It depends on the type you grow and where you grow it. Common fennel is a fennel perennial in mild areas, so the herb does come back year after year from the same root. But a Florence fennel grown for its bulb is an annual. It will not return once you pull that bulb in fall.
You can watch the difference play out in one bed. Leave common fennel standing through a mild winter and its crown pushes fresh feathery growth the next spring. Harvest a Florence fennel bulb in fall and that plant is done. The root went into the bulb you ate, so nothing is left to regrow. This is the quickest way to see whether your fennel come back habit is real or just self-seeding.
The botany is simple once you know the two plants apart. Common fennel, Foeniculum vulgare, lives a few years in warm zones. It acts like a true perennial herb. Gardeners in cold climates grow that same plant as an annual because hard freezes kill the crown. Florence fennel is a different story. It is bred for one fat bulb and grown for a single season only. So the same name covers a tough little perennial and a one-and-done bulb crop. Knowing which one you planted answers most of your worry.
So is fennel annual or perennial? The honest answer is both, and your zone settles it. Common fennel is hardy in USDA zones 4 to 9. There it can live on as a perennial and seed itself around the garden, per Wisconsin and NC State Extension. In those zones the plant seems to return two ways at once. The crown survives the winter, and dropped seed sprouts fresh plants nearby. That double trick is what makes a fennel perennial patch look so reliable.
That self-seeding habit is why even annual fennel can fool you into thinking it survived. The old plant dies, but the seed it scattered last fall comes up as a thick patch of seedlings in spring. You did not replant a thing, yet there it is again. Pull the extra seedlings early if you want to keep the patch in bounds. Left alone, a few plants can spread into a wide stand within a couple of seasons. Thin them while they are small and the roots come up with one easy tug.
Here is what to do based on your climate. In a mild zone, leave common fennel in place and cut the dead stalks back in late winter. The crown handles the cold on its own. In a cold zone, treat fennel as an annual. Or pile 3 to 4 inches of mulch over the crown to give it a fighting chance against freezes. That mulch can be the difference between a dead root and a living one come spring.
For Florence fennel, plan to replant each year no matter where you live. The bulb harvest ends that plant for good, so there is no crown left to overwinter. Let a few common fennel plants flower and drop seed if you want a steady supply with no work. You then get two ways back at once. An overwintering crown holds the same plant. A carpet of self-sown seedlings fills in around it. Together they keep a fennel perennial stand coming back for years from one planting. That mix of habits is the real reason fennel feels like it never quite leaves the garden.
Read the full article: Fennel Plant: Grow, Care, and Harvest Guide
