A couple of .edu sites, Clemson, Arizona say yes.

I grow cilantro, dill, and fennel in the same garden and allow them all to self-sow. I have never observed seed purity problems. The cilantro and dill plants look the same as the original planting several years ago. The fennel is less of a self-sower in comparison. My bronze fennel is the best variety for self-sowing and I have maintained a small population of bronze fennel plants for many years.

I recently heard this as well.

I guess I'll start some seeds from this years set to see if I get an odd tasting plant. I'd really rather not have licorice flavored pickles....

No - they can't. They are in the same family (Apiaceae or Umbelliferae) but are completely different genuses. Intergeneric hybrids are very, very rare. If they could cross they'd be as likely to interbreed with coriander, carrots, parsnips or celery as fennel. The relationship is just as close - and just as distant. The crossing story is widely disseminated on the web by people who don't know a family from a genus from a species but it just isn't so. Read from .... 'Meanwhile, in the Mental Gardening Department....'

Here is a link that might be useful: Dill and fennel can't cross.

This post was edited by floral_uk on Mon, Oct 6, 14 at 16:45