I'll make a suggestion since your question hasn't had any activity. I am unfamiliar with what fennel looks like, but I wonder if the cottage forum might have suggestions since some of them use herbs in with the perennials.

Good luck

Bronze fennel looks great as a contrast in texture to almost any other bronze or purple foliaged plant, or combined with chartruse foliage of a different type, and I use it with white lillies.

Green fennel would work as a great textural contrast with any larger green leaved foliage plant.

I think they fennels have such pretty foliage they soften/or look nice with anything..... but since bronze fennel has a vaguely purple tone maybe placing it nearest the white flowers would be nice....... ?????? :).... :)

I have bronze fennel in a bed behind some Bergarten Sage, Catherine Woodbery DL, a giant lavender iris and a Abe Darby rose. The texture of the fennel looks really pretty next to silver foliage.

I grow the Bronze Fennel for cut flower arrangements and paired it in container with Tanacetum vulgare "Isla Gold"...sort of gaudy but fun. josh