RHS Growing Guides
How to grow fennel
Our detailed growing guide will help you with each step in successfully growing Fennel.
Getting Started
Fennel is a large attractive plant with soft feathery leaves and tall stems topped with flat clusters of tiny yellow flowers in summer. The leaves, flowers and seeds have an aniseed flavour and aroma and can be infused to make tea, added to salads or used in cooking.
Fennel is easy to grow, hardy and drought tolerant, and needs virtually no maintenance once established. It likes free-draining soil and full sun. Fennel dies down in autumn and re-sprouts from the base in spring. If you allow it to self-seed, new fennel plants will pop up around the garden.
As well as being edible and aromatic, fennel looks great in many settings, from formal herb gardens to flower borders and gravel gardens, and it mingles particularly well with ornamental grasses. The flowers are attractive to a many beneficial insects, as well as to flower arrangers.
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Choosing
Two forms of fennel are widely available – green-leaved common fennel (Foeniculum vulgare) and the bronze-leaved variety F. vulgare ‘Purpureum’. They are both grown in the same way and are equally tasty and aromatic, although bronze fennel is slightly hardier.
You can see many herbs, including fennel, growing in the herb plots at all the RHS gardens, so do visit to see how they are grown and pick up useful tips and inspiration.
What and where to buy
Fennel seeds are widely available in garden centres and online from gardening retailers and herb nurseries.
You can also buy young plants in spring and summer from the same sources. This is often a time-saving and cost-effective option if you only want one or two fennel plants.
Recommended Varieties
Foeniculum vulgare 'Purpureum'
The bronze- purple variant of the generic green...
Foeniculum vulgare
The generic green common fennel whose stems, feathery leaves and seeds are widely...
Foeniculum sativum
The wild perennial form which can be grown from seed. A sparser plant for stem and leaf...
Sowing
Fennel is best sown straight into its final growing site in mid- to late spring. It can also be started off indoors, but dislikes root disturbance, so use modules to keep the rootball intact when transplanting. Alternatively, you can buy young fennel plants in pots and carefully transfer them to their growing site without disturbing the roots. Buying young plants is often a good option if you only want one or two of these ultimately large plants.
Choose a sheltered, sunny growing site with light, free-draining soil. Fennel grows fast and makes a big, tall plant, so allow it plenty of space, with at least 30cm (1ft) between neighbouring plants.
Sowing indoors
Once seedlings appear, keep them in good light and water regularly.
Sowing outdoors
You can sow fennel seeds outdoors from mid-spring, into their final growing site, in the ground or in a large container filled with multi-purpose compost.
Fennel seeds should be sown about 1cm (½in) deep.
Thin out fennel seedlings as they grow, until they’re at a final spacing of at least 30cm (1ft). Use the thinnings in salads.
Protect fennel seedlings from slugs and snails.
Transplanting
Young fennel plants grown from seed indoors should be moved outside as soon as possible, before their tap root forms. Harden off first, to acclimatise them to outdoor conditions.
Water young fennels well before transplanting, then gently lift them out of their plastic module and pop them straight into the ground without disturbing the roots. Position them at the same level they were previously growing at and space them at least 30cm (1ft) apart, then water in well.
Bought fennel plants should be planted outside in late spring or early summer using the same method.
Alternatively, you could plant one young fennel into a large container, at least 30cm (1ft) wide and deep, filled with multi-purpose compost.
Protect young fennel plants from slugs and snails.
Plant Care
Watering
Fennel is drought tolerant once established, so shouldn’t need watering. But you should water young fennel plants regularly for the first couple of months, until growing strongly.
Weeding
Keep fennel seedlings and young plants weed-free, to reduce competition for light, water and nutrients.
Supporting
In windy locations, fennel’s tall flower stems may need supporting with a cane.
Cutting back
Fennel seeds ripen from late summer onwards. If you leave them to scatter, they will often germinate around the garden. So if you don’t want more fennel plants, cut off the faded flowers or immature seedheads. Still, the seeds are a useful food source for various birds and other small creatures.
If you only want fennel leaves, not the flowers or seeds, then you may prefer to cut off any developing flower stems down at the base, so the plant puts its energy into producing new leaves. You can also trim back the whole plant to about 30cm (1ft) tall several times during the growing season, to encourage a regular supply of fresh young foliage for harvesting.
Fennel’s stems and leaves naturally die down in autumn, then new ones sprout from the base in spring. Tidy up the plant by cutting out the old dead stems at ground level before the new shoots appear, so you don’t damage them. Even though you could cut them back in autumn as soon as they fade, it’s a good idea to delay until early spring if possible, as the tall stems can make an attractive architectural feature through winter, especially when laced with frost. The dead hollow stems are also a valuable overwintering site for insects.
Propagating
Gather the seeds once they ripen and dry in late summer or autumn.
If you leave the seeds in place, they will scatter and germinate around the garden – these young fennel seedlings can be carefully dug up and moved to wherever you want them to grow, but do your best not to disturb the roots (see Transplanting, above).
Tranplanting self-sown fennel seedlings or seed saving and sowing are better propagation methods than trying to dig up and divide established clumps.
Harvesting
Harvest sprigs of young leaves as required from spring to autumn. The more leaves you harvest, the more will be produced.
The flowers can be added to salads, and the seeds can be used fresh over the summer months or dried for later use.
To dry the seeds – for use in cookery or for sowing – cut whole seedheads in late summer when they ripen. Place in a paper bag until the seeds dry out and fall. Then store the seeds in an air-tight container.
Problems
Fennel is usually vigorous and healthy, when given plenty of sun, warmth and free-draining soil. Avoid waterlogged sites, as the roots may rot. Fennel is drought tolerant, so is generally unaffected by dry conditions.
Fennel is troubled by very few pests or diseases – just look out for:
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Slugs and snails – these like to eat fennel seedlings and young plants, so put deterrents in place. They may also feed on the fresh new shoots of established plants, but shouldn’t do any serious damage.
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Aphids – these may colonise the shoots in spring and early summer, but generally require no treatment
Get involved
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