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Growing Fennel: Fronds, Bulbs, And Seeds

Fennel is an attractive plant 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 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.

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Quick Care Guide

Growing fennel
Growing fennel provides you with more than just herbs! Source: jorge zapico
Common Name(s)Sweet or common fennel; Florence fennel, bulb fennel, finocchio 
Scientific NameFoeniculum vulgare; Foeniculum vulgare var. azoricum
Days to HarvestHerb 60-70 days; bulb 80-115 days 
LightFull sun
Water:Regular, consistent
SoilSandy loam
FertilizerYoung plants: balanced NPK. Maturing bulbs: high nitrogen feed
PestsSlugs, snails, aphids
DiseasesPowdery mildew

All About Fennel

Fennel herb
Fennel fronds are popular herbs used in cooking. Source: dichohecho

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. Fennel plants originate from the sunny coastal regions of southern Europe and the Mediterranean. 

Bulb fennel was first introduced in Italy in the 17th 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 12inches (30cm) in length. 

In mid to late summer common fennel develops large flat flower heads of compound umbels at the end of stems, 4inches (10cm) 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, 3-4 inches (7-10cms) across, and grows to an average stocky height of 2-3ft (60-90cms).  Herb fennel grows to the majestic height of 6ft (1.8cm) with thicker sturdier stems and no bulb.

The bulb, herb greens, and seeds can be used as a garnish, in salads, bread, stews, 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 is not invasive.  Common fennel is a prolific self-seeder and can become quite a nuisance. In domestic settings, seedlings are easily weeded and flowers deadheaded. However, in favorable climates such as in California, fennel plants are deemed as an invasive species.

If you are short of space, grow fennel in large, deep pots filled with lots of organic matter.  In fact, container growing is highly recommended as fennel is an allelopathic plant which means it can inhibit the growth of neighboring plants, in particular beans, tomatoes, and some brassicas.  

Varieties to try:

  • 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

Planting

Buds of fennel flowers
From the buds of the fennel flowers emerge the seed when developed. Source: tinkiak

The key to growing fennel is the consistency of temperature, sunlight, and moisture.  Plant 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 somewhere with a Mediterranean type climate, you can start sowing seeds under cover in mid-spring. Seedlings should be ready for planting out after 4-5 weeks when they are 2-3inches tall and all risk of frost has passed.  Space plants 12 inches apart (30cm), and 18inches (45cm) between rows.  The crop should be ready for harvest from mid-summer onwards.  Seeds may also be sown directly when soil temperatures are a minimum of 50ºF (10ºC). 

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 of the plants 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.

Fennel Care

Florence fennel
The fronds, bulbs, and seeds of fennel are all edible. Source: willsong

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.

Sun and Temperature

Both types of fennel require a minimum of 6-8 hrs of direct sunlight per day. They will not tolerate shade so be careful not to overshadow with another crop in your garden.  They grow best in USDA zones 6-10 with ideal temperatures ranging from 60-70ºF (15-21ºC).  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.  Plants benefit from mulches to retain moisture in summer and heat in winter.

Water and Humidity

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. Do not allow the soil to dry out.  Suitable to grow in pH 6–8.

Fertilizing

Apply lots of well-rotted organic matter such as farm-yard 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.

Pruning

Stems of common fennel can be pruned back to ground level after seeds are harvested. Fennel plants 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 ½inch (1½cm) deep and rows 12 inches apart (30cm) after the last frost date and when soil temperatures are a minimum of 50ºF (10ºC).  When seedlings are 2-3inches (5-8cm) high, thin to 18inches (45cm) between plants.  

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

Bulb fennel: Start sowing seeds indoors from mid-spring. Sow into small pots or module trays filled with compost, ½inch (1½cm) deep.  Germination usually takes 1-2 weeks.  Sow two seeds per pot/module cell and nip out the weaker seedling when they are around 2-3 inches high (5-8cms) and ready to plant out.  Acclimatize young plants to outside conditions for about a week before planting into final growing positions.  Space plants as above for common fennel. Florence fennel seeds may be sown directly in the same way as common fennel.

Harvesting and Storing

Fennel flowers
Fennel flowers are not distinctive, but pretty nonetheless. Source: tinkiak

The leaves, seeds, and fennel bulbs are easy to harvest and store for short and long term use.  

Harvesting

Herb fennel leaves can be harvested throughout the growing season while 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 4inches (10cm) 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 similar to herb fennel.  Remove the green stems to prevent the bulb from dehydrating and use the trimmings to flavor soups and stocks.

Storing

Use a damp paper towel or plastic wrap to store herb greens and bulbs in the fridge.  They will stay fresh for around 5 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.

Troubleshooting

Bulb fennel
The large bulb of a bulb fennel is a culinary delight. Source: bourgeoisbee

These plants are pretty resilient and can resist most garden issues. Still, there’s 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.  As a last resort use organic slug/snail pellets.  Read the label carefully to ensure they will not harm other wildlife.

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 (cococinella 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 can affect 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 fungicide such as sulfur, neem oil, or potassium bicarbonate, prior to or on first sight of disease.

Frequently Asked Questions

Q: Does fennel come back every year?

A:  Common fennel is a herbaceous perennial and will grow back each year.  Bulb fennel is treated as an annual vegetable and is harvested when the bulbs are the size of a tennis ball.

Q: What can you not plant near fennel?

A:  Beans, tomatoes, and some brassicas should not be located next to a fennel plant.