Poncirus, they're definitely not zone 6 hardy - perhaps Lotka and Korela are closer to zone 8 hardiness and Sorfo maybe on the border of 7b/8a. But they might be valuable for the potentially better fruit quality. I have their copies grafted in pots, and maybe some day they they'll fruit.

Meanwhile, the Poncirus-type seedlings of 899O/Q have survived, also above the protection mounds. They got some bark cracks, but not serious and not harmful to them in a visible way. They may be nucellar variants, and/or self-polinated O/Q. Hardiness would be at least to zone 7 and maybe they'll thrive here in zone 6 too. I have five such seedlings in ground, each looks a bit different and one (C2) is apparently tetraploid:


Another one looking diploid (C1):

[Skandiberg]

Marcin,

Thanks for all the useful information.