I'm just asking, I really don't know the answer, but Fern grows in acidic soil, correct? If so, would spreading pellet lime kill 2 birds? It would reduce conditions for fern and improve conditions for more "desirable" plants, and you wouldn't need (as much) herbicide. I don't have ferns so I don't know much about fighting them. Maybe changing the soil so it isn't compatible with fern is a way to reduce fern without using as much herbicide.
I'm doing something similar to fight Creeping Charlie. I'm not using lime, but I am using Borax. So far, it seems to be working.