Valentine and Cocktail pomelo cold response

[Pancrazio]

I'm thinking to drop few lines about an experience i made last winter regarding those two plants.
Is hard to fine accurate comparision between two clones because one never knows how they were spaced and especially how much their rootstock/nutrition can vary.
Well, in my case, i guess, i have kept as close as possible two varieties in rigorously the same environment.
I grated both valentine and coctail on same rootstock, at the same height in the same spot, so everithing they got was rigorously equal. Just... i'm not sure what rootstock it is since i reused a failed clementine which i'm not sure about the origin, but i guess it must be citrumelo.



They grew nicely during summer then winter hit.



In the first column you have the average temperature in celsius and in the second the minimum temperature in celsius. As you can see we got till 17F with about two weeks of sub freezing nights (and i mean, not just light frosts).
During all this time both the valentine and the cocktail were outside with just a light frost cloth.
Well somehow they manage to survive with just little dieback and not becoming completely defoliated. No flowering this year but they bounced back.

Here a picture of them just after the frost at the end of january (the right part of the plant is cocktail and the left part is valentine, frost cloth moved up to show leaf damage):



They managed to survive! Pretty beaten but alive. Of course some degree of defoliation was to be expected and so happened (BTW you can see on right bottom corner of the pic a twig of ilya11's voss bloomsweet grapefruit completely unfazed by the cold)
This is them a month later:



The cocktail grapefruit got beaten a little harden than Valentine, even if i have to admit that on a small sized plant like this one is hard to make conclusive statements.
What can be said is that Valentine and Cocktail when dormant are pretty cold tolerant. Also im not completely sure but i suspect that keeping some leaves alive on more exposed twigs may be useful to help the plant in avoiding entire twig dieback. It's just a thought though.
Right now coctail has also a flower while valentine is a little more shy, but they completely recovered.

[tve]

Interesting! I have a 1-year old valentine in the ground and just grafted cocktail, but not on the same tree... -8C is possible here, but very uncommon, I hope the trees grow big before that happens, if it does.
What frost cover do you use, is that spun polyester or plastic? I can't quite tell from the photo. Also, I wonder whether it would help to keep it a little away from the plant?
I did some measurements with a medium-weight spun poly floating row cover draped over a small banana. The result was the same inside temperature as outside, but those measurements were above freezing. On their own these covers do not increase the minimum temperature. There has to be something else that raises the temp and then the cover helps a little to keep that in. I have the feeling it's quite tricky to ensure that there is this "something else" and a tiny tree like you have or the small banana that I was trying to protect just don't have it. For next winter I'll have to read-up on the best way to combine irrigation with cover and stuff like that, or resort to xmas illumination under the covers...  Fortunately we mostly don't really need it, but I'd like to understand it better.

[Ilya11]

The result was the same inside temperature as outside, but those measurements were above freezing. On their own these covers do not increase the minimum temperature. There has to be something else that raises the temp and then the cover helps a little to keep that in

These covers are more efficient in wet conditions, sprinkling them water before the frost helps a lot. Also it is better to put them on the cage around the tree to avoid a direct contact.

[SoCal2warm]

I'm assuming that's in climate zone 8b. Citrus fruits have long been grown in Italy (with some difficultly in Italy's North).
I don't think pomelos would ever survive outside like that in my zone 8a Pacific Northwest climate. My Bloomsweet barely survived (with a cover, in a warm spot up against the south-facing side of the house) and right now it looks like it could go either way, I wouldn't be surprised if it dies. (Although temperatures have warmed up by now so any damage should have fully manifested itself by now, I would think)

Besides, I'm a little bit further North of you, about the same latitude as Northern France.

(I have read of at least two instances of grapefruit/pomelo trees surviving outside in the middle of London, but I looked it up and the temperatures inside London do not dip down as low as they do here)

I'm not that surprised that pomelos could manage to survive in Florence, Italy with a cover over them.
The fact that you appear to be in the middle of a city and they are up against a fence probably helps as well. That fence acts like a windbreak. The concrete bottom of that fence probably helps store a little heat too.

[Pancrazio]

Interesting! I have a 1-year old valentine in the ground and just grafted cocktail, but not on the same tree... -8C is possible here, but very uncommon, I hope the trees grow big before that happens, if it does.
What frost cover do you use, is that spun polyester or plastic? I can't quite tell from the photo. Also, I wonder whether it would help to keep it a little away from the plant?

