From Houzz Fennel is allelopathic to most garden plants, inhibiting growth, causing to bolt, or actually killing many plants. Like black walnut or elder, it produces some chemical or other which does bad things to its neighbours. Although it causes coriander to bolt, the coriander does help to prevent fennel from becoming too much of a weed. It depends which one you want to sacrifice, I guess! It will cross-pollinate with dill, resulting in a horrible-tasting plant. I think fennel will be OK if grown in a pot, but you'll need to be vigilant to stop it from going to seed (a pity if you want those seeds!) - those seeds can travel far and wide, making it a very invasive plant indeed.

- Mike