How to Plant, Grow, and Care for Fennel

Fennel is an attractive herb that looks great in any garden and can be grown as a vegetable or an ornamental. Growing fennel can be tricky, especially the bulb variety. It tends to be fussy about how, when, and where you grow it, but don’t let that put you off. Follow our simple growing instructions to help you grow fennel at home. You won’t regret it.

There are two main types of fennel. The common or herb fennel is a herbaceous perennial, grown for its leaves and seeds. Bulb fennel is grown mainly as an annual vegetable. Both are similar in appearance and have a strong licorice or anise flavor and aroma. If the bulb variety is too daunting, start with common fennel, which is less demanding and will come back every year bigger and stronger. 

Fennel has been used throughout history for its medicinal benefits but is most renowned for aiding digestion and calming stomach spasms. In folklore, it was thought to give a person courage, strength, and a long life. The Greeks and Romans ate vast quantities believing it would ward off obesity. In more modern times, fennel is used mostly in the kitchen.

What is Fennel?

The two types of fennel are Foeniculum vulgare, known as common, herb or sweet fennel, and Foeniculum vulgare var. azoricum which is known as bulb or Florence fennel and finocchio. Both come from the carrot family, Apiaceae, and are related to parsnips, celery, and coriander.

Native Area

Fennel originates from the sunny coastal regions of southern Europe and the Mediterranean. It thrives in dry, sunny environments with well-drained soils and has been widely cultivated and naturalized in many parts of the world due to its culinary and medicinal uses. 

Characteristics

Bulb fennel was first introduced in Italy in the 17^th century and is the swollen stem of common fennel, bred to grow as a bulbous biennial vegetable, but are more commonly grown as an annual. Both varieties have green, smooth, hollow stems and finely dissected feathery leaves reaching 12 inches in length. The entire plant can grow to 6 feet in ideal conditions.  

In mid- to late-summer common fennel develops large flat flower heads of compound umbels at the end of stems, four inches across, each with 20-50 flowers on each umbel section. The flowers attract a wide variety of pollinating insects such as wasps, bees, lacewings, and swallowtail butterflies. Seeds are produced when the flowers fade and are green/brown in color turning grey with age and grooved along the sides. Fennel bulbs will bolt and produce similar flowers and seed if not harvested.

The main differences between the two fennel types are; ‘Florence’ fennel develops a large flat or round white bulb at its base, three to four inches across, and grows to an average stocky height of two to three feet.  Herb fennel grows to the majestic height of six foot with thicker, sturdier stems and no bulb.

The bulb, herb greens, and seeds can be used as a garnish, in salads, bread, stews, and soups or diffused as an anise flavor tea. However, the sweetest, most intense, and sought-after anise-like flavor comes from the tiny dried yellow flowers and is referred to as ‘fennel pollen’.

Bulb fennel is harvested before it flowers or sets seed so it is not invasive. Common fennel is a prolific self-seeder and can become quite a nuisance. Deadhead flowers to prevent seeding. In some regions, fennel is invasive.

Fennel is allelopathic which means it can inhibit the growth of neighboring species, in particular beans, tomatoes, and some brassicas.

Varieties

There are some hybrid varieties of fennel that have been bred to be more resistant to diseases, heat, or cold. These are worth trying in areas that have a difficult climate. Other varieties, like the bronze fennel, are grown mainly to be ornamental and are used as a contrasting color and texture in landscape design. These are a few to look out for:

• ‘Rhondo F1’: Quick to mature -good quality bulbs.
• ‘Amigo’: Uniform flattened white bulbs that are bolt-resistant.
• ‘Perfection’: Delicately flavored medium-sized bulbs, which are bolt-resistant.
• ‘Purpureum’: Bronze fennel that is mainly grown as a garden ornamental.
• ‘Rubrum’: Bronze fennel with red tones used as a garden ornamental.
• ‘Romanesca’: Bulbous stem with good flavor.
• ‘Dragon’: Big bulbs with succulent flesh.

Planting

The key to growing fennel is the consistency of temperature, sunlight, and moisture. Grow fennel in the garden in moisture-retentive, well-drained soil, improved with lots of organic matter, and located in full sun.

Common fennel develops a long taproot and does not like root disturbance. Sow seeds directly into their final growing positions in mid to late spring after the last frost date.  

Bulb fennel can be sown from mid-spring to late summer, depending on your climate. If you live in an area with a Mediterranean-type climate, you can start sowing seeds under cover in mid-spring. Seedlings should be ready for planting out after four to five weeks when they are two to three inches tall and all risk of frost has passed. Space 12 inches apart and 18 inches between rows. The crop should be ready for harvest from mid-summer onwards. Seeds may also be sown directly when soil temperatures reach a minimum of 50°F (10°C). 

If you live in a northern cool climate, sow fennel seeds directly in mid-summer when the weather is warm and temperatures shouldn’t fluctuate too suddenly. Bulbs will be ready for harvest in autumn. 

A great tip to grow show-stopping bulbs is to earth up soil around the base as the stem begins to swell. This provides stability as they grow and gleaming white, sweet bulbs.

Both varieties of fennel can be grown under cover in climatically controlled environments where the risk of bolting and wilt is managed. Garden greenhouses and polytunnels tend to overheat even in the cooler months and are not suitable for growing fennel.

How to Grow

The key to growing bulb fennel is sowing seeds at the right time and avoiding heat waves!  Follow the care tips below on how to grow fennel in your own garden.

Light

Both types of fennel require a minimum of six to eight hours of direct sunlight per day. They will not tolerate shade, so be careful not to overshadow them with another crop in your garden. Container grown fennel also need lots of sunlight, but make sure the pots don’t dry out too much and keep them hydrated.

