I harvest it. I have cut it up to first frost. It is best I think. About a month before the first frost. As by then it is tougher. I like to wait until after it seeds. I then get the seeds for replanting next year and for making tea.
There are two types of fennel: those that bulb (Florence fennel) and those that produce only leaves and flower stalks. You might look to see if the seed packet says which kind you have -- I've never had the leaf type make bulbs, but I'm sure the bulb kind could bolt and not make bulbs, either.
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