Test of cold hardy citrus in winter 2022
After successfully overwintering some hardy citrus varieties in an unheated greenhouse in the winter of 2021, I decided to try these proven hardiest citrus varieties outdoors in the following winter of 2022 to test their true limits in the climate of central Europe, roughly matching USDA zone 6B. In the greenhouse, I continued to test some new varieties, which surprised positively and ranked among the best candidates for planting in open soil in an unheated greenhouse…
Conditions of my Cold Hardy Citrus Trials
One group of citrus was left in buried containers in the soil all winter in an unheated greenhouse covered with 4mm polycarbonate, a more detailed description of which is given in previous frost tolerance tests, these citrus were in the greenhouse without additional protection.
The other hardiest varieties were in containers buried in the ground on the west side of this greenhouse, see photo, covered with several layers of non-woven fabric during December, later completely without protection.
Winter temperatures 2022 – 2023
You can see the temperature trend in winter 2022- 2023 on the temperature graph of the probe located 20 km from the test site, at my place the temperatures are usually about 2°C higher. As you can see the first major frost came in December when temperatures briefly dropped to -10°C, followed by a fairly mild January, but the worst period came in early February when it was continuously freezing for 7 days and temperatures dropped to -10°C (14F) for 5 days, the minimum for the whole winter was -12°C (10.4F).
Cold hardy citrus in the greenhouse without additional protection
The varieties tested in the greenhouse are marked in the photo taken on December 18, 2022 after a night minimum of -4°C (24.8F).
Sudachi
It is a variety from the group of Japanese acid-fruited citrus, my plant comes from seeds bought from the USA, it grows very slowly and bushy on its roots, much more vigorously on the trifoliate, but I have not seen flowers yet. Sudachi is claimed to have a lower frost tolerance than Yuzu, which has not done well in my greenhouse in previous trials (only Yuzu from the USA has been tested), but growers from Switzerland and Canada have successfully tested Sudachi with lows of -12°C without major damage.
Eremorange
It is a descendant of the Australian wild citrus species Eremocitrus glauca, which is believed to have been pollinated by a variety of orange in USDA Indio Date Station. Eremocitrus glauca is called Australian Desert Lime in English and grows in the harsh conditions of the Australian semi-desert areas where it can withstand large temperature fluctuations, even below freezing, and prolonged drought, and fortunately these resistance genes are also dominant in the hybrid offspring. Eremorange should theoretically be able to withstand -12°C without major damage, and there are reports of its survival at -14°C and 5 days of continuous frost.
In the greenhouse, the minimum of -9.8°C (14.4F) was measured on February 7, 2023, the condition of the plants immediately after this frosty night is shown in the photo gallery below…
Condition of plants tested inside the greenhouse on March 3, 2023
1. No damage
Eremorange and the possible hybrid FD „VS“ x Citrumelo 5 Star had no frost damage, but both had a white fungal rot in the middle of one shoot, probably due to high humidity in the greenhouse and possibly also to honeydew secreted by the Scale insects in that area from where it started to spread, after cutting off the affected shoots this fungal infection did not appear again.
Eremorange thus overwintered in the best condition of all the plants and its distinct stiff, narrow leaves are probably better able to withstand the winter physiological drying of the leaves caused by increased evaporation on clear days by the sun, just as the leaves are also dried by frost or wind, especially in combination with frozen soil, when the plant cannot take up enough water to replace the evaporation losses and this is what causes the winter drying and leaf drop.
Eremorange inherited this resistance from the desert Eremocitrus glauca and it would be good to use it in future crosses, it seems to be similarly effective in our winter conditions.
Sudachi
Apart from a few minor leaf drops, Sudachi overwintered in good condition with no damage and the leaves were in the best condition of the common citrus tested, I therefore chose this variety for planting in soil in an unheated greenhouse, it looks very promising for my conditions.
2. Minor leaf damage
Citrumelo 5 Star hybrid seedling with narrow leaves
This seedling survived the winter with more dried leaves and one damaged shoot tip, so quite moderate damage, but given Citrumelo’s supposed theoretical higher frost tolerance compared to Sudachi or Eremorange, it is rather a slight disappointment and it is clear that the large temperature fluctuations in March, increased by the greenhouse, are not good for some citrus, including this hybrid.
3. Greater loss of leaves and shoots
Clem-Yuz 2-2 and SP Urban F2
These two varieties have suffered similar damage with loss of more leaves and some shoot tips, with a minimum of -9,8°C probably already close to the limit where more significant damage starts, or they are slightly less able to tolerate the large temperature differences between day and night in spring when the sun heats the greenhouse to 35°C (95F) but it can still be freezing at night.
N1 Tri Voss F2
These seedlings repeatedly fail in an unheated greenhouse and have extensive leaf damage and some also have damaged shoots, this may be due to temperature fluctuations or possibly due to the hybrid origin, where the pollen donor may have been a common citrus and thus reduced the frost tolerance of these N1 Tri Voss F2 seedlings. Nevertheless, one seedling survived the winter without damage, and I have chosen that one for further testing outdoors the following winter.
