I think ferns might have a similar result to sugar cane bales which have
been used around here (Queensland, Australia). The difference is that
the bale is made of leaf rather than stalks and consequently it does not
give as good a key for the render as compared to straw bales laid flat,
so I've been told. I suspect it also doesn't trap air as well so would
have a lower R value. Cane bales are usually used for lower-quality
constructions like farm sheds, and only because they are cheap.
However, if they have an abundance of ferns there's no harm in
experimenting.