以下の文献によるとどうやらコンパニオンプランツになるものはないみたい。。ほんとかな？危うく混ぜるところだった。ほ。

http://www.seedaholic.com/fennel-zefa-fino-florence-or-bulbing-fennel.html

Companion Planting:
The crucial thing to know about fennel is that you do not want it growing close to any other plant, and most especially nowhere near other members of the Umbellifrae family, which is a large one. It inhibits the growth of most plants, and readily cross-pollinates with its kin to give useless results on both plants.
Grow it by itself off in a corner of the garden, or in pots placed well away from the herb bed (and your carrots!) If you see large, black & green caterpillars on your fennel, don't be alarmed. Fennel is also a host plant for swallow butterfly caterpillars.