Florence fennel

Light and exposure

Florence fennel likes full sun and mild warmth. Set aside a clear bed for it, with no shade cast by a hedge or taller plants, because a bulb forms better when the plant gets light all day long. A spot sheltered from wind is a plus, young plants do not like being shaken.

Under our climate, the warmth of the late season suits it well. Avoid, however, spots that bake and dry out too fast, such as the foot of a south-facing wall with no chance of watering. Fennel wants sun on the foliage but coolness at the base, which naturally points to good garden soil rather than a dry corner.

Watering

This is the gesture that decides everything. Florence fennel needs constantly cool soil, never dry. A shortage of water, even brief, is perceived as a stress and triggers bolting: the plant runs up a flower stem and the bulb stays stringy or non-existent. Water regularly and generously, especially during hot summers and while the bulb is forming.

In practice, it is better to water abundantly two to three times a week than a little every day, so the water reaches deep down. A mulch at the base, straw or dried clippings, keeps the moisture and limits the swings. Our Belgian summers are often damp, which helps, but the dry spells of July remain the most dangerous moment, to watch closely.

Soil and repotting

Fennel calls for rich, deep soil, and above all soil that can hold water. Good garden soil enriched with ripe compost before sowing gives it the fertility and coolness it needs. Sandy soils that dry fast should be corrected with organic matter, otherwise bolting looms.

Drainage counts too, because permanently waterlogged soil rots the collar. So you look for the balance of a worked garden soil, supple, that stays moist without being saturated. A neutral to slightly limey pH, common in the Herve area, suits it perfectly.

Temperature and humidity

Florence fennel grows well between 15 and 25 degrees. It is a plant sensitive to extremes in both directions. A cold snap early in life, below 10 degrees for several days, acts as a winter signal and pushes the plant to run to seed as soon as the warmth returns, hence the failure of too-early sowings in the Belgian spring.

At the other end, strong dry heat also speeds up bolting. The ideal compromise here is the mildness of late summer and early autumn: still-warm days, cool nights, moist soil. The bulb withstands light autumn frosts, but not real prolonged frost, which sets the end of harvest around the first hard frosts.

Pruning and upkeep

Fennel is not pruned like a shrub, but one gesture matters a lot: earthing up. When the base begins to swell and reaches the size of a small egg, draw soil around the bulb to cover it partly. This blanching makes it more tender, milder and less fibrous. Repeat the operation as the bulb swells.

You can also take a few fine leaves as you go to flavour cooking and salads, without ever stripping the plant, because the foliage feeds the bulb. If a plant bolts to flower anyway, leave it: its yellow umbels attract beneficial insects and you will harvest aniseed seeds, in place of a fleshy bulb.

Pests and diseases

In the Belgian garden, fennel is fairly robust on the disease front, but a few troubles recur. Damping-off and collar rot strike when the soil stays waterlogged and cold, especially on too-early sowings. Foliage that yellows and a base that softens are the signs. The solution lies in well-drained soil and sowing on the right date.

On the pest front, slugs love tender young plants in spring and in a damp autumn, to watch from emergence. The carrot fly, which affects the Apiaceae family, may lay at the collar; an insect-proof mesh protects young crops effectively. Aphids sometimes take refuge in the heart of the bulb, a jet of water and ladybirds see them off.

Propagation

Florence fennel is propagated from seed, and the best is to sow directly in place. Its roots do not like being disturbed, a rough transplant often ends in bolting. Sow in rows spaced 40 centimetres apart, at one centimetre deep, then thin to leave one plant every 20 to 25 centimetres.

If you insist on sowing in pots to save time, use deep pots or soil blocks, and prick out very young, before the root coils, keeping the block intact. Direct sowing from late May to July nonetheless remains the surest route here. Emergence takes one to two weeks in soil kept moist.

Month-by-month care calendar

Green Hub tips

• Never sow before the end of May in Belgium. An April sowing takes the night cold for winter and runs to seed instead of forming a bulb.
• Never stop watering. The slightest drought triggers bolting, keep the soil cool with a good mulch.
• Earth up the bulb as it swells to blanch it: it becomes more tender, milder and less fibrous.
• Sow in place rather than in pots. Fennel hates having its roots disturbed and often answers a transplant with a run to flower.
• Do not confuse it with aromatic wild fennel nor with dill: here you are after the fleshy bulb, not just the leaves.