That's a new one on me, we have several varieties of Fennell/Fenal (SP) growing here in the South East U.S. but I have not heard of them being used as a snake repellent. Does your Fenal grow in the wild or is it a cultivated variety? Some things work well on some snakes and not so well on others, the combination of napthalene (7.5%) and Sulphur (25%) used in the commercial repellant I sell works well on Rattle Snakes and most Non-venomous snakes but not as well on Copperheads and not at all on Water Mocassins.
The Brown Snake you refer to has a nasty reputation from what I have seen, but I don't pretend to know anything about them beyond what is shown on television and most programming today tends to be slanted in favor of the snakes. I know everyting has its place, but snakes, particularly venomous snakes, are better off in the wild places and not in my backyard. I'll walk around them in the woods, but if one should slither into my yard it is likely to end up with its hide stretched on a board.