Why is it hard to ferment fennel?
After dumping out a couple of jars of failed not-quite-fermented fennel I wondered, "Could fennel have antimicrobial properties?"

&lt;&lt; facepalm >>

Why, yes. Yes it could:


An article on PubMed
Another on PubMed
One on Science Direct
One on Research Gate


Granted, these are about the essential oil and not the whole plant. So I can't be sure this is the problem, but it seems very likely. 

Maybe a little fennel mixed in with something else won't muck up the fermentation process, whereas a whole jar of fennel can.

And though it's hard to know, I suppose there could be a safety concern. Lacto fermentation protects us through (1) salt, (2) oxygen depletion, and (3) acid production. Based on what I saw with the failed batches, (2) and (3) may have been below normal. If salt-tolerant critters were to survive... that could be bad.