Citrus > Cold Hardy Citrus

Accuracy of cold hardiness temperatures?

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Isaac-1:
Over the last few weeks thanks to google  I have been reading scans of a variety of older 20th century scholarly works regarding growing citrus, and one of the things I noticed was a significant difference in reports of cold hardiness for various common citrus cultivars.  Not so much the absolute temperature values, but instead their relatively sequence  from most to least cold hardy.    One in particular that I noted, several of these mid 20th century works list Meyer's lemon as being almost as cold hardy as Owari Satsuma, which matches my observation of many people near where I live on the 8b/9a line having mature inground Meyers lemons growing in their yards that get no formal freeze protection yet continue to survive.

All of which leads me to ask, does anyone know of any modern detailed studies on cold hardiness of common citrus cultivars, I know there are many guides published by universities that list the often repeated numbers, but I have to wonder if that is all they really are, since based on comments on this and other web forums there seems to be a dependency in real world observations on cold tolerance of certain types of citrus, particularly Meyers lemons, and certain types of Grapefruit.

Millet:
At best the information on temperatures that a citrus cultivar can survive are general estimations.  Many, many variables contribute to the survive-ability during a cold spell.  Such as----the temperature just before the cold spell, the water content in the root zone, the age of the tree, the thickness of the trunk and branches, the wind, the length of the freeze, the health of the tree, the root stock, and the particular location the tree is growing in.    Grapefruit is generally listed as one of the more tender varieties, but one hears stories of the Dunstan grapefruit, which in reality is not an actual grapefruit, but rather a Citrumelo.

Walt:
Ilya11 had a related post on another thread a week or two ago.  It got me to thinking, and I've spent hours studying research papers on cold hardiness of apples and plums.  Similar papers came up on pears, peaches, and apricots when I searched on google.
So there is an amazing amount of information that  might matter.  Much of it rather discouraging to me, as in breeding hardier citrus, I need to be able to sort out those in each generation that are a degree or two more hardy.
First the good news.  My idea of testing cold hardiness of scion wood works with apples.  Good.
Now the bad news.  For such a test to work, the trees the cuttings are taken from must be from the same location.  And we all know that microclimates can vary even in small areas.  Fertility must be good in the ground where every tree was grown.  Rootstocks need to be the same.  If the cuttings were taken from own-root plants, then root differences may effect twig cuttings.
One study treated freeze damage and cold damage seperately.  Freeze damage is damage from sap freezing in the xylem.  Cold damage is damage done though the sap in the xylem hasn't frozen.  Spring and fall damage is often cold damage, not freeze damage.
I would like studies to be made of cuttings, scionwood-like cuttings, of different varieties and different species of citrus from different locations, tested different times during spring, winter, and fall.  I think the resulting data would be quite useful.  A little dorm-type refridgerater would be enough to test twigs of several trees at once.  I think a couple of days might be enough to test freeze damage at a given time and place.
Testing for tolerance to quick drops in temperature, then the temperature going right back up, like single overnight freezes, will be harder to sort out.

Walt:
Last winter many of pure P. trifolata seedlings died to the ground over the winter.  These seedlings had been planted in pots as seeds as soon as the fruit were ripe.  They were grown in a greenhouse over the first winter, then planted in the ground as soon as last frost date.
I had treated seedlings the same every year for 4 years, building up a supply of rootstocks, for 4 years with no problems so I blamed drought stress, not cold.  But since they had survived underground and came back up, I decided it was cold and drought stress combined, not one or the other.  Such little things can affect winter hardiness I think.
All seeds were from the tree and I think they are all geneticly identical.

So this year I am irrigating over the winter.  And yesterday I was out carrying water to the seedlings, since the summer automatic irrigation is shut off for the winter.  I noticed 3 out of 94 seedlings still had green, healthy leaves.  I have had over a week below freezing, dropping to -15C during 2 nights.  And green leaves still on my P.t?  The cold had broken and the last 5 days have been quite warm, dropping below freezing most nights but just barely freezing.  Freeze damage should have been seen by now if it had happened.
So I looked at my 3 and 4 years seedlings.  14 of 25 4 year old seedlings still have green healthy leaves.  Nothing I have been told or read prepared me for this.
I think that any report of trees living or dieing at a given temperature is of very limited value in another place,  I don't say it is of no value, because we often have no better information.  But there are many variables other than temperature and variety.

Ilya11:
It is very common for the young seedlings of PT to keep green  leaves over the winter.
Moreover, in poncirus  nucellar seed formation  is not obligate. It depends on the season and variety, many of  seedlings are zygotic, giving plants different from the mother.

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