I read this thread title as 'Bottling fennel' and was curious ¦..  What sort of fennel are you growing?  I don't get on very well with the Florence fennel chosen for its bulb-type base, for salads and cooking, but the herb-type fennel grows prolifically in my garden " shoots up everywhere.  I am blessed with a cold, wet garden and the herby fennel seems to thrive in it.  Although I don't get good bulbs, I still use the stalks, thinly sliced, and the feathery leaves in cooking.  I do also bottle fennel stalks and flowers in olive oil, with garlic and slices of lemon, to give a lovely flavoured oil.  To return to your question - regular and copious amounts of water might be the answer.