This is a rather bizarre phenomenon that has been occurring sporadically throughout the area for a number of years. I believe it was first noticed in Seattle's Seward Park about 5-6 years ago and that initial die off has now spread to 15 acres! Various scientists and environmentalists from the UofW have been involved as has some fern expert from back east but no cause for the die off has yet been established.

Seward Park sword fern decline.

Spore-adically?

I've been noticing for the last couple of years the late Autumn into December has been warmer, sunnier, and drier than it has previously been. I wonder if that could have something to do with it. However, Springs have also been wetter.

(But then again, Fall precipitation levels may be more important than Spring levels because temperatures in this climate are warmer in Fall than Spring, so more conducive to photosynthesis)

I haven't noticed any die off here.

When I lived in South Seattle I used to frequent Seward Park. The area where the die off is occurring is in a heavily forested area. Maybe it's getting so dense the ferns can't handle the lack of light and water. I realize Sword Ferns grow in light and water challenged areas, but maybe this is too challenging. I'd like to go see it myself. I'm not too far away.

I haven't noticed any die off here...........yet

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Sword ferns are extremely versatile with regards to their tolerance of varying light conditions (heavy shade to full sun) and the degree of drought they can withstand. If it were something that simple, it seems that the horticulturists and other environmental scientists studying this problem would have figured that out by now. Yet the die-off remains a mystery........