A different clones of PT has a different hardy. Such ex. from Sochi (is about 3-4 years old) has a big damages in common winters here in 6b (-16 -18C) and died in severe winter with -27C. But I have clone from Pyatigorsk which shows good winter hardy and even one year old seedlings was alive (and without serious damages) after terrible winter 2011-2012 when temperature low to -27C and frosts was over 1 month without pause.

Well, I went back and checked the USDA zone rating on the two coldest locations of mature Poncirus that I know (Connecticut and central Massachusetts). It looks like according to the new USDA zone map the one in CT is now zone 6a and the one in central MA is now zone 5b (up from 5b and 5a/5b, respectively). The FD in Connecticut receives no protection, is completely exposed to the elements in the middle of a display garden at a rural nursery (Quackin' Grass Nursery), and has seen some quite cold temperatures just in the past few years. The nursery owner told me that he has never seen damage to his tree.

Here's a description of the weather at the nursery that was included with the listing of one of the other plants they carry:

Quote:

Aucuba japonica var. borealis – This is the hardiest of all among the growing list of Japanese Aucuba cultivars. It’s a beauty with a display of rich, glossy dark green foliage on a mounding and compact plant. It survived the hard USDA zone 5b winter of 2009 planted in a sheltered spot along the north foundation of our home. We had abundant snowfall but very many cold days and nights with at least 3 nights bottoming-out in the lower minus teens. When the snow melted it looked perfect. Only after the seemingly endless rains that followed well into July that very odd year did several branches blacken presumably from too wet conditions. But in the following December at the time of this writing the remainder of the shrub is unscathed and looks beautiful. Under cover of snow in the hard winter of 2011 also with several nights down to minus 10F and lower it emerged from the melting snow with out a single damaged blade, as fresh and green as it had looked the previous autumn.

So from the wording of the above description I would be conservative and interperate "lower minus teens" in 2009 as -13F (not -17 to -19F, that's crazy cold for the area). So this would be in the -25C range. Then two years later in 2011 the tree saw several days with similarly cold temps (-10F and lower). The nursery owner is very honest and knowledgeable, I have no reason to doubt these temperatures.

The central Massachusetts FD has also seen a lot of cold. I believe this tree has been in the ground since at least 2000. Here are the low temps since 2000 from an airport ~3 miles from the trees location:
2000 -11F (-24C)
2001 -2F (-19C)
2012 0F (-18C)
2003 -11F (-24C)
2004 -8F (-22C)
2005 -14F (-25/-26C)
2006 3F (-16C)
2007 0F (-18C)
2008 -9F (-23C)
2009 -11F (-24C)
2010 1F (-17C)
2011 -11F (-24C)
2012 3F (-16C)
2013 -2F (-19C) so far

In addition to the above trees, there are a handful of other Ponirus in the area that have seen pretty cold temps in the past decade:

Two PT's at the Arnold Arboretum in Boston situated in a relatively exposed location saw -7F (-22C) in 2004 as recorded at Logan airport. The Arboretum has had these trees since the 1980's, I believe.

One PT in Beverley, MA saw -9F (-23C) and three consecutive days with lows of -7, -8, and -9F in 2004 as recorded at an airport ~3-4 miles from the trees location.

My FD and an older, flowering age one in my next door neighbors yard went down to -2F (-19C) in 2011 without any damage. The tree I have is a seedling of the central Massachusetts FD.

Tim's tree in southeast MA has surely seen multiple nights in the -2 to -4 F range also.

I agree with Achillides' opinion that there may in fact be a significant variation in cold tolerance among Poncirus. Climatic conditions leading up to the cold temperatures surely also play some role. In any case, at least for New England, I would say that Poncirus could be considered to be reliably hardy in zones 6a and much of 5b without special care (with frequent lows of -10F to -13F). Whereas in 5a and 5a/5b areas (such as southeast Vermont) trees would benefit from being planted in a good microclimate, as mentioned by Hardyvermont.

There are many factors that affect the hardiness of Poncirus. From observations of them growing in different locations on my property and a friend's I conclude that some of them are:

1) Drying out from the wind when the temperatures are below freezing and being unable to transport sap.

2) Drying out from the sun when the temperatures are below freezing and being unable to transport sap.

3) Age of plant. Young trees seem to keep their leaves through the winter, making them more susceptible to dessication.

4) Possible variations between plants.

I never recognized damage to the plants from the cold. It seemed to come from drying out. Plants that looked great in January looked badly damaged in March. I attributed this to the location where they were growing. The best looking plants grew in a narrow valley that had limited sun and a lot of pines growing nearby. They came through the winter unscathed.

I am completely agree with hardyvermont.

Here is mother-plant of PT from Pyatigorsk. Unfortunately tree is died a couple years ago (winter mushroom made his black purpose).
Seedlings are quite small and not blooms yet (they grows rather slowly).

http://piccy.info/view3/2191545/d828daf33d1d677b9485ebb262eb49bf/

http://piccy.info/view3/2191549/14d5a748e63ffda7f85737922fb43827/

http://piccy.info/view3/2191555/f2f47ce42529973e124b0333f241136e/

To me it seems that this plant has been somehow protected. Pyatigorsk is probably z5a climate?

No, this tree never been protected and survived -26C in 2005-06 (with easy damage). Pyatigorsk is also 6b zone.

I've got a reference to a poncirus in Chelsea, MI. Chelsea's a 5b, on the edge of the Ann Arbor-Detroit 6a heat island.

http://www.plantmaps.com/interactive-michigan-usda-plant-zone-hardiness-map.php
http://greenstreettree.com/poncirus-plum-and-a-cat-2/

That region has seen -30 C in the last decade.

I suspect one of Michigan State University's gardens has one, but cannot confirm it. That would be 5a, bordering on 5b.

Certainly there are more northern examples -- Vancouver has some, but Vancouver is USDA zone 8. Niagara has been cited, but that's 6a/6b. I've also seen Toronto and Halifax noted, but those are 6b.
(Halifax: http://palmsnorth.com/forum/viewtopic.php?t=3724&start=30&sid=dccf9452edcfb8ba0991678559811abc)

Any PT in Vermont has my attention.......there are no warm spots up there!

My PTs are ripe now.......they look good enough to eat!

So you're volunteering to tell us how "good" they taste?????

If you carfully squeeze and keep the resin apart from juice there will be no problem. PT-juice tastes sour and there is nothing against mixing it with soda or cola.