For the last dozen years or so, I've thought that Washies (at least Filifera) are frond hardy to 17F or 18F. Same with CIDPs.

What do I base that on? The fact that Washies and Canary Island dates are Zone 9a trees. Zone 9a, by definition, has an average winter minima between 20F and 25F. If that's "average", then there certainly are years where the minima goes even colder. Into the mid to upper teens. Basically, an 8b winter in a 9a location. One wouldn't expect these trees -- so common in places like Savannah, St. Mary's, and Jacksonville -- to burn during a "normal" (i.e., Zone 9a) winter. Hence, my thinking that the fronds are hardy at least to 20F.

But maybe I've been wrong all this time.