Why Are My Oregano Leaves Curling?

Spider mite

Two-spotted spider mite (Tetranychus urticae) is the most common pest of oregano grown under cover or in warm, sheltered UK garden positions. The mites are too small to see with the naked eye but their feeding damage is distinctive: a pale, bronze-green, mottled stippling on the upper surface of the leaves, caused by the mites piercing individual cells and extracting the contents. In heavier infestations, very fine silky webbing becomes visible on the underside of the leaves and between stems. The leaves curl, yellow, and eventually drop. Spider mites thrive in hot, dry conditions and are most common on oregano during July and August, particularly on plants growing in pots or under glass.

What to do

• Inspect the underside of oregano leaves from June for the earliest signs of spider mite: tiny pale or yellowish specks (the mites themselves and their eggs) and the characteristic stippled, bronze mottling on the leaf surface.
• Increase humidity around affected plants: spider mites cannot thrive in genuinely humid conditions. Mist the leaves with water daily or place pots on a tray of damp gravel. Move oregano out of sheltered, hot, dry positions if possible.
• The predatory mite Phytoseiulus persimilis is available as a biological control and provides excellent spider mite control when introduced early in the season before populations build up. It is particularly useful for oregano grown under cover in a greenhouse or conservatory.
• Insecticidal soap or a plant oil spray provides contact control of spider mites when applied to the underside of leaves, covering all surfaces thoroughly. Repeat every 5 to 7 days.

Overwatering

Overwatering is the most common way to kill oregano in UK gardens and the most frequent cause of yellow, wilting, curling leaves on plants grown in pots. Oregano is native to the dry, rocky hillsides of the Mediterranean and has evolved for conditions of summer drought and well-drained, often poor soil. Its roots are adapted to periods of dryness and begin to rot in persistently moist or waterlogged soil, leading to root rot and the rapid deterioration of the above-ground growth: yellowing leaves that wilt, curl, and eventually drop.

What to do

• Water oregano only when the top 2 to 3 centimetres of compost or soil are completely dry. Between waterings, the plant should be left to dry out rather than receiving frequent light irrigation.
• Ensure oregano in pots has adequate drainage holes and is planted in free-draining compost: a mix of regular compost and horticultural grit (approximately 50:50) is ideal for oregano in containers.
• Garden-grown oregano on heavy clay soil benefits from the addition of coarse grit at planting to improve drainage, and should be planted on a slight mound or raised position where water does not pool.
• If root rot has already set in, check whether any healthy root tissue remains: if so, repot into fresh, very free-draining compost, remove any dead, blackened roots, and allow the compost to dry out completely before the next watering.

Aphids

Greenfly and blackfly colonise oregano growing tips in spring and early summer, causing the young leaves to curl inward around the feeding colonies. Aphids on oregano are usually controlled quickly by natural predators, particularly ladybirds and hoverflies that are attracted to the oregano flowers later in the season.

What to do

• Inspect oregano growing tips from May for aphid colonies. Small colonies can be squashed by hand or removed with a jet of water. Aphid infestations on established oregano plants rarely require insecticidal treatment as natural predators typically arrive quickly once the aphid colony establishes.

Powdery mildew

Powdery mildew on oregano produces white, powdery patches on the leaves, particularly in late summer when warm days and cool nights create ideal conditions for the fungus. Affected leaves curl slightly and may yellow around the white patches. Powdery mildew on oregano is favoured by dry soil conditions combined with humid air, and is more common on plants growing in hot, sheltered positions.

What to do

• Remove and destroy affected leaves. Cut the plant back hard to promote fresh, clean growth. Improve airflow around the plant. Watering the soil (not the leaves) during dry spells reduces the root stress that makes plants more susceptible to powdery mildew, while not creating the leaf wetness that promotes other fungal problems.

Cold damage

Oregano leaves turn purple, then brown, and collapse in hard frosts. Common oregano (Origanum vulgare) is fully hardy in the UK and recovers well from winter die-back, re-shooting from the woody base each spring. Greek oregano is slightly less frost-tolerant and the top growth may be killed entirely in a cold winter, though the roots survive and re-shoot if not waterlogged. Winter waterlogging is more damaging than cold temperatures alone.

What to do

• Cut oregano stems back to 5 centimetres above the ground after the first frosts. This is not essential for garden-grown common oregano but improves the appearance of the plant and encourages vigorous new growth from the base in spring.
• In pots, ensure drainage is excellent over winter: move pots against a wall or under cover if very prolonged heavy rain is forecast, to prevent waterlogging of the root system.

Frequently asked questions

Why are my oregano leaves curling?

Oregano leaves curl most often from spider mite infestation, overwatering, or aphids. Spider mites (Tetranychus urticae) cause a characteristic pale, bronze, mottled stippling on the leaves with fine silky webbing on the underside; affected leaves curl and yellow as the infestation progresses. Overwatering causes root rot, which manifests as yellowing, wilting, and curling of the leaves from the base upward. Oregano is a Mediterranean herb evolved for dry conditions and is far more tolerant of drought than of wet soil. Aphids colonise the growing tips in spring and cause the young leaves to curl inward.

How often should I water oregano?

Oregano needs far less water than most garden herbs. Established oregano in garden soil can go 2 to 3 weeks without watering in dry weather and prefers to dry out between waterings. The most common cause of oregano dying or producing yellow, wilted leaves in UK gardens is overwatering, not drought. Water oregano only when the top 2 to 3 centimetres of soil are completely dry, then water thoroughly and allow to dry out again before the next watering. In pots, ensure the container has drainage holes and never allow water to sit in a saucer beneath the pot. Garden-grown oregano on well-drained soil rarely needs watering at all once established beyond the first few weeks after planting.

Is Greek oregano stronger than common oregano?

Yes. Greek oregano (Origanum vulgare subsp. hirtum) has a much stronger, more pungent, intensely aromatic flavour than common oregano (Origanum vulgare), which is the wild form native to the UK. Greek oregano has smaller, hairy, white-green leaves and is the variety used in Mediterranean cooking and dried as the oregano sold in supermarkets. Common UK oregano (sometimes called wild marjoram) has broader, flatter, slightly less aromatic leaves and a milder, sweeter flavour better suited to fresh use. For cooking purposes, Greek oregano is almost always the better choice; for ornamental use in a herb garden, common oregano is more attractive and easier to grow in UK conditions.

How do I overwinter oregano in the UK?

Common oregano (Origanum vulgare) is fully hardy in the UK and overwinters in the ground without any protection in most areas: it dies back to a woody base and re-shoots vigorously from the crown each spring. Greek oregano (Origanum vulgare subsp. hirtum) is slightly less hardy and benefits from protection in cold areas: cut the stems back to about 5 centimetres from the ground in late autumn and cover the crown with a layer of grit or coarse compost to protect the root crown from waterlogging. In very cold areas or in pots, move Greek oregano under cover for winter. The main winter risk for oregano in the UK is not cold but waterlogging: wet, heavy soils kill oregano more reliably than frost.