The OP's question rang a bell but I've only just remembered, rather
late, what I'd heard about fern (or rather bracken) and eyes.

In west Scotland, farmers used to cut bracken to use dried as free
cattle bedding in winter. If you've ever handled dry bracken you'll know
it easily splinters. Apparently, the little sharp bits of broken bracken
would cause trouble when they got into the cows' eyes in the byre.

This could be the basis of the threat to children's eyes he'd heard
about; a child walking through bracken is being brushed by it at
face-level.

Janet.