This Allotment Pro editorial draft summarises the cited RHS guide. Check the source and adjust timings for local conditions.RHS source guide
Modules — Sow one seed per module for an earlier crop or a head start in colder areas, avoiding early cold.
Direct sow — Sow once conditions are warm, using the crop to follow early potatoes or another cleared spring crop.
Florence fennel, *Foeniculum vulgare* var. *azoricum*, is an upright vegetable with feathery foliage and a sweet, mildly aniseed-flavoured white base. The part usually called a bulb is not a true botanical bulb: it is formed by the overlapping, swollen bases of the leaf stalks at soil level.
This is a compact relative of herb fennel and matures within a few months. It needs little ground space and suits small plots and large containers, but it can be a demanding crop. Cold, disturbed roots or dry soil can trigger bolting, leaving no usable swollen base or making it woody. Warm, damp summers and steady growth give the best results.
Full-sized bulbs usually take three or four months, while closely spaced plants can be cut as mini vegetables after about six weeks. Slice or grate the bulb raw, or steam, grill, roast or braise it. The young feathery foliage is also edible and useful as an aromatic herb.
Select seed labelled Florence or bulb fennel rather than herb fennel. Most modern varieties have some resistance to bolting, although no cultivar can compensate fully for prolonged cold, severe root disturbance or drought. Check the recommended sowing season because some cultivars perform better as early or later crops.
A compact Florence fennel with excellent resistance to bolting and yellowing.
A quick-growing early cultivar producing uniform bulbs and suitable for protected cropping.
A quick-maturing cultivar that forms medium to large bulbs with a slightly flattened shape.
Choose the warmest sheltered site available, with full sun and fertile, free-draining soil. Florence fennel is less successful in cold districts and dislikes heavy clay or waterlogged ground. Where those limitations cannot be corrected, use a large container and choose a compact cultivar.
Add plenty of well-rotted manure or garden compost in autumn, winter or early spring. This improves soil structure, drainage, fertility and water retention before the crop is sown or planted. A bed enriched for an earlier spring crop is ideal, because Florence fennel can take over as that crop is cleared. In a no-dig system, apply an organic mulch and sow or plant through it.
Florence fennel resents root disturbance, so either sow directly where plants will mature or put one seed in each module indoors. Early sowings are especially liable to bolt. For reliable outdoor crops, wait until early summer when soil and air have warmed.
Sow on a warm windowsill or in a greenhouse from late spring, placing one seed in each cell of a modular tray. Keep seedlings in good light and water regularly. Individual modules preserve the complete rootball when plants are moved, reducing the check that can prompt bolting.
Do not rush plants outdoors into cold spring conditions. Harden them off and wait until after the last frost; planting can begin from early May where weather is suitable.
Direct-sow during June or July. Prepare a shallow drill about 15mm (½in) deep, water its base, then sow thinly. Space rows 30cm (1ft) apart. Gradually thin full-sized crops to 30cm (1ft) within the row, taking care not to disturb the roots of retained seedlings. Use the thinnings in salads.
For mini fennel, thin to 10–15cm (4–6in) and harvest when the swollen base is about 5cm (2in) wide or tall. Guard seedlings from slugs and snails, especially in damp weather.
Fill a large container with moist peat-free multipurpose compost. A pot about 40cm (16in) wide accommodates up to three mature plants; for baby vegetables, use 10–15cm (4–6in) spacing. Container compost dries quickly, so the smaller volume needs particularly regular checking and watering to prevent bolting.
Harden off indoor-raised seedlings or bought plug plants before planting from early May onwards, after the last frost. Prepare the ground or a large container first so roots spend the shortest possible time exposed.
Move each seedling with its rootball intact and set it at the same depth at which it was growing. Space plants 30cm (1ft) apart for large bulbs or 10–15cm (4–6in) apart for mini vegetables. Firm gently, water thoroughly and protect young foliage from slugs and snails. Root disturbance, cold and a subsequent pause in growth all increase the risk of premature flowering.
Once established, Florence fennel needs little training or feeding. Its main requirement is uninterrupted access to moisture, accompanied by weed control and mulching so the swollen stem bases can develop rapidly.
Keep soil or compost consistently moist. Never allow it to dry completely: drought is a major trigger for bolting and can ruin the harvest. Containers are particularly vulnerable, so inspect them frequently in warm weather. Water generously and, where practical, use collected rainwater early in the morning.
Once plants are growing strongly, spread a thick layer of garden compost or well-rotted manure around them. Mulch slows evaporation and suppresses weeds. Keep the bed weed-free so fennel does not compete for light, moisture and nutrients, removing weeds by hand close to the crop to avoid cutting the developing bulb with a hoe.
When the overlapping stem bases begin to swell, mound soil around them. Earthing up excludes light so the bulb becomes whiter and sweeter. It also steadies the upright plant and gives the swollen base some protection from early frost. Add soil gradually without burying or damaging the feathery central growth.
Harvest from late summer into autumn according to sowing time. Full-sized bulbs are normally cut at 10–15cm (4–6in) across, but any size from about 5cm (2in) is usable. Closely spaced mini crops may be ready after roughly six weeks, whereas mature bulbs need three or four months.
Cut before the bulb elongates and sends up a flower stem, since a bolting base becomes woody and inedible. Either lift the whole plant or use a sharp knife just below the swollen base. If the basal plate and roots remain in the ground, they may produce small leafy shoots for salads, but they will not make a second bulb. Trim off the feathery foliage, retaining young leaves for kitchen use.
Premature flowering is the principal challenge. Cold spring weather, damaged roots during transplanting and dry summer soil can all interrupt growth and initiate bolting. Choose resistant cultivars, sow indoors only in modules, keep early plants protected until weather warms, delay outdoor sowing until early summer, and water without allowing the root zone to dry.
Slugs and snails are most troublesome around seedlings and young plants. Keep nearby weeds under control, inspect in damp conditions and protect new sowings without harming the wider garden ecosystem.
Bolting causes the flowering stem to elongate before a usable swollen base forms, or turns an existing bulb woody. Avoid cold checks, root disturbance and drought, while remembering that even resistant cultivars require steady conditions.
Problem sourceSlugs and snails can remove seedlings and damage young fennel in damp weather. Monitor direct sowings and newly planted modules closely, and use growing practices that reduce sheltered feeding sites.
Problem source
