Fennel is one of those plants where buying the wrong packet of seed can change the whole result. Ordinary herb fennel gives you tall stems, feathery leaves, yellow flowers and aromatic seeds. Florence fennel is grown for the swollen, crisp base that shops sell as a fennel bulb. They're closely related, but they aren't grown or harvested in quite the same way.
If you want leaves for fish dishes or seeds for cooking, choose herb fennel. If you want to slice a white bulb into a salad or roast it with vegetables, choose Florence fennel. The distinction matters much more than any complicated gardening technique.
Choose the right kind of fennel
Common fennel, Foeniculum vulgare, is a hardy perennial herb. It can reach about 2m tall, die back in autumn and produce fresh stems the following spring. Green and bronze-leaved forms are both useful in cooking.
Florence fennel, Foeniculum vulgare var. azoricum, is usually treated as an annual vegetable. Its overlapping leaf bases swell into a rounded structure that looks like a bulb, although it isn't a true underground bulb.
Look for varieties described as bolt-resistant when buying Florence fennel. The RHS recommends choices including 'Dragon', 'Rondo' and 'Zefa Fino'. These are more suitable for producing bulbs than a packet simply labelled 'fennel herb'.
Where fennel grows best
Both kinds prefer sunshine and soil that drains well. Herb fennel tolerates relatively dry conditions once established, making it useful in a sunny herb border. Give it room; the mature flower stems can tower over neighbouring plants.
Florence fennel needs more consistent moisture to form a good swollen base. A warm, sheltered vegetable bed with fertile, moisture-retentive but well-drained soil is ideal. Heavy, wet clay can make growing it difficult.
Don't place tall herb fennel immediately in front of smaller plants that need sunshine. Its height is attractive in a mixed border, but it can also cast considerable shade.
When to sow herb fennel
The RHS recommends sowing common fennel directly outdoors from mid-spring. Prepare the soil, remove weeds and make a shallow drill about 1cm deep. Sow thinly and cover lightly with soil.
Water gently while seeds germinate. When the seedlings have developed, thin them until the plants are at least 30cm apart. Young thinnings from correctly identified edible fennel can be used in salads.
You can start seed indoors in spring, using one seed per module. Move young plants outside after hardening them off, while they're still small. Fennel develops a taproot and dislikes having its roots disturbed.
When to sow Florence fennel
Florence fennel is particularly sensitive to growing conditions. A cold spell, dry soil or damaged roots can encourage bolting: the plant sends up a flower stalk before forming a useful bulb.
For direct sowing in Britain, June and July are often the most reliable months. The RHS advises waiting until the soil has warmed rather than sowing too early. This makes Florence fennel useful after an early crop has been cleared.
If you want to start plants indoors, sow in individual modules from late spring. Keep the seedlings in good light and transplant carefully once the weather is suitable. Don't pull several seedlings apart from a shared pot.
How to sow and space Florence fennel
Make a shallow drill approximately 15mm deep in prepared soil. Water the drill, scatter seed thinly, cover it and keep the ground moist as seedlings appear.
For full-sized bulbs, thin to roughly 30cm between plants and allow similar space between rows. Handle the remaining seedlings gently while removing extras so you don't disturb their roots.
You can grow baby Florence fennel at closer spacing, around 10–15cm, and harvest it while the swollen bases are small. This is useful if you have a narrow bed or want a quicker crop.
Growing fennel in containers
Herb fennel can grow in a large, stable pot at least 30cm wide and deep. Because it becomes tall, choose a heavy container and place it somewhere sheltered from wind.
Compact Florence fennel can grow in a container about 40cm across. Use fresh peat-free multipurpose compost with drainage holes below. The RHS says a pot of that width can hold up to three plants grown for mature bulbs.
Containers need closer attention to watering than open ground. Florence fennel in particular may bolt if its compost dries out repeatedly, so check moisture below the surface during warm weather.
Keeping Florence fennel growing steadily
Florence fennel benefits from a reliable supply of moisture throughout the growing season. Water deeply during dry spells and mulch around plants to reduce evaporation and weed competition.
Avoid damaging roots while weeding. Pull weeds by hand near developing bulbs instead of scraping a hoe against the base.
Once the swollen bases develop, you can gently draw a little soil around them to blanch the outside and protect them. Keep the growing point clear. Consistent moisture and warmth matter much more than extra fertiliser.
When and how to harvest herb fennel
Snip young feathery leaves from early summer onwards, taking a few sprigs at a time so the plant can keep growing. They have a mild aniseed flavour and are useful with fish, potatoes and salads.
The flowers are also edible. If you want seeds, allow some flowerheads to mature during late summer. Cut ripe heads into a paper bag, let them dry thoroughly and store the seeds in a clean, airtight container.
If you mainly want leaves, remove some developing flower stems or trim the plant back to about 30cm during the growing season to encourage new shoots.
When is Florence fennel ready?
Harvest Florence fennel while its swollen base is still firm and tender, often when it reaches about 10–15cm across. Smaller bulbs can be used too, including those around 5cm wide.
Use a sharp knife to cut the swollen base just above the roots, or lift the entire plant. If the base begins stretching upwards into a flower stalk, it is likely to become woody and less pleasant to eat.
The remaining base may produce edible leaves, but not another bulb. Use harvested fennel fresh where possible for the best texture.
Why did my Florence fennel bolt?
Bolting is the most common disappointment. The plant may look tall and healthy, yet the white base stays narrow because energy has gone into producing flowers.
Cold weather after early planting, irregular watering and transplant damage are frequent triggers. Next time, sow outdoors in early summer, choose a bolt-resistant variety and keep moisture steady.
A bolted plant won't reliably turn back into a good bulb. You can use suitable young foliage, but it's usually better to harvest what remains and try a later sowing.
Looking after fennel through winter
Herb fennel normally dies back in autumn. You can leave its dry stems standing for winter interest and insects, then cut them to ground level before new shoots emerge in spring.
It also self-seeds readily. Remove unwanted seedlings before their taproots grow deep, or collect seeds before they scatter if you want to keep the plant in one part of the garden.
Florence fennel is generally cleared after harvesting. Don't expect an annual vegetable crop to behave like a long-lived herb plant.
Which should you grow first?
For an easy, long-term herb, choose common fennel and give it a sunny corner where its height won't cause trouble. For crisp white bulbs, try a small summer sowing of Florence fennel and pay particular attention to warmth, spacing and water. Both are rewarding, provided you know which harvest you're growing towards.
Sources
Royal Horticultural Society: How to grow herb fennel and How to grow Florence fennel.