There are two parts to the fennel, the frond (top) and the bulb (bottom). As the bulb starts to grow, you are supposed to keep it covered with dirt. I also keep the fronds trimmed so that the plant spends more of its energy developing a large base.
|You can see the white bulb developing on the bottom of this plant. After taking this picture, I mounded up some dirt around the base of the plant to cover it up.|
Also, if you let the tops grow and go to seed (and they grow like crazy... several feet tall if you don't cut them back), you can harvest fennel seed from them and use it to grow more fennel, or in cooking. I've never tried this, mainly because I forget to let one go that long, but this year I'll see if I can!