I have a fennel forest in my yard. My kid loves fennel and also the butterflies that come with it. Every year many many caterpillars hatch, and transform turning into beautiful pollinators for the other plants. Some of the caterpillars turn into bird food too. It's important to plant host plants. Fennel can grow almost anywhere as deer, woodchuck, and other "nuisance" animals that eagerly consume other parts of the garden don't like it's strong scent and flavor. The deer eat everything in my yard, except fennel, lavender, calendula, foxglove (poisonous), daffodils (poisonous) and hellebores (poisonous.)
If you plant fennel and don't want a fennel forest, I suggest that you trim the seeds off after they are set.