Cold hardy citrus tested outside
I have tested at least in theory the most cold hardy citrus in the past 2 winters in an unheated greenhouse where most of them overwintered without damage, so I decided to test these varieties outdoors to determine their actual limits in my climate. The citrus were buried in containers into the ground and the surrounding area was mulched with leaves. I covered them with several layers of non-woven white fabric in December, but due to relatively mild temperatures and no visible damage, I removed this fabric in late December and the plants remained unprotected for the rest of the winter and spring.
Citrus varieties in the test
6 varieties grafted onto one Flying Dragon rootstock: Ichangstar 60, Citrumelo Swingle F2, Sunki x Poncirus, Dragon Lime, Ichangquat and Cicitrumelo 17-01 (Citrumelo 5* x Flying Dragon).
Ventura Lemandarin grafted on FD rootstock, another Sunki x Poncirus grafted on FD, Yuzu seedling from the USA that did not do well in the greenhouse and dwarf, poorly growing F2 seedlings Citrumelo 5 Star and N1 Tri Voss.
On December 6, 2022 the citrus still looked fine and the whole experiment started out promisingly…
All the citrus trees were together on about 1 square meter near the western wall of the greenhouse, in this place the wind blows quite often from the west, it is protected from the north by a building and the eastern side is partially protected by the greenhouse, photo taken on December 10, 2022, when the tested citrus trees were protected by several layers of non-woven white fabric until end of December…
February 7, 2023 after a frosty night with a minimum of -12°C ( 10.4F )without any protection, the damage to the citrus trees is already visible, how much will become apparent later in March…
On March 20, 2023, I removed the citrus from the ground and began to determine the extent of the damage…
1. Large loss of leaves and immature shoots on FD rootstock grafted with 6 varieties
Cicitrumelo 17-01
Cicitrumelo 17-01 (Citrumelo 5* x Flying Dragon) retained a few undamaged leaves, but most of them fell off, as well as 2 relatively long immature shoots that grew late in the summer and did not have time to harden off before winter, but the same damage to immature shoots is also shown by poncirus, so no conclusions can be drawn for the resistance of this new variety.
However, the shoots have retained a healthy green colour similar to poncirus and due to the relatively mild winter it is not yet possible to verify whether this hybrid with 75% poncirus is noticeably more resistant than the 50% poncirus hybrids, for this a hard winter with at least -17 °C or a long-term test by grafting onto outdoor poncirus planted in open soil will be needed.
Citrumelo Swingle F2
The smaller branch of this Citrumelo from Adavo made it through the winter quite well and kept a few leaves intact, so basically this Citrumelo met the expectations for a 50% poncirus hybrid.
Ichangquat
A pleasant surprise was this hybrid, which has no poncirus genes, as it was a cross between Ichang papeda and Kumquat, so it probably combined well the frost hardiness of Ichang with the deep dormancy of Kumquat. Again, a few leaves on a short shoot survived without damage, so Ichangquat overwintered similarly well as the previous two varieties, but the question is how it would have coped with even lower lows below -15°C (5F) in comparison to poncirus hybrids…
Ichangstar 60
Almost all leaves and the ends of some shoots were lost in this hybrid from a Citrumelo 5* x Ichang papeda cross that combines 50% Ichang papeda with 25% Poncirus and 25% Grapefruit, so even just 25% Poncirus genes combined with 50% Ichang provided similar resistance to the 50% and 75% Poncirus hybrids in this test.
Ichangstar also overgrew all the other grafted varieties, so it had the disadvantage of greater distance from the soil surface, which may have resulted in the loss of all the leaves at the top, as a few leaves did survive on the lower branches of Ichangstar.
Sunki x Poncirus
It is kind of the Schrodinger’s cat of this test because it both survived and did not survive the winter of 2022… the small weak branch grafted onto FD with the others survived, although it lost all its leaves, but the other vigorously growing SunkixPT grafted onto the weaker poncirus rootstock eventually died completely up to the grafting point and only the rootstock grew out.
A possible explanation is the additional inclusion of this grafted PT to the tested group, when I brought it from a cold wintering site in December, but still it did not undergo the same „frosty“ acclimatization as the other varieties…
2. Failed the frost resistance test
Dragon Lime F2
The only variety of the 6 grafted on one FD that completely died to the point of grafting is this 50% hybrid with FD, Dragon Lime, although my plant comes from Dragon Lime seed from the USA, so further pollination of Dragon Lime with another variety or a different combination of genes after self-pollination of the original Dragon Lime mother plant in the F2 generation is not out of the question.
So this result is true for my particular Dragon Lime F2 seedling and I can only hope that with reduced hardiness it will at least get a better flavour against the original variety, but I will find out when it fruits, for now it is just flowering and has not yet set fruit.
Yuzu from USA
The same applies to this Yuzu as to Dragon Lime, again it comes from seeds from the USA, so pollination by another variety is not excluded and it may be a hybrid Yuzu, so the result of this test cannot be applied to other Yuzu varieties available in Europe, I will test them in the next years.
Anyway, this Yuzu seedling showed some damage already in the greenhouse in the last 2 winters, so it is not a big surprise that it did not cope with the outdoor conditions and died completely, it did not even grow from the roots and I am definitely done with this Yuzu, as you can see it is not a big shame.
