Many people think that they taste different.
First, let’s be clear, here we are talking about florence fennel, that’s the one with the bulb that is eaten, that italians call finocchio. This is not the bronze leafed or the giant fennel, that provides lovely foliage and tasty seeds used for flavouring italian sausages.
Now, there is a bit of folklore about the gender here and this can be a cause of disagreement, depending on what your grandmother told you.
The fatter, rounder bulbs, which I refer to as female, taste sweeter and are certainly less stringy than the longer skinnier bulbs, which I refer to as the male. The male bulbs can have a more savoury flavour.
For cooking, the difference is not so obvious, but shaved really thinly, in a salad or eaten fresh, the female bulbs are my preference as they are sweeter and more juicy.
Some people say that the male bulbs are more prevalent toward the end of the season, but the two photos above are of three plants that were sown together, then harvested at the same time. I often see both types sold together and make my choice depending on my fancy.