Why Are My Spring Onion Leaves Curling?

Onion fly

Onion fly (Delia antiqua) is the most serious pest of spring onions in UK gardens. The adult fly resembles a small grey housefly and lays eggs at the base of young plants in spring and early summer. The white larvae tunnel into the base of the stem and developing bulb, causing the leaves to yellow and wilt from the base upward. Affected plants pull from the ground easily, revealing a rotten, maggot-infested base. Spring onions are more susceptible than bulbing onions because their stems are thinner and the larvae can cause complete collapse quickly.

What to do

• Cover spring onion sowings with fine insect mesh immediately after sowing, securing all edges to the ground to prevent adult flies from reaching the soil surface. This is the single most effective preventive measure.
• Remove thinnings from the growing area promptly: the smell of crushed onion foliage attracts egg-laying females from considerable distances.
• Sow spring onions after mid-June when the main onion fly egg-laying period (late April to June) has passed: later sowings are substantially less affected.
• Rotate allium crops each year. Onion fly pupae overwinter in the soil of previously infected beds, and returning alliums to the same ground builds up the soil population over time.

Downy mildew

Downy mildew (Peronospora destructor) on spring onions produces pale, oval or elongated patches on the leaf surface, with a grey-purple, furry fungal coating visible on the patches in humid conditions. Affected areas yellow and the leaf may droop or curl around the infection site. The disease progresses rapidly in cool, humid, overcast conditions and is most common in spring and early summer and in autumn sowings. Dense sowings with poor airflow are most susceptible.

What to do

• Sow spring onions at the correct density: thinly enough that the emerging plants do not overlap or compete, as overcrowding creates the humid microclimate that favours downy mildew.
• Remove and destroy affected plants promptly to reduce spore spread within the sowing.
• Rotate allium crops each year. The downy mildew oospores can persist in the soil and on crop debris for several seasons.

Onion thrips

Onion thrips (Thrips tabaci) rasp the surface cells of spring onion leaves, producing a fine, silvery-white streaking and stippling on the surface of the hollow tubes. In heavy infestations the leaves may curl or kink at the damage sites, and the overall appearance of the plant becomes bleached and papery. Thrips are tiny (less than 2 millimetres), pale, and difficult to see; look for them in the shelter of the base of the leaf tube where it meets the next leaf. Thrips populations peak in hot, dry summer conditions.

What to do

• Water spring onions consistently during hot, dry spells: thrips thrive in drought-stressed plants and populations build fastest in dry conditions.
• Apply insecticidal soap spray in the early morning when thrips are less active, directing the spray into the base of the leaf tubes where the insects shelter. Repeat every 5 to 7 days.

Onion white rot

Onion white rot (Sclerotium cepivorum) causes yellowing and collapse of spring onion leaves combined with white, fluffy fungal growth at the base of the plant and on the bulb. The disease is persistent: the tiny black sclerotia that develop in the white mycelium can survive in the soil for 20 years or more, making affected beds unsuitable for all allium family crops for the very long term.

What to do

• Remove and destroy infected plants immediately, removing as much surrounding soil as possible with each plant.
• Avoid growing any allium crops (spring onions, garlic, leeks, onions, chives) in confirmed white rot beds. The sclerotia are activated by the root exudates of allium plants, and growing alliums in infected soil increases the active spore count over time rather than reducing it.

Drought

Spring onions are shallow-rooted and dry out quickly, particularly when sown in light, free-draining soil. Drought causes the leaf tips to yellow and the leaves to droop, kink, or lose their upright habit. Drought-stressed spring onions also become pungent and harsh-flavoured rather than the mild, sweet flavour expected at harvest.

What to do

• Water spring onions regularly in dry weather, keeping the soil consistently moist. Light, frequent watering is more appropriate for shallow-rooted spring onions than the deep, infrequent watering suited to root vegetables.
• Sow into well-prepared, moisture-retentive soil with plenty of incorporated organic matter. Spring onions sown into compacted or very sandy soil have shallow, inefficient root systems and are more susceptible to drought stress.

Frequently asked questions

Why are my spring onion leaves curling and yellowing?

Spring onion leaves curl and yellow most often from onion fly, downy mildew, or onion thrips. Onion fly (Delia antiqua) lays eggs at the base of the plants and the larvae tunnel into the bulb and stem, causing the leaves to yellow and collapse from the base upward; affected plants pull from the ground easily with a rotten base. Downy mildew (Peronospora destructor) produces pale oval patches on the leaves with a grey-purple downy coating, causing yellowing and curling. Onion thrips (Thrips tabaci) cause silvery-white streaking and stippling on the leaves, with curling in heavy infestations.

How do I protect spring onions from onion fly?

Onion fly (Delia antiqua) is the most damaging pest of spring onions and is very difficult to control once larvae have entered the plant. Prevention is far more effective than treatment. Cover spring onion sowings with fine insect mesh immediately after sowing, securing the edges at ground level to prevent the adult flies from reaching the soil to lay eggs. The main egg-laying period is from late April to June; sowings made after mid-June are less affected. Remove thinnings promptly and away from the growing area: the smell of crushed onion foliage and roots attracts egg-laying females from a distance. Do not grow onion family crops on the same ground in successive years.

How often should I sow spring onions for a continuous harvest?

Spring onions can be sown every 3 to 4 weeks from February (under cover) through to September for a near-continuous harvest from April to November. Successional sowing is the key to avoiding a glut: a single large sowing produces all the spring onions at once, and they deteriorate rapidly once ready. Small sowings every few weeks provide a steady supply of pencil-thickness onions at the perfect stage. 'White Lisbon' is the most widely grown UK variety and is available as a 'winter-hardy' form that can be sown in late summer for an overwintered crop ready from March.

When are spring onions ready to harvest?

Spring onions are ready to harvest when the stem at the base is approximately pencil thickness and the green tops are 15 to 20 centimetres long. This is typically 8 to 10 weeks after sowing in warm conditions, or longer in cool spring and autumn weather. Harvest by pulling individual plants from the row as needed rather than cutting: pulling allows the surrounding plants more space to develop. Spring onions become tough and pungent if left too long after reaching harvestable size, so check the crop regularly and harvest promptly.