Assessing the damage
This is a report on the extent of frost damage to inground citrus growing in Poland, frost hardiness zone 6.
The problem of this winter was not the lows, but rather the duration of frost. Also a warm period in January wasn’t beneficial. During that warimng the temperatures raised up to +15°C. It was followed by a cooling with the low of approximately -13°C, and at least 2-3 weeks of frozen soil. The plants other than poncirus were protected by mounds made of peat, leaves, soil etc., of various width and height.
Such conditions proved easy for poncirus and plants similar to it, but difficult for most other varieties. Some of the plants tested this winter are listed below:
Edited May 2025.
Survived:
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) ‘Orla’.Survived, but with considerable damage:
Yuzu N4, yuzu Cologne small leaf, citrandarin Forner-Alcaide 5, Ichang papeda IVIA F2, citrumquat Maroon, Non-twisted Flying dragon hybrid (probably with yuzu) ‘Yolar’, two citrandarin HRS899O/Q hybrid seedlings: Sorfo, I4.No new growth until May 2025, probably didn’t survive:
Other citrandarin HRS899O/Q hybrid seedlings, Ichangstar60.
Discussion
It was another opportunity to test winter performance of hardy citrus varieties. Many of them are out of their zone here, and I don’t expect them to grow big or produce fruit, because every year they are severely damaged. Still, with mound protection, they somehow thrive here (for instance yuzu N4, for which it was its third winter in ground).
I think they are not only fun to grow, but can be also used as reference points for estimating the hardiness of other varieties, particularly in breeding. “Hardier than …”, “Less hardy than …” often means more than just knowing what was the lowest temperature.
A positive suprise is the good survival of Poncirus hybrid from Schleipfer. Its origins are mysterious for me, but it might be related to citrandarin US852, and this would explain its high hardiness. The seedlings of citrandarin HRS899O/Q ,which resemble Poncirus phenotypically, are probably the result of self-pollination or nucellar ones. They can be expected to be very hardy, such as their mother plant.
The hybrid seedlings of HRS899O/Q (Sorfo, Lotka, Mazurek, Korela) are a different matter. They are clearly hybrids based on appearance, leaf scent and vigor. New Zealand Lemonade is one of the possible pollen parents. Because those seedlings are less than 50% poncirus, lower hardiness is predictable, but also better fruits can be expected. They were selected out of ~20 such seedlings, on the basis of surviving two previous winters in ground, always with some damage, but regrowing well during the season.
A negative suprise is the poor performanace of Maroon hybrid, which survived three previous winters with less damage. This was probably due to poor root condition, and because I replanted it last autumn. Also Ichangstar60 and IVIA F2 were planted only last autumn, which may have impacted the result (but so was planted the multivariety tree, and at least Schleipfer hybrid survived well).
Photos
