I have lost only one Phormium planted outside in the cold, when temperatures dropped to -14 here. Generally even if the top growth looks unhappy they make an effort to grow again from the epicormic roots. I find that it is best (providing you are strong enough) to tug out the brown or rotting leaves as if you leave cut bits of leaf at the base they gather water and encourage rot. I managed to salvage one by taking off all the damaged growth (this left about three leaves!) and potting up the healthy remnant in free-draining compost. I think that wet is probably more damaging then cold, especially as you live in the south-west, and if you have heavy clay soil I would keep Phormiums in pots and shelter them in the winter. They do need very big pots to grow well and regular repotting or division...and that is another story, although I think they are marginally easier to split than Agapanthus. Your saw is the best tool for this, although I wouldn't worry too much about your plant as I am sure that it will soon look much better when the weather warms up, and is already looking much better than some of mine have in the past.
Yes- persistent wet cold soil is a far bigger problem for plants like Phormiums, and it's why I don't plant directly in the ground. Our climate here is completely unsuitable for that. Mine have managed plenty of cold, wet winters [I've been growing them for decades] because they're in suitably prepared raised beds and pots, but the persistent cycle of wet/freeze/thaw/freeze of this winter was different. Dry cold, even when below minus 10, is much easier for them.
It's a place where beautiful isn't enough of a word....
I live in west central Scotland - not where that photo is...
I’ve got another Yellow Wave growing directly in the ground - chalky soil, sunny, south facing and reasonably free draining - but that one looked completely decimated 4 weeks ago so I just cut the foliage to the ground in a sort of kill or cure treatment. It’s probably a bit early to expect anything, but it’s just sitting there doing nothing yet. Because the leaf bases looked a bit mildewed, I assumed that particular damage was a result of mealy bugs rather than wet, so thank you for pointing out that moisture can be an issue too.
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Mine have managed plenty of cold, wet winters [I've been growing them for decades] because they're in suitably prepared raised beds and pots, but the persistent cycle of wet/freeze/thaw/freeze of this winter was different. Dry cold, even when below minus 10, is much easier for them.
I live in west central Scotland - not where that photo is...