Fennel grows best during warm summers and needs an open, sunny site. Prepare a seedbed in fertile, well-drained soil, adding plenty of well-rotted organic matter the winter before planting. Fennel thrives on warm, moist, fertile, sandy soils.
Fennel dislikes root disturbance. Sow in cooler climates or, for early crops, better to sow in modules as single seedlings to avoid root damage. Plant out modules as soon as possible once the roots fill container from April, ‘harden off’, then plant out once the soil is warm and there is no danger of frost, from early May onwards. Early sowings are very liable to flower prematurely (bolt); there are bolt-resistant cultivars.
Alternatively, sow directly into the soil, 1.5mm (1/2in) deep in rows 30cm (12in) apart, thinning to 30cm (12in) apart in the rows when the soil is warm from May to early July. Use bolt-resistant cultivars from mid-June to mid-July sowings.
Provide plenty of moisture throughout the growing season, keep weed free and mulch to conserve moisture.
Earth up (mound soil) round the bulbs as they start to swell, from mid-summer until mid autumn, until the bulb is mature and about 7-10cm (3-4in across) to blanch bulb and to exclude autumn frosts.
Feed with high potassium fertiliser every two weeks once established.
The largest bulbs are formed in warm, sunny, moist summers.
Around 20 days after ‘earthing up’, cut the bulbs off at ground level. They will then re-sprout and the small shoots can be used in salads.
'Perfection’: Produces medium sized bulbs with a delicate aniseed flavour. Resistant to bolting and ideal for early sowing.
‘Cantino’: Has a refreshing aniseed flavour. Bolting resistant.
‘Amigo’ AGM: Produces uniform, slightly flattened bulbs. Bolting resistant.
‘Heracles’ AGM: Fast maturing, with moderately sized, flattened bulbs.
Bolting: Crops start to flower and produce seeds prematurely; leaves are unusable. This is most often caused by dry soil or hot weather but with fennel, it can be caused by day length and transplanting shock.
Remedy: Ensure the soil or compost in pots is kept moist, especially during hot, dry spells. Using a temporary shade screen or sheets of shading material will help during very sunny weather. Keep the soil moist at all times.
Find out more information on bolting
Slugs: Eat leaves and young shoots.
Remedy: It’s impossible to completely eradicate slugs and snails, so protect vulnerable plants. Non-chemical controls including hunting by torchlight on mild, damp nights, or making traps consisting of a jar half-filled with beer sunk into the ground near plants. Iron phosphate-based pellets are effective and less likely to harm other wildlife than other pellets.
Find out more information on slugs
Small bulbs: Small bulbs grown; only a small part of the bulb is white.
Remedy: Ensure that they are earthed up properly and maintain moisture levels in the soil.