RHS Growing Guides
How to grow Florence fennel
Our detailed growing guide will help you with each step in successfully growing Florence Fennel.
Getting Started
Florence fennel is an attractive plant that forms a white ‘bulb’ at ground level made up of the overlapping, swollen bases of the leaf stalks. This has a sweet, mild aniseed flavour. It is also known as bulb fennel, although botanically it’s not a true bulb.
As Florence fennel
The key to success is to provide regular water, never letting the soil dry out, so plants keep growing strongly. Early to mid-summer is the best time to sow, once temperatures have warmed up. The bulbs take three or four months to mature, but you can harvest at any size – baby bulbs can be enjoyed after only about six weeks. Fennel bulbs can be eaten raw, sliced or grated into salads, or cooked in all kinds of dishes. The young feathery leaves can also be used as a flavouring.
Florence fennel is a handsome, upright plant that takes up little ground space, so is ideal for small gardens and containers, as well as open ground. With its green feathery foliage, it’s ornamental as well as edible, making an attractive feature in any garden.
Month by Month
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Choosing
There are several varieties of Florence fennel – most are bolt resistant, which is useful if growing conditions aren’t ideal. Some varieties are more suitable for sowing earlier or later.
Look in particular for varieties with an RHS Award of Garden Merit (AGM), which shows they performed well in trials – see our list of AGM fruit and veg for recommended varieties of Florence fennel and many other crops.
If you visit the veg plots in the RHS gardens, you’ll also see many crops being grown, including Florence fennel – it’s a great way compare different varieties and pick up useful tips and inspiration.
What and where to buy
Seeds of Florence fennel are widely available in garden centres and from online seed suppliers. Be sure to select Florence fennel, not herb fennel.
Recommended Varieties
Large crisp white bulbs, with a mild aniseed flavour. Fast to mature.
A quick-growing variety that produces uniform bulbs. It's an early cropper that can be grown under cover.
Preparing the Ground
Choose a warm, sheltered, sunny site with fertile, moist but well-drained soil.
Add plenty of organic matter, such as well-rotted manure or garden compost, ideally the winter before sowing, to improve soil structure, drainage and fertility.
Florence fennel dislikes heavy clay and waterlogged ground. If your soil is unsuitable, or you’re short on space, it can be grown in large containers, at least 40cm (18in) wide, filled with multi-purpose compost.
Sowing
Fennel dislikes root disturbance, so sow seeds either direct outdoors or singly into modules indoors. Early sowings are very liable to bolt (flower prematurely), so if you want to give this a try, choose a bolt-resistant variety.
It’s best to wait until early summer (June) when sowing outdoors. Due to this late sowing time, Florence fennel is useful to fill gaps after crops such as early potatoes have been harvested.
Sowing indoors
To get an earlier harvest or a head start in colder regions, you can sow Florence fennel in a greenhouse or on a warm windowsill from late spring – don’t be tempted to sow too early, as it dislikes cold temperatures so shouldn’t be transplanted outdoors until early summer.
Florence fennel particularly dislikes root disturbance, so use a modular tray and sow just one seed into each module. This avoids pricking out and means the young plants can be transplanted outdoors with minimal disruption to the roots.
Keep the seedlings in good light and water them regularly.
From early May onwards, after the last frost, prepare to transplant young fennel plants outside by gradually acclimatising them to outdoor conditions by hardening off.
Then plant them into prepared ground (see above), doing your best not to disturb the roots. Space plants 30cm (1ft) apart and water them in well.
Sowing outdoors
Sow Florence fennel outdoors in June or July, once temperatures have warmed up. It’s an ideal crop to follow early potatoes or to fill any gaps in your veg plot where spring crops have been harvested.
Prepare the ground as detailed above, then make a shallow drill about 15mm (½in) deep. Water along the base, then sow the seeds thinly along the row. Space neighbouring rows 30cm (1ft) apart. For full details, see our guide to sowing outdoors and our guide to sowing veg seeds.
Thin out the young seedlings to 30cm (1ft) apart, taking care not to disturb the roots of the remaining plants. The thinnings can be used in salads.
If you’re short on space, you can grow baby bulbs at a closer spacing of 10–15cm (4–6in). These can be harvested when the bulb is about 5cm (2in) wide or upwards.
Fennel can also be sown into large containers filled with multi-purpose compost. You can grow up to three mature plants in a container about 40cm (16in) wide. Do bear in mind you will need to water these regularly, as containers can dry out very rapidly, which can cause fennel to bolt.
Plant Care
It’s essential to keep Florence fennel well watered, to ensure it grows without any checks. It tends to bolt (flower) in dry conditions, which ruins the harvest.
Once the bulbs start to swell, mound (or earth up) soil around them to blanch them, which produces whiter bulbs with a sweeter flavour. Earthing up also helps to stabilise them and protects the bulbs from early frosts.
Watering
Provide plenty of moisture consistently throughout the growing season. Dry conditions can cause bolting.
Mulching
Apply a thick layer of mulch, such as well-rotted manure or garden compost, around the plants to help hold moisture in the soil and deter weeds.
Feeding
Feed with a high potassium fertiliser every two weeks once established.
Weeding
Keep the ground weed-free, to reduce competition for light, water and nutrients. Weed by hand close to the plants, rather than hoeing, so you don’t accidentally damage the developing bulb.
Protecting from pests
Take care to protect seedlings and young plants from slugs and snails.
Harvesting
Florence fennel is ready to harvest in late summer and autumn, depending on sowing time. It is usually harvested when the swollen bulbs are 10–15cm (4–6in) across, but can be harvested at any size, from just 5cm (2in) upwards.
Make sure you harvest before the bulb starts to elongate and send up a flower stem, at which point it becomes woody and inedible.
To harvest, either dig up the whole bulb or slice it off just above the base with a sharp knife, leaving the lower part in the ground. This should then send up small leafy shoots that can be used in salads.
You can steam, grill, roast or braise fennel bulbs and serve with cheese sauce or butter. They have a sweet, mildly aniseed flavour and go particularly well with fish.
When using Florence fennel raw in salads, you can enhance the flavour by slicing the bulb and putting it in a bowl of water and ice cubes, then placing in the fridge for an hour.
You can also use the chopped young leaves to flavour salads and other dishes.
Fennel is best used fresh, but the bulbs can be stored in a fridge or other cool, dry place for several weeks.
Problems
The most common problem with Florence fennel is bolting (flowering), which leads to the bulb not forming or turning woody. This can be caused by various issues, including cold temperatures in spring, root disturbance when transplanting and dry conditions in summer. Florence fennel is most successful in warm, damp summers.
To deter bolting:
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Water regularly and generously, never letting the soil dry out completely
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Choose bolt-resistant varieties, especially for later sowings – however these can still bolt in dry or cold conditions
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Delay sowing outdoors until early summer, and protect with cloches or fleece if temperatures dip
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Use modular trays if sowing indoors, to reduce root disturbance when transplanting outdoors later
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Keep early sowings indoors until the weather has warmed up, then harden off carefully and protect with cloches or fleece
Few pests trouble Florence fennel, apart from slugs and snails, which will eat seedlings and young plants, so take steps to deter or control them. See our tips on how to stop slugs and snails.
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