Why Are My Fennel Leaves Curling?

Aphids

Carrot-willow aphid (Cavariella aegopodii) and related species from the carrot family (Apiaceae) commonly colonise fennel, clustering at the growing tips and the bases of the feathery leaf fronds. The aphids are often pale yellow or green and may be difficult to see against the fine fennel foliage. Aphid feeding causes the fine leaves to curl and bunch together at infested areas, and the sticky honeydew they secrete promotes sooty mould growth. On Florence fennel, heavy aphid infestations at the growing tip can prevent the bulb from forming properly.

What to do

• Inspect fennel growing tips and leaf bases weekly from May for aphid colonies. Small colonies on herb fennel rarely require action: natural predators, particularly hoverfly larvae and parasitic wasps that are attracted to fennel flowers, control them effectively.
• Apply insecticidal soap spray to larger colonies on Florence fennel, where aphid damage to the growing tip can significantly impair bulb formation. Cover all leaf surfaces thoroughly and repeat every 5 to 7 days.
• Avoid broad-spectrum insecticides on fennel, as the flowers are extremely attractive to beneficial insects and pollinators that would be harmed by them.

Drought

Florence fennel in particular requires consistently moist, never dry, soil to form a good bulb. Drought causes the feathery leaves to droop and curl, and significantly impairs bulb development: drought-stressed Florence fennel produces small, poorly formed bulbs with a more intense, sometimes bitter flavour. Drought also increases the tendency of Florence fennel to bolt prematurely. Herb fennel is more drought-tolerant but performs best in well-watered conditions.

What to do

• Water Florence fennel regularly and consistently from transplanting through to harvest: every 5 to 7 days in dry conditions. Apply a mulch of compost around the plants to conserve soil moisture.
• Fennel prefers a well-drained but moisture-retentive soil. Incorporate compost before planting on free-draining soils. Avoid waterlogged positions, which cause stem and root rot.

Leaf miner

Celery leaf miner (Euleia heraclei) occasionally attacks fennel, as it does other plants in the carrot family. The larvae mine between the leaf surfaces, but on fennel the very fine, feathery leaves mean the damage appears as shrivelled, brown areas rather than the distinct blistered patches seen on broader-leaved crops. Affected leaf fronds curl and dry around the mined areas.

What to do

• Remove and destroy affected leaf fronds promptly. On herb fennel, which produces abundant foliage, leaf miner damage is cosmetic and does not affect the long-term health of the plant.
• Cover Florence fennel transplants with fine insect mesh to prevent leaf miner (and aphid) damage during the growing season.

Bolting

Florence fennel bolts (runs to flower) prematurely when it is sown too early in spring, exposing young plants to the long days and temperature fluctuations of April and May. Once Florence fennel has bolted, no bulb forms: the plant produces a tall, branched flower stem instead. This is the single most common problem with Florence fennel in UK gardens.

What to do

• Sow Florence fennel no earlier than late June to July in the UK for reliable bulb formation. The shortening day length from midsummer onward is essential for the plant to form a bulb rather than bolting directly to flower.
• Sow direct rather than transplanting: Florence fennel dislikes root disturbance and transplanting shock can trigger bolting even in correctly-timed late sowings.
• Choose bolt-resistant varieties such as 'Romanesco' and 'Victorio' for earlier sowings if you wish to extend the season slightly.

Cold damage

Fennel leaves brown and collapse at the tips in cold or frosty conditions, particularly in autumn as temperatures fall. The feathery foliage of Florence fennel is damaged at temperatures below about -2 degrees Celsius. Browning and wilting of the foliage in October and November does not affect Florence fennel bulbs that are ready to harvest; the bulbs are somewhat frost-hardy and can be left in the ground until needed.

What to do

• Harvest Florence fennel bulbs before a hard frost is forecast: damaged foliage is a sign to lift and use the bulbs promptly.
• Herb fennel dies back completely in winter and re-shoots vigorously from the crown the following spring.

Frequently asked questions

Why are my fennel leaves curling?

Fennel leaves curl most often from aphid infestations or drought. The carrot-willow aphid (Cavariella aegopodii) and the willow-carrot aphid are the most common aphids to colonise fennel, clustering at the growing tips and on the underside of the feathery leaves, causing them to curl inward. Florence fennel (the bulbing variety) is also sensitive to drought: insufficient water causes the bulb to develop poorly and the fine, feathery foliage to droop and curl. Fennel leaves also curl and brown in cold, frosty conditions, particularly in autumn when Florence fennel plants are close to harvest.

Why is my Florence fennel not forming a bulb?

Florence fennel fails to form a bulb (bolts without bulbing, or produces only a narrow, scrawny base) most often because it was sown too early in spring, exposing the young plants to cold temperatures that trigger premature bolting. Florence fennel is extremely sensitive to day length and temperature: plants sown before midsummer (before the longest day has passed) in the UK detect the lengthening days of spring and bolt to flower rather than forming a bulb. For reliable bulb formation, sow Florence fennel no earlier than late June or early July, when the days are beginning to shorten. Drought also prevents bulb formation: keep the soil consistently moist throughout the growing season.

Does fennel affect other garden plants?

Fennel (both the herb and Florence fennel) is allelopathic, meaning it produces chemicals from its roots and leaves that inhibit the growth of many neighbouring plants. It is particularly antagonistic to tomatoes, peppers, courgettes, and most brassicas. In a kitchen garden, fennel is best grown in a pot or in an isolated bed well away from other vegetable crops. Herb fennel allowed to spread and self-seed can suppress neighbouring plants significantly over several seasons. Despite its allelopathic tendencies, fennel is highly attractive to beneficial insects, particularly hoverflies, parasitic wasps, and lacewings, when in flower.

When do I harvest Florence fennel?

Florence fennel bulbs are harvested when they reach the size of a tennis ball to a cricket ball: approximately 7 to 10 centimetres across. Cut the bulb at soil level with a sharp knife, leaving the root base in the ground; this often produces secondary shoots (fennel tops) that can be used as a herb. Do not leave Florence fennel bulbs in the ground too long: they become stringy, pithy, and more strongly flavoured with age. In the UK, fennel sown in late June or July is typically ready to harvest from August to October. The fine, feathery foliage can be harvested and used as a herb at any point throughout the growing season.