Check out the palm forum. Some people over there do some amazing zone pushing, usually consisting of enclosure, lights, and insulation. One user in particular that has great info is artictropic

Here is a link that might be useful: Zone pushing winter protection

I have some seedlings that I planted out two springs ago - this is their second winter. So far they are slow growing but did not have any winter damage last year. They are about 8" tall.

I flip a tall styrofoam container over top of them from mid fall to mid spring - no supplemental heat source. I know they are destined to fail at some point, as the trees grow beyond the height at which protecting them becomes impractical. More of a fun experiment for me! They saw -10F last winter under their styrofoam house. Even though our minimum temps line up with zone 6, our winters stay below freezing for a month at a time, often hovering in the single digits to teens for days at a time.

Give yours a try and see how it goes!

Here is mine with their winter house on:
{{gwi:2124327}}

and after winter was over:
{{gwi:2124329}}
{{gwi:2124331}}

Thanks for the tips and encouragement. I will check out the other forum and link

There is a smallish (3-4') Monkey Puzzle tree growing at the Philadelphia Zoo, it had survived the "polar vortex" when I saw it in mid-summer last year. I think the zoo is probably zone 7a, but that cold killed back every single butterfly bush in the zoo.

Hardiness zone temperature ranges used by USDA to represent typical conditions in each zone are average minimums based on specific periods of multiple years - it is not possible for a single minimum temperature or small number of minimum temperatures to correspond with a specific zone. Only sets of temperatures large enough to effect the annual averages of the minimum number of years these are based on have any relation to hardiness zone designations - somebody doesn't have "a Zone 6 winter" or become located in USDA 6 because it got down to -10F.

Bboy - if your reply was in response to my comment above, I was just mentioning that they survived -10F which is a cold winter for my area. We rarely go below -5F. I wasn't calling myself zone 6 because this year or last lined up with the range of average lows for zone 6. I completely understand the averaging of annual extreme minimum temperature readings over many years being the basis for the USDA zone system. I reviewed 60+ years of temperature data from my local weather station (I'll be surprised if the monkey puzzle trees survive long term for me though because I don't see them tolerating 4-6 weeks of temps in the teens and 20's F with overnight lows dipping into the single digits or occasionally below 0F, which is pretty typical for us. They would probably have a much better chance if we hovered in the high 20's to low 30's with less often and less severe dips in temperature. We'll see how they fair!

Here's a Monkey Puzzle Tree - Araucaria araucana - growing in the Royal Botanical Garden, Edinburgh, Scotland. I'm fairly certain it gets quite cold there in the winter.

"Royal Botanical Garden, Edinburgh, Scotland. I'm fairly certain it gets quite cold there in the winter"

Not very cold - zone 8/9 border. Anything below -10°C is exceptional there. Britain's winters are kept mild by the surrounding ocean.

Resin

>4-6 weeks of temps in the teens and 20's F with overnight lows dipping into the single digits or occasionally below 0FNot necessarily a problem for this mountain tree. But -10F could be a different story.

There are two in my zone 6, planted miles apart but both about 10 years old.
One wrapped religiously, one never wrapped.
Last winter we had a terribly cold snap with no snow on the ground, I figured even the wrapped one would die... Nope both still good. No noticeable browsing visible from road either.

Of course this means little in just 10 years. But they are tougher than I had imagined.

Bboy - I thought it would be the prolonged cold that would do them in. Perhaps it will be the occasional "bad winter" where we go below -5F that limits their success. I always assumed they experienced cold winters in their native habitat, but not extended deep freeze. Interesting.

Yes, in general freezing of the root zones is what excludes so many plants that "ought to be hardy" (based solely on tolerance of brief episodes of low temperatures) from northern areas. I don't specifically know if what you describe would kill a monkey puzzle or not, but am not surprised that some tolerance of similar conditions may have been shown.

I noticed that plant for sell at a garden center near me and I'm in zone 6b-7.