When to sow Florence fennel

Florence fennel is grown for its swollen leaf bases, the "bulb". Its big weakness is bolting: instead of forming a bulb, the plant shoots up and flowers. Early sowings are the most likely to bolt, because long days in late spring push the plant towards flowering.

That is why the classic advice is to sow in June or July, so the bulbs swell as the days shorten in late summer and autumn. Raised in modules, you can start from late April or May, but choose a bolt-resistant variety for anything sown before June.

Direct sowings go in from mid-May to the end of June or early July. Modules raised indoors are planted out after the last frosts, from mid-May, and young plants can still go in until mid-July.

Fennel forms a taproot and hates being disturbed, which is another trigger for bolting. Either sow directly where it is to grow, or sow one seed per module so the rootball can go out intact. Transplant young and promptly, before the root fills the cell, and harden plants off first.

Seed germinates best at 68–72°F and comes up in about 10–14 days. Outdoors, sow thinly in a shallow drill about 0.4–0.8 in deep and thin the seedlings rather than moving them.

Give fennel a sunny spot with fertile, free-draining soil that warms up quickly; it dislikes heavy clay and waterlogged ground. Dig in compost before sowing or planting.

Space plants 10–12 in apart in rows 12–16 in apart. In a 120 × 80 cm raised bed that means two rows of four at the wider spacing, or three rows of four, 12 plants, at 12 in each way. For baby bulbs you can go down to 4–6 in.

Keep the soil evenly moist the whole time. Dry spells, and summer heat together with drought, are among the most common reasons fennel bolts. A mulch of grass clippings or straw keeps moisture in.

Once the bases start to swell, draw a little soil up around them. It blanches the bulbs and keeps them tender; the drawback is some soil between the layers, so rinse well in the kitchen.

Fennel grows fast once it gets going: bulbs are ready about 12 weeks after sowing, or about eight weeks after planting out. Pick them at any size from about 2 in across; a full-sized bulb is 4–6 in across.

Cut the bulb just above soil level; the feathery leaves can be used like a herb.

Florence fennel only tolerates light frost. Cover or earth up late plantings when frost threatens, usually from October, and finish harvesting by the end of October. Fennel is best eaten fresh.

Florence fennel is an annual crop grown for the bulb. For spring and early-summer sowings, pick a bolt-resistant variety; in the UK, 'Dragon', 'Rondo' and 'Zefa Fino' are recommended.

Herb fennel, including bronze fennel, is the same species but a tall perennial, grown for its leaves and seeds, and it forms no bulb. It can reach about two metres and seeds itself freely, so give it a spot at the back of a border rather than in the vegetable bed.

Bolting is the number one problem, and it usually comes down to sowing too early without a bolt-resistant variety, a dry spell, or roots disturbed at planting. A bolted plant is no use for the kitchen, though the flowers feed insects.

Slugs and snails go for seedlings and young plants and can damage the bulbs. Powdery mildew and bacterial rots also occur, mostly in damp, crowded plantings.

Fennel belongs to the carrot family (Apiaceae), with carrots, parsnips, parsley, celery and celeriac, and dill. Don't grow it straight after any of these; leave about four years.

Fennel has a reputation as a bad neighbour that holds back plants around it. There is a little support for this: wild fennel is invasive in some countries and may release substances that inhibit other plants. Whether that matters for Florence fennel in a mixed vegetable bed is not established; treat it as a caution rather than a rule.

The rule with a real reason is rotation: keep fennel apart from other carrot-family crops. Seed savers also keep it away from dill.