Spun polyester! It is cheap and id doesn't has the dowsides of plastic which doesn't really allows air circulation.

I did some measurements with a medium-weight spun poly floating row cover draped over a small banana. The result was the same inside temperature as outside, but those measurements were above freezing. On their own these covers do not increase the minimum temperature.

In my understanding they just avoid (reduce) radiative cooling. That sort of cooling that happens when a flat surface is exposed towards the dark cloudless night sky and the entire infrared radiation of the object doesn't get replenished by radiative heating from sun and atmosphere. In this case, flat surfaces exposed towards the sky gets cooler than air (that's the reason why on nights around 32°F you can see cars with frozen roofs even if temperature never got under 32). Covering the plant, you reflect back to the plant part of the infrared radiation, thus keeping the plant at the same temperature of the air, and in several cases this is a net gain.
They are less useful instead in cloudy nights (which incidentally are also warmer on average because of said effect, cloud behave like a giant blanket).

I'm assuming that's in climate zone 8b. Citrus fruits have long been grown in Italy (with some difficultly in Italy's North).
I don't think pomelos would ever survive outside like that in my zone 8a Pacific Northwest climate. My Bloomsweet barely survived (with a cover, in a warm spot up against the south-facing side of the house) and right now it looks like it could go either way, I wouldn't be surprised if it dies. (Although temperatures have warmed up by now so any damage should have fully manifested itself by now, I would think)

Oh. USDA climate zone shouldn't be taken seriously in consideration when describing european climate. Sure, the mathematical calculation still works and as general rule, you can get an idea of what kind of winter you can expect, but they are aimed to tie a plant to a particular climate and as general rule they fail in taking in account the effect of consistent low temperature on some tropical and subtropical plant. This has been a major topic of people on italian forums. I think is important to point this!
Over the years i have found pretty big discrepancies from what you people from US tell us about citrus and when i see from other italian members.Because of that i calculate a usda map zone for italy based on several weather stations, and as you can see i'm between zone 9a and 8b (Florence).

However a lot of the stuff people says it works for usda zone 9a at my place doesn't grow. Like bananas: i have been growing bananas for quite some times and attempted them in ground for several winter but more often than not they die to the corm (dwarf namwah).

Besides, I'm a little bit further North of you, about the same latitude as Northern France.
I'm not that surprised that pomelos could manage to survive in Florence, Italy with a cover over them.
The fact that you appear to be in the middle of a city and they are up against a fence probably helps as well. That fence acts like a windbreak. The concrete bottom of that fence probably helps store a little heat too.

Well, keep in mind that the citrus growing zone in Italy starts around naples, and in Tuscany there isn't even a single plantation of citrus. Definitively citrus has been grown historically all over Italy but northern than Rome, it took quite some effort,and it was reserved to nobles who could afford to heat them in winter in ages when common folk had to endure the frost. Definitively in some places of Italy citrus aren't at home.

[SoCal2warm]

Over the years i have found pretty big discrepancies from what you people from US tell us about citrus and when i see from other italian members.Because of that i calculate a usda map zone for italy based on several weather stations, and as you can see i'm between zone 9a and 8b (Florence).

I have no doubt that is true. The U.S. Pacific Northwest climate is probably closer to the European climate than the climate of the U.S. South is.
I'm not saying you can completely extrapolate how things will perform in Europe based on how they performed in the Pacific Northwest, but it's definitely closer.
Also I think Italy is in a special climate area all its own, there's really nowhere else quite like it.
What I mean is that Northern Italy is already so far north in latitude (Rome would be somewhere between New York and Boston), and it's influenced by the Mediterranean sea, not very much influenced by an actual ocean, yet the climate is still relatively mild.
Europe is a climate zone anomaly compared to other parts of the world, because of all that water and the Gulf Stream current. No other major big civilization developed in the world so far north in latitude.

However a lot of the stuff people says it works for usda zone 9a at my place doesn't grow. Like bananas: i have been growing bananas for quite some times and attempted them in ground for several winter but more often than not they die to the corm (dwarf namwah).

Probably because you do not get enough heat. (Well, these days you do, but the season of high temperatures is not long enough)

That's kind of similar to the American Pacific Northwest region (particularly between about the middle of Oregon and Seattle).