Growing Fennel in the Home Garden
Fennel (Foeniculum vulgare) is a Mediterranean plant in the carrot family with feathery leaves and a sweet anise flavor. Two types are grown in gardens, and they are grown differently.
| Florence (bulb) fennel | Common (herb) fennel | |
|---|---|---|
| Botanical name | F. vulgare var. azoricum | F. vulgare; bronze fennel is a purple-brown form |
| Grown for | The swollen leaf bases ("bulb"), plus fronds | Leaves, seeds and flowers; no bulb |
| Life cycle | Grown as an annual | Short-lived perennial in roughly zones 5 to 9, often grown as an annual |
| Height | 2–3 feet | 3–5 feet or more |
| Main challenge | Bolting before the bulb forms | Self-sowing |
The bulb is a good source of vitamin C, potassium and fiber. Slice it raw into salads or roast it; the fronds go in salads and sauces, and the seeds season sausage, breads and spice blends.
Growing bulb fennel
The usual disappointment with Florence fennel is bolting: the plant sends up a flower stalk and the bulb stays flat and tough. Heat, lengthening days, drought, a cold snap after transplanting and root disturbance all trigger it. Timing and steady care prevent most of it.
- Fall crop (easiest): sow in midsummer, about 12 weeks before your first fall frost, so the bulbs swell in the cool, shortening days of fall. Mature plants tolerate light frost. Find your frost date with the zone and frost date lookup.
- Spring crop: sow around your last spring frost date and choose a bolt-resistant variety such as 'Zefa Fino'.
- Sow directly 1/4 inch deep and thin to 8 to 12 inches apart, in rows 18 to 24 inches apart. Fennel has a taproot and resents transplanting. If you start it indoors, use individual cells or soil blocks and transplant young, at 3 to 4 weeks, without disturbing the roots.
- Sun and soil: full sun (at least 6 hours) and fertile, well-drained soil with a pH of about 6.0 to 7.5. Work in compost before planting.
- Water evenly. Dry spells are a common cause of bolting and tough bulbs. Mulch to keep the soil moist.
- Fertilizer: compost at planting is usually enough; a light side-dressing helps in poor soil.
As the bulb swells, some gardeners mound soil around it to keep it pale and tender.
Harvest when bulbs are about 3 to 4 inches across, usually 65 to 90 days from sowing depending on variety. Don't wait too long; bulbs that start to stretch upward are about to bolt. Cut the bulb at the soil line. The stump often resprouts a few small shoots you can use like the fronds.
Growing herb fennel
Common fennel is easy. Sow it in spring after the last frost, 1/4 inch deep, and thin to 12 to 18 inches apart. Once established it tolerates dry spells. Snip leaves any time. For seeds, let the yellow flower heads ripen until the seeds turn from green to tan, then cut the heads into a paper bag to dry.
It self-sows heavily. Cut off flower heads before the seeds drop unless you want fennel seedlings throughout the garden. Common fennel has escaped gardens and become an invasive weed in parts of the West, especially California, so don't let it go to seed near wild land.
Pests and problems
- Black swallowtail caterpillars (green with black bands and yellow spots) feed on fennel, dill and parsley. Fennel can usually spare the leaves; hand-pick them or move them if a small plant is being stripped.
- Aphids collect on the tender tips; knock them off with a spray of water.
- Slugs and snails chew young plants in damp weather; hand-pick or trap them.
- Root rot in wet, poorly drained soil.
Fennel has a reputation for stunting nearby plants. Whatever the reason, it is simplest to give it its own spot at the edge of a bed, where tall herb fennel won't shade other crops. Its flowers draw many small beneficial insects.