"Details are welcome. For instance one morning frost at -12*C is not the same as one week with temperatures night and day between -6*C and -8*C."

No it's not. And it's not going to like either one. Based on the fact it's been getting through the recent mild winters in Wash. DC, in an incredibly sheltered spot, I'd say it's almost certainly 8b hardy, possibly 8a if mature, and almost certainly not 7b hardy. Based on, of course, what those statistics mean in the US. It may not survive for example, in some cold part of Japan that is 8b but has many many 9a nights per year. The problem the US is our excursions between averages, USDA zone temps, and record lows, is huge.
It generally isn't surviving summers hotter than DC's because of root rot, although for whatever reason that isn't affecting them in Brisbane, which is roughly as warm as DC in summer. I certainly wouldn't plant one in Florida unless it were grafted onto something else.

Mind you my thoughts are still on the optimistic end of the spectrum. If they were still $100 a piece, I wouldn't plant one unless I were in at least zn 9a. Their native area is 9b at worst, and since they've been through a genetic bottleneck and were on a continent not really affected by the ice ages, I wouldn't expect them to have, for example, the anamalous hardiness of various Asian conifers. I seem to recall hearing some, but not all, of them died in zn 8 parts of the UK that actually had zn 8 temps in recent winters.

This post was edited by davidrt28 on Thu, May 16, 13 at 23:10

Since USDA zones are based on averages over periods of years there is no such thing as Zone 8 etc. temperatures - temperatures outside of the core ranges indicated for each zone occur, without the zone then becoming another one. Unless enough of them occur to change the average.

Hillier manual had the tree marked as tender by 2002. Elsewhere I have seen it designated Zone 9. Temporary successes in colder zones would just be the same pattern that has been repeated over and over by experimenters for years with other kinds of plants outside of their ranges of adaptation - these live as long as winters are mild enough, and as soon as the spell is broken they die back or die out.

"Since USDA zones are based on averages over periods of years there is no such thing as Zone 8 etc. temperatures "

Amused by this. I'm reminded of Mike Myer's Linda Richman character on SNL saying "The Industrial revolution was neither industrial, nor a revolution...talk amongst yourselves". Of course there ARE USDA zn 8 temperatures...it a temperature scale defined hardiness system. It's just a matter of decided on the frame of reference of those numbers for a give plant or set of plants, and an applicable timeframe.

As you have said yourself, or perhaps Resin too, of course for a tree you want it to go many years before a cold winter would make it an expensive giant matchstick. It's not like the many tender subshrubs or smaller plants that I've tried in my garden. To me the criteria would be "to the point that something else is as likely to finish it off." GIven that these trees have reluctantly survived upper single digits in DC, I can imagine certain 8b locations that infrequently get that frigid could support them until something else like lightning or storms is as likely to take them out. Whether we have such an 8b in the country is another question. Most of the 8b areas of the south have gotten notably colder than 8F; of course the DC tree was not mature so it's difficult to know how much that would matter. Not far from its native range, Canberra AU is exactly the sort of place where USDA zoned plants could be expected to never encounter temperatures much colder than their zone designation: it has a record low of 14F but is definitely zn 9 and not zn 10. There are many other such locations around the world and the OP didn't specify a location.

Given that the most similar plant the same general geographic area offers (Araucaria bidwillii) is zn 9 hardy, I'd agree that's the safest bet. Could it be a little hardier? Maybe. I remind people I was the naysayer trying to warn people not to be fooled about 10 years ago when the National Geographic Society was outrageously selling them as zn 7 hardy.

This post was edited by davidrt28 on Sat, May 18, 13 at 8:43

Also as Osprey's posts from NM indicate - as well as prior known anecdotes about such dry climates - not every severe freeze is as severe as it would be somewhere else. There are reports of Phoenix canariensis recovering from well below 0F in the 1960s in the Las Cruces/El Paso area and I can believe them. The USDA system - as well our records of weather history - only record the lowest temp. as a single scalar quantity. The actual type of that freeze, its duration, etc. will influence whether the plant can survive it.

The USDA system isn't saying it doesn't get below 10 in Zone 8 and so on. A particular zone doesn't "own" a particular temperature, that's my point. When it gets below 10 in Zone 8 it is still part of the normality of Zone 8: 10 F-20 F is is an averaging, not a totality. 5 F is not peculiar to Zone 7.

"The USDA system isn't saying it doesn't get below 10 in Zone 8 and so on."

Right. And neither was I.

I was pointing out that depending on location around the world, the chances of appreciable variance from the USDA zone temperature varies. In much of the US, there's a high chance of sometimes going well below it.

Hi,

My experience from Copenhagen/Denmark: I've read that it should endure down to -12, but already at -3 foliage started to get brown.

So my conclusion is that it may very well be able to survive down to -12C, but if you want a healthy looking tree keep it above 0C.

I move it to a frost-free location every november and move it outside again when night temperaturs stay reliably above 0. It seems to thrive with this procedure.

Hans Olav

I agree with Hans Olav

I had 2 planted out in the North of Germany, a bit south of the border to Denmark. Both survived -12ðC with much snow. But much brown needles and ugly looking.The next winter, also cold, gave both the rest.

If you want healthy plants you need the climate of South-England.

Wolfgang

Well I know of one in Lantzville, British Columbia that has been outside in the ground for several years without any protection, and has continued to grow well the entire time. I believe the owner said his lowest reading was -8C including some protracted cold periods.

If I got the right conversion it was 17.6F. To make a lasting specimen in this region a tree needs to be able to go a lot lower than that.