AFAIK the straight species P. cookianum (colensoi) and P. tenax are the hardiest. Next hardiest are giant, bronze forms of P. tenax and a particular yellow-variegated P. cookianum cultivar ('wings of gold' or something). Then after that comes the purple giant P. tenax ('Atropurpureum')... then the big stripey ones (Maori series, Pink Stripe, etc)... last of all the smaller, really colorful stripey ones. Unless someone else has a different experience.

The ones that have done the best for me are the ones planted in extremely well drained soil, the grittier the better. I can't figure this out as I have seen them growing in water in New Zealand!

Mary,
I'm guessing it's the combination of 'cold and wet' that does a phormium in, and so many others.
In New Zealand, the wet factor is present, but not the cold.
I'm a phormiophile (?) too, and mine are looking pretty beat up from Winter...
My big P. tenax has frozen to the ground at least twice before, but each time it's come back. I'm hopeful this time will be no exception.

Hi Sedum gal, wet and cold is why I think supreme drainage might be more of an issue in our climate, at least where I live. It sounds like you might garden in a warmer zone 8 climate pocket than I do. I am curious if you plant your Phormiums in any particular manner that might ensure a better chance of their survival or is it that you are just that much warmer where you garden? Have they grown back in one season to look garden worthy or does it take awhile? Sorry for so many questions but I think anyone who loves and tries to grow these awesome plants would love to know what has worked! It would be nice if we didn't have to think of them as expensive annuals that may or may not make it. :-$

At my previous garden in Olympia, I planted them directly into heavy clay and mulched them with fresh grass clippings. The species, and bronze forms loved it and are now huge. The little stripey ones eventually declined - so in my experience it's those that are really fussy. I also planted a hedge of the bronze ones in a seasonally swampy area. They have grown well too, though not as quickly. It hasn't been terribly cold there though - I think 19 degrees last December was the coldest it's been there since 1998.

In my observation of other people's plants, it seems like it often takes 3 years for them to fully recover from heavy freeze damage. Perhaps they could be sped up a bit with regular fertilizing.

The purple one here on Camano that has now frozen twice is so diminished I'll probably take it out. Since these are evergreen plants it might be expected that defoliation would knock their energy budget out of whack.

The biggest ones I've seen locally might be the green one and the purple one at Oasis Water Gardens in Seattle. Being on a valley floor the site might get pretty frosty. On the other hand being in Seattle it might be less frosty than other areas with cleaner air.

There used to be a number of comparatively bigger, older ones in the landscaping at Bellefields Office Park in Bellevue. Another low-lying site plus it is out some miles from the Sound.

Locally otherwise unsuited plants can be induced to grow for a time by planting in coarse soils like berms of pit run sand. Up on a mound the plant may also be elevated above the level of the most frequent hard ground frosts. But all plants have a minimum temperature below which they are damaged, regardless of other factors. When site conditions make a plant "more hardy" it is because the plant's full level of hardiness is being achieved. Unsuitable sites are often causing the plant to be damaged at higher temperatures than those to which it is genetically hardy.

It only takes a short time below the plant's hardiness level for it to be damaged. Although generally hardier than Cordyline australis phormiums still just aren't hardy enough to not be damaged periodically anywhere it gets below something like 15 degrees F.