Can't speak for your ferns but I can tell you about my experience with Southern Wood Ferns. Neighbors across the street from me have mostly ferns covering their front yard. Planted in the 40s. I have a standing invitation to take as many as I like.
It took me a few (or more) times to get the hang of it. Now, as soon as I see the little fiddleheads starting to pop up, I know that I'd better get ready to dig.
So I vote for Spring.
