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Herb growing guide
Bulb fennel is easy to germinate but quick to bolt: cold starts, dry soil, long hot days and root disturbance all push it into flower before the bulb swells. This guide shows how to sow and time it so you harvest firm, white bulbs.
Fennel seed is sown dry. Know which type you have: Florence (bulb) fennel such as Mantovano is grown for the swollen leaf base, while herb or seed fennel (saunf) grows 1-1.5 m (3-5 ft) tall and never forms a bulb. The sowing method is the same, but the timing advice below is for bulb fennel.
Not needed. Fennel seed takes up water readily.
Not needed. Cold actually works against bulb fennel: cold spells after sowing are a common trigger for bolting.
Not needed. In standard seed tests fennel germinates readily on a plain moist substrate at 20-25 °C (68-77 °F) without any pre-treatment.
Seed can germinate anywhere between about 5 and 35 °C (41-95 °F), with an optimum soil temperature of 16-18 °C (61-64 °F) in Australian field guidance. Brazilian seed tests found constant 20 and 25 °C (68 and 77 °F) best and a constant 30 °C (86 °F) unsuitable. In Indian summer heat, sow in the evening or wait for cooler weather.
1.5-2 cm (about 1/2-3/4 in) in a drill; RHS gives 1.5 cm (1/2 in). In modules, cover with the same depth of compost.
A fine, well-drained seed compost or a rich, crumbly bed with plenty of organic matter. Drainage matters because wet, cold soil invites damping-off.
Keep the soil moist until emergence. Germination drops sharply under even mild drought stress (from about -0.1 MPa in seed tests), so do not let the drill dry out.
Fennel germinates with or without light, so cover the seed to keep it moist.
About 8-10 days at the optimum 16-18 °C (61-64 °F); field emergence is generally 12-14 days after sowing when moisture is maintained.
Fennel has a taproot and dislikes root disturbance, which is one of the main triggers of bolting. Sow one seed per module, plant out young after hardening off, and keep the root ball intact. Do not prick out from seed trays.
Thin or space plants 20-30 cm (8-12 in) apart in rows about 30 cm (12 in) apart. Water regularly: dry soil both stops the bulb swelling evenly and triggers bolting.
When the base starts to swell to about egg size, draw soil up around it. This blanches the bulb, making it whiter, sweeter and more tender, and steadies the plant.
Bulb fennel sown too early bolts in long, hot days; sown later it matures in cooler, shorter days. In the Indian plains this means sowing after the monsoon for a winter harvest; in hill areas, early summer sowings maturing in autumn work best.
Bulbs take about 3-4 months from sowing to reach full size; mini bulbs for baby veg can be picked after about six weeks at 10-15 cm (4-6 in) spacing.
Pick a few feathery leaves for the kitchen as the plants grow, without stripping them. Flower heads left to dry on the plant can be cut, dried in a paper bag and threshed for seed.
Seed fennel is a long crop: about 62-85 days from sowing to 50% flowering and 155-185 days to seed harvest, depending on temperature. Field sowings use about 4-7 kg of seed per hectare at 1.5-2 cm depth.
Sow Mantovano bulb fennel in the cool season for crisp, white, aniseed-flavoured bulbs.
Both are Foeniculum vulgare. Florence or bulb fennel is a shorter type grown for its swollen leaf base, eaten as a vegetable. Herb or seed fennel (saunf) grows tall, does not form a bulb, and is grown for leaves and seeds; as a seed crop it takes about 155-185 days.
Cold weather soon after sowing, long hot days, dry soil and root disturbance during transplanting all trigger early flowering. Sow so the bulb matures in mild weather, water regularly, and avoid moving seedlings bare-root.
Yes, if they are raised one per module and planted out small with the root ball intact. Seedlings pulled from a tray or open bed usually check and bolt.