Ventura Lemandarin
Another hybrid without poncirus genes was created by accidental pollination of a relatively hardy Citrus taiwanica (down to -15°C = 5F) with a very likely mandarin, possibly a frost-resistant Keraji. The plant tested was grafted onto FD, the graft came again from my seedlings originally from the USA, but they all look the same, so they are probably nucellar seedlings, i.e. true clones of the parent variety.
Ventura Lemandarin survived -14°C (7F) in the USA with damage, in my case it had no problems overwintering in an unheated greenhouse for the past 2 winters, so I would expect it should be able to cope outdoors at a similar level to 50% poncirus hybrids. Unfortunately this was not the case and Ventura Lemandarin died completely to the grafting point and only the FD rootstock sprouted in the spring.
Ventura Lemandarin thus does not reach the same resistance as Citrumelo, Ichangquat and others that survived the test in our conditions of prolonged frosts with a minimum of -12°C, its resistance is probably noticeably lower, how much, I will find out in the next years.
Hybrid seedlings N1 Tri Voss F2
These poorly growing dwarf seedlings were a write off anyway, so I at least tried using frost to find the hardiest individuals, unfortunately this failed and they all died. Again, this does not reflect the hardiness of the original N1 Tri Voss variety, which I will have to finally test in future years.
Citrumelo 5 Star F2 hybrid seedlings
The same is true for these Citrumelo seedlings, except that a few survived and I am testing them now in this winter 2023, but they are already showing a lot of damage and loss of all leaves, so their increased resistance will not be proven in the next winter.
Satsuma mandarins overwintered in a polystyrene cover inside a greenhouse without heating.
In the greenhouse I tested the possibility of overwintering relatively hardy Satsuma mandarins in a polystyrene cover without additional heating. Both are in 20+ litre pots so I had to dig quite deep into the ground to fit them into the 50cm high polystyrene enclosure, dumping the soil behind the polystyrene walls, secured only by indented wooden slats and one side brace.
Traditionally I built this enclosure at the last minute before the threat of frost in December, so it was assembled very quickly and is somewhat fiddly, but it lasted all winter without any problems, and the enclosure was dismantled just as quickly in the spring.
Experience with polystyrene cover
As a last minute makeshift protection it served well and fulfilled its purpose, the shortcoming was poor sealing of the ceiling, due to the unevenness it created gaps and some of the uppermost branches lost a few leaves through frost, otherwise overall it kept temperatures above freezing for most of the winter.
Another disadvantage is the high humidity in this closed enclosure, which is conducive to the development of fungal diseases, and the noticeable black coating of mould that lives on the honeydew of the Scale insects, so that unless the greenhouse is freezing, the ceiling needs to be removed and ventilated frequently.
The Skorospelyj mandarin survived the winter well except for the top few leaves, but stagnated the following season with only 3 fruits, which is unusual for this plant, so far it has had a good crop every year
The other variety of satsuma mandarin Kalinin 5 is a poor producer and has alternate fruiting, so I grafted scions of Ventura Lemandarin, Aber’s Narrow leaf, Lalangha into the crown to speed up flowering of these seedlings and all these varieties overwintered without problem, Kalinin 5 also had a decent crop the following season.
Overall, this method of protection in a polystyrene cover inside an unheated greenhouse can be recommended, it just needs to be ventilated as often as possible, so it is not maintenance free and better to solve the joints between the boards for example by inserting them into a tight wooden frame and further compressing them, the gaps in the ceiling can also be solved by further covering with many layers of non-woven fabric.
Summary
Growing even the hardiest cold hardy citrus hybrids outdoors without protection in the climate of the Czech Republic is not very promising and every winter with a week of continuous frosts, even with a minimum of -12°C, they will probably lose all their leaves and some of their young shoots, so it does not make much sense in the long run. However, some varieties with additional protection against the wind and probably winter sun might be able to make it somewhere sheltered, the question is whether the fruit would ripen outside in our short growing season.
The only solution is to achieve a similar resistance to poncirus in hybrids, which I will try to do by backcrossing selected hybrids with the most resistance and acceptable flavour with selected poncirus with more edible fruit.
It is said that a greenhouse will create the same climatic conditions as 500 km south and the excellent condition of some citrus repeatedly proves that in an unheated greenhouse these varieties could already be grown successfully, so this winter 2023-24 I am testing Eremorange and Sudachi already planted in the ground and so far they look good again without damage as described in this article from the test of winter 2022-23.
This year I want to finally graft a lot more varieties onto small poncirus and test them in future winters to possibly select more suitable varieties for my conditions.
In a polystyrene enclosure inside the greenhouse, most of the hardier citrus such as the Satsuma mandarins could be grown, but in future winters I want to try the even hardier Changsha, Keraji, Bendizao and actually more Satsumas, if they can handle the winters as well as the Sudachi, maybe even a polystyrene cover wouldn’t be necessary, or I want to try replacing it with a thick geotextile, which is more breathable after all, so there is still a lot to test and try in the future.
Translated with www.DeepL.com/Translator (free version)
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