Fennel (bulb)
Fennel (bulb)
Foeniculum vulgare var. azoricum
📋Quick Facts
Height
0.6-0.8m
Spread
0.3-0.4m
Water
💧💧 Average watering
Hardiness
Zone 4-9
About
Sow Florence fennel seed direct from April to July; bulbs swell in cool moist weather and bolt in heat or cold check, so timing matters more than for most crops. Florence fennel is half-hardy (RHS H3–H4) and the bulb-forming form distinct from herb fennel (same species, different cultivar group, same slug needs to be disambiguated — herb fennel under separate slug `fennel-herb`). Plant 30 cm apart in rich moist soil; thin from clusters; earth up the developing bulb to keep it tender and white. Pick bulbs when they're fist-sized (8–12 cm across) — checking weekly once they start to swell. The classic UK bolting risk: cool dip after planting, hot summer, or simply growing the wrong cultivar — Romanesco and Mantovano are the most bolt-resistant.
How to grow fennel (bulb)
- 1
Sow direct from April
Sow direct from late April through to July. Florence fennel hates root disturbance — direct sowing or careful module-raising only. 1 cm deep drill; sow thinly; cover lightly. Successional sowings every 3 weeks for continuous picking.
- 2
Choose bolt-resistant cultivars
Romanesco, Mantovano, Finale, and Perfection are bred to resist bolting. Older cultivars (Zefa Fino) bolt readily in UK conditions. Bolt-resistant is the difference between a crop and a wasted bed.
- 3
Thin to 30 cm
When seedlings have 4 true leaves, thin to 30 cm apart in rows 45 cm apart. Eat the thinnings (small fennel plants are tender in salad).
- 4
Water consistently
Florence fennel bulbs split or bolt under drought stress. Water deeply twice a week from planting through to harvest; mulch with grass clippings or straw.
- 5
Earth up the developing bulb
Once the bulb is the size of a golf ball, draw soil up around it loosely (3–5 cm), keeping the top exposed. Earthing-up keeps the bulb white, tender, and stops it going green and tough.
- 6
Watch for bolting
Cool checks (cold weather after planting), drought, or simply day-length sensitivity in older cultivars can trigger bolting (early flowering stem). Bolt-resistant cultivars help. Pull bolted plants; the bulb stops developing once flowering starts.
- 7
Pick when fist-sized
Cut bulbs at ground level with a sharp knife when fist-sized (8–12 cm across) — checking weekly once swelling starts. Smaller is more tender; bigger gets fibrous. The cut stump sometimes resprouts giving secondary shoots that can be picked young.
- 8
Save feathery foliage
The feathery aniseed-flavoured foliage above the bulb is edible too — chop into salads, fish dishes, or use as a garnish. Pick lightly through the season; pick heavily when you harvest the bulb.
Common questions
Pest Resilience
Aphids may cluster on flower heads; generally robust.
Companion Planting
Visual Characteristics
Fruits
Harvest: Summer to autumn / fall
Culinary
Roasted, salads, risotto, gratin, braised, pasta, fish accompaniment
The fennel (bulb) year in your garden
How to Propagate
This plant produces viable seeds for propagation
Hardiness Zones
USDA 8–9 equivalent