Crossbow (2-4-D) and Roundup mixed together will torch anything...

the problem with ferns is the spores, and the roots, you have to really kill the roots before you go waving them around (after pulling them) as the spores will spread all over the place creating new sets...

if you cant find Crossbow, Lily Miller Weed Be Gone has 2-4-D in it as well, mix it with concentrate Roundup (per directions), and spray the plants not dripping, but cover them, and they should be close to dead in 2 days, you can yank em out a couple days after that...

good luck, they can be a pain in the ass...