Why Are My Marjoram Leaves Curling?

Overwatering

Overwatering is the most common cause of marjoram leaf problems in UK gardens. Sweet marjoram evolved in the dry, rocky conditions of the Mediterranean basin and its roots are poorly adapted to persistently wet soil. When the soil or compost remains wet for extended periods, the roots begin to rot. The symptoms above ground are yellowing leaves that wilt, curl, and eventually drop. The problem is worst in poorly drained pots and in heavy clay soils, and is more common in the UK's wet autumn and winter than in summer.

What to do

• Water marjoram only when the top 2 to 3 centimetres of compost or soil are completely dry. Never water on a schedule; always check the moisture level first.
• In pots, ensure there are drainage holes and that water cannot accumulate in a saucer beneath the pot. Use a free-draining compost mix of regular compost and horticultural grit (approximately 50:50).
• In the garden, marjoram on heavy clay soil benefits from the addition of coarse grit at planting and should be grown on a slight mound where water drains away from the crown.

Spider mite

Two-spotted spider mite (Tetranychus urticae) attacks marjoram in hot, dry conditions, particularly on plants grown under cover or in sheltered garden positions. The first sign is a pale, bronze, mottled stippling on the upper leaf surface; in heavier infestations, fine silky webbing appears on the underside of leaves. Affected leaves curl, yellow, and drop. Spider mites on marjoram are most common from July to September.

What to do

• Increase humidity around the plant: mist leaves in the morning, place pots on damp gravel trays, or move plants to a less sheltered position. Spider mites cannot thrive in humid conditions.
• The predatory mite Phytoseiulus persimilis is an effective biological control for marjoram under glass. In the garden, natural predators (ladybirds, hoverflies) provide useful control once established.
• Insecticidal soap or plant oil spray applied to the underside of leaves provides contact control. Repeat every 5 to 7 days.

Aphids

Greenfly colonise marjoram growing tips in spring and early summer, causing the young leaves to curl inward around the feeding colonies. Aphid populations on marjoram are usually controlled quickly by natural predators once the herb comes into flower, as the flowers attract large numbers of beneficial insects.

What to do

• Squash small aphid colonies by hand or knock them off with a jet of water. Insecticidal soap is effective for larger colonies. On a culinary herb, avoid systemic pesticides.

Powdery mildew

Powdery mildew on marjoram produces white, chalky patches on the leaves, particularly in late summer. Affected leaves may curl slightly at the edges and yellow around the white areas. Powdery mildew is more common when marjoram is stressed by drought from the roots while humidity is high around the foliage.

What to do

• Remove and destroy affected leaves. Improve airflow around the plant. Water the soil during dry spells to reduce root stress without wetting the foliage. Cut affected plants back hard to encourage fresh, clean growth if the infection is widespread.

Cold damage

Sweet marjoram is killed by hard frosts (below approximately -2 to -3°C). Cold damage causes the leaves to turn dark green, then grey-brown, then collapse. Unlike oregano, which overwinters as a hardy perennial, marjoram does not reliably survive UK winters outdoors without protection.

What to do

• Bring marjoram indoors before the first frosts: move pots to a cool but frost-free windowsill or conservatory. Cut the plant back by about half before bringing it in to reduce the transition stress.
• If growing marjoram in the garden rather than in pots, take cuttings in late summer to overwinter on a windowsill and treat the outdoor plant as a disposable annual.
• In very mild, sheltered positions (southwest England, coastal areas), marjoram may survive a mild winter with the protection of a thick mulch over the crown and horticultural fleece.

Frequently asked questions

Why are my marjoram leaves curling?

Marjoram leaves curl most often from overwatering, spider mite, or aphids. Sweet marjoram (Origanum majorana) is a Mediterranean herb that evolved in warm, dry conditions and is far more sensitive to overwatering than drought. Persistently wet soil causes root rot, which manifests as yellowing, wilting, and curling of the leaves from the base upward. Spider mites cause a pale, bronze, stippled mottling on the leaves with fine webbing on the undersides; the leaves curl and drop as the infestation progresses. Mites are worst in hot, dry summer conditions. Aphids colonise the growing tips and cause young leaves to curl inward around the feeding colonies.

What is the difference between marjoram and oregano?

Sweet marjoram (Origanum majorana) and oregano (Origanum vulgare) are closely related plants in the same genus, and the distinction between them causes considerable confusion. The key differences are: Flavour: marjoram has a sweeter, more delicate, slightly floral flavour that is less pungent than oregano; oregano has a stronger, more robust, often spicy flavour. The two are not interchangeable in cooking, though they are sometimes used together. Hardiness: oregano is fully hardy in the UK and returns reliably from a perennial crown each spring; marjoram is half-hardy (tender perennial) and is killed by hard frosts in the UK, meaning it is typically grown as an annual here. Appearance: marjoram leaves are slightly smaller and more rounded than oregano; the plant is more compact and upright.

Is marjoram a perennial or annual in the UK?

Sweet marjoram (Origanum majorana) is a perennial in its native Mediterranean habitat but is half-hardy and treated as an annual in most of the UK, as it is killed by hard frosts (below about -2 to -3°C). In very sheltered, mild gardens in the south and west of England, marjoram may survive the winter outdoors if cut back and protected with fleece, but it will not survive a typical UK winter without protection. The solution most UK gardeners use is to bring marjoram indoors to a cool but frost-free windowsill for the winter, or to treat it as a half-hardy annual and raise new plants from seed each spring. The related common oregano (Origanum vulgare) is fully hardy in the UK and behaves as a true perennial.

How do I harvest marjoram to keep it producing?

Harvest marjoram regularly from the growing tips rather than cutting stems from the base. Taking 5 to 10 centimetres from each growing tip when the plant is in active growth encourages the plant to branch and produce more stems, extending the harvest period significantly. The best time to harvest for the strongest flavour is just before the plant flowers: marjoram leaves have the highest concentration of aromatic oils at this stage. Once the plant flowers, the flavour of the remaining leaves declines. To delay flowering and extend the leaf harvest, cut off any flower buds as they appear. Marjoram can be harvested more heavily in late summer (taking up to a third of the plant) to provide material for drying, then allowed to produce a flush of new growth before being brought indoors or protected for winter.