Water

Fennel requires consistent and regular watering throughout the growing season. Dry soils will cause common fennel to wilt and reduce seed production. It will also trigger bulb fennel to flower. Timed soaker hoses early in the morning, or drip systems are perfect for providing consistent irrigation. Avoid wetting the foliage during high summer to prevent sun scorch.  Overwintered common fennel does not require regular watering, but do not let the soil dry out.

Soil

Grow fennel in fertile, well-drained, moisture-retentive sandy loam soils, improved with lots of well-rotted organic matter. The soil needs to be friable to allow them to grow well. Do not allow the soil to dry out. Suitable for growing in pH levels of 6–8.

Temperature

They grow best in USDA zones 4-9 with ideal temperatures ranging from 60-70°F (16-21°C). As a perennial, common fennel will overwinter outside but may not survive prolonged freezing temperatures. Summer-sown bulb fennel can be harvested right up until the first frosts. In milder climates, it can be overwintered and treated as a biennial for seed production. Fennel benefits from mulches to retain moisture in summer and heat in winter.

Fertilizing

Apply lots of well-rotted organic matter such as farmyard manure or compost to the planting site at least six weeks before planting. A balanced slow-release fertilizer can be added to the seedbed prior to planting as an extra boost. When bulbs begin to swell, apply a nitrogen-rich liquid fertilizer once every two weeks until harvest. Mulch herb fennel in springtime for added nutrition.

Maintenance

Stems of common fennel can be pruned back to ground level after the seeds are harvested. Herbs that are grown for ornamental use often retain the dried seed heads to provide winter structure in the garden. This can result in self-seeded plants popping up around the garden in spring, but these are easily weeded out. Prune any developing bulb fennel flower spikes to stop them from bolting.  

Propagation

Common fennel: For best results, common fennel seeds should be sown directly into pre-watered drills half an inch deep and rows 12 inches apart after the last frost date and when soil temperatures are a minimum of 50°F (10°C).  When seedlings are two to three inches high, thin them to 18 inches between herbs.  

Mature common fennel may be propagated by division with varied success due to problems with disturbing its taproot. Herbs may be dug up in spring when shoots begin to appear. When removing, ensure the entire root system remains intact. Viable side shoots can be removed from the main crown, potted in a sandy compost mix, and grown in pots until new roots have been established. Once roots have filled the pot, set into a new growing position.

Bulb fennel: Start sowing seeds indoors from mid-spring. Sow into small pots or module trays filled with compost, half an inch deep. Germination usually takes one to two weeks. Sow two seeds per pot/module cell and nip out the weaker seedlings when they are around two to three inches high and ready to set out. Acclimatize young seedlings to outside conditions for about a week before planting them into final growing positions. The space is above for common fennel. Florence fennel seeds may be sown directly in the same way as common fennel.

Harvesting

The leaves, seeds, and fennel bulbs are easy to harvest and store for short and long-term use. Fennel leaves can be harvested throughout the growing season while they are green and fresh. Seeds are ready to harvest when they are brown and dry.  

Cut flower heads from the main stem and place them upside down into a paper bag and hang in a cool, dry room. As the seeds become completely dry, they will drop from the seed head into the bag and are ready for storing. 

Florence fennel can be harvested when bulbs are around four inches across. Cut the bulb away from the root just below soil level. If you leave the root in the ground, it may sprout new greens that can be used similarly to the herb fennel.  Remove the green stems to prevent the bulb from dehydrating, and use the trimmings to flavor soups and stocks.

Storage

Use a damp paper towel or plastic wrap to store herb greens and bulbs in the fridge. They will stay fresh for around five days. The bulb can be sliced, blanched, and frozen to add to cooked dishes at a later date. Seeds may be stored in an airtight container for up to a year.

Common Problems

These herbs are pretty resilient and can resist most garden issues. Still, there are a few tips you’ll need for true success!

Growing Problems

Common fennel is easy to grow and more or less problem-free. Wilting is the main growing problem you might encounter and is easily rectified through adequate irrigation.

Increases in temperature and lack of water will cause bulb fennel to try and flower early. Snip off any flower stalks as soon as they develop and mulch to lock in moisture. 

Keep areas around your crop free from weeds to reduce competition for nutrients, space, and water.

Pests

Slugs and snails attack young fennel seedlings, devouring them entirely and leaving holes in stems, leaving a glistening slime trail as evidence. They mostly feed at night and are more active in damp weather. Reduce slug and snail populations by removing their daytime hideaways and breeding grounds like damp, wet wood and weed matting.

Remove by hand on sight (best results at night) or leave beer or oatmeal traps, which can be collected and discarded in the morning.

Aphids (Aphidoidea) are small and sticky white, yellow, green, and black flies. They feed on the sap of new growth and spoil the edible parts. Treat biologically, through the release of beneficial insects that feed on aphids, such as ladybug larvae (Coccinella septempunctata). Or, spray with a good organic insecticidal soap or neem oil. Squishing aphids with fingers or a quick blast of water can help reduce numbers. 

Diseases

Powdery mildew is a fungal disease that affects fennel grown in hot, humid, shaded conditions.  It grows as a white, thick dust on leaves, inhibiting photosynthesis and hindering growth. Foliage eventually turns yellow and dies. To prevent powdery mildew, maintain good garden hygiene, removing infected foliage to prevent the disease from spreading and reinfection in subsequent years. Provide adequate sunlight and avoid conditions the disease thrives on. Provide adequate space to allow good air circulation. Spray with an organic neem oil at the first sight of disease.

Frequently Asked Questions