I find that bronze fennel reseeds itself, sometimes in the same spot, other times it pops up unexpectedly somewhere else. I have also had them simply die on me after 2-3 years. So, maybe let some purposely go to seed every year. I currently don't have any in my actual herb garden, but one is growing in my chicken yard, where I often toss my weed bucket. The birds are leaving it alone, so I'll nab some seeds this fall and sprinkle them back where they belong.