I may be conservative on the hardiness numbers . Even when my fronds are completely bronzed the petioles are green . There may be something to be said for the milder Southeast coastal areas that see some real cold during a decade but get above 32F during the day . Filiferas are also hardier than Robustas and many of the Washys on the Southeast Coast might have more Filifera in them than Robusta .
I think my Washy has a lot of Robusta in it since the trunk is relatively thin .
My Washy might be trunk hardy way lower than 17F but the fronds definitely have some burn at 25F , and 22F is about the temp that bronzes them totally .
I wish I had protected my young Washy filifera better and could check the hardiness of my Filibusta and a Filifera . My Filifera was poorly protected last winter and it never recovered after that 9 days below 32F last winter .