The fennel herb will grow in most North American vegetable gardens--fennel thrives in USDA hardiness zones 5 through 10--and you can use it as both an herb and a leafy vegetable. The bulb is a prized part of the herb and, if harvested at the right time, is soft in texture and sweet in flavor. Harvest your fennel's bulb properly, as incorrect harvesting times or poor harvesting methods can ruin the tasty bulb.
Measure the fennel bulb with a ruler. The bulb should measure at least 5 cm in length but no more than 7 cm, as larger bulbs begin to dry out and become tough.
Use pruning shears and cut off the fennel plant's stalks and leaves. Don't discard these, since they can be used in salads and similar dishes as a simple leafy green. Leave approximately an inch of stalk left on the top of the bulb.
Harvest the bulb. Hold the bulb and cut it off of the plant's root base by inserting a knife at soil level and sawing away. Remove the cut bulb and rinse it under cold water to remove dirt and debris.