In some areas Fennel is considered a noxious and/or invasive weed. But for me, Fennel has always been a nice, well-behaved multi-use plant. It even smells good in the dye pot. (And I can identify it correctly : see post about the Woad that turned out to be Weld.)
Fennel has also become one of of my seasonal markers. Wood Sorrel runs through half of the year and as it dies off the Fennel starts up. And visa versa. So I am always stocked with yellow dye.
Quick note: Fennel by my experience works only well with protein fiber (i.e. wool), I’ve never successfully dyed plant fiber such as cotton. Wood Sorrel on the other hand dyes anything I’ve tried. The only exception has been corn fiber.