I doubt you will have many problems overwintering phormiums successfully - your very maritime climate will likely prohibit too many instances where temps would drop to dangerous (to phormiums) levels. And the fact that you have very good drainage is a big plus - phormium failure in the NW is generally due to overly wet soils combined with colder than ideal temperatures. Lose the poor drainage and your chances of root rots resulting from the wet/cold combo is greatly reduced. You may see some foliar damage. Even in very moderate locations phormium foliage can get chewed up a bit in winter and if exposed to winter winds or storms, the damage could be considerable. But trimming off a few torn up blades is a lot easier to deal with than replacing an established plant.

The good news is that phormiums are becoming more and more readily available and at much less cost and they grow like weeds, so even replacement is not a huge issue. I've been growing various phormiums in my Shoreline garden for years and the only reason I've replaced them is because they simply got too dang big!

Thanks, gardengal. I've been attracted to phormium for years and now that, as you said, they are more readily available, I am taking the plunge and putting them out in the landscape.

I wouldn't worry about it. I've got several kinds planted on a comparatively high part of Camano Island (just below top of Monticello Drive) and will probably continue to expand the collection. Purpleleaf ones in particular are very common in local communities (our oldest is nearly head-high now) and hardy most winters near the Sound. Last time I remember seeing New Zealand flax freezing back in western WA was 1990, coldest winter in 30 years. So, they are not fully hardy but live for long periods--how long depends on when the next killer winter comes. Some of the biggest specimens I have seen have been in low-lying, flat sites where frosts must sometimes be pretty severe (for instance Oasis Water Gardens in Seattle).

"Rugged P. tenax varieties are sturdy and fast growing. They take almost any soil, little or regular watering, hot or chilly conditions; do well at seacoast. They will take poor drainage to a point; crown rot can be a problem if they are planted too low in poorly drained soil...P. tenax, P. cookianum, and their forms and hybrids are all harmed by temperatures below 20F/-7C.*" -- Sunset WESTERN GARDEN BOOK

*Seems a bit high to me, but maybe not