Why Are My Oregano Leaves Curling?

Waterlogging and root rot

The primary cause of origanum decline in heavy UK soils; like lavender, thyme, and rosemary, oregano is native to dry, free-draining, calcareous Mediterranean hillsides and extremely intolerant of persistently wet, cold, oxygen-depleted soil. Leaves turn from healthy green or grey-green to a dull yellowish-green then yellow or pale brown; plant progressively loses vigour; root rot can progress rapidly in oregano due to its small fibrous root system.

What to do

• Improve drainage radically before planting: incorporate very large quantities of coarse horticultural grit (40 to 50% grit by volume in the planting hole); or grow in a raised bed of gritty lean compost; or in terracotta containers of gritty free-draining compost with large drainage holes; choose a south-facing, open, fully sunny position; do not water established in-ground plants in autumn, winter, or spring; keep mulch away from the main stems and crown.

Aphid infestations

Aphids attack the young, tender shoot tips of oregano in spring and early summer from April to June; feeding causes the young leaves to curl, pucker, distort, and become stunted. Small, soft-bodied aphid colonies are typically visible on the shoot tips and on the undersides of the curled young leaves.

What to do

• Squash or rub off aphid colonies with fingers, or knock off with a jet of water; for culinary oregano, hand control is strongly preferred over insecticide sprays; tolerate for a few days initially to allow natural predators (ladybirds, lacewing larvae) to colonise; the aromatic oils in oregano leaves may deter some aphid species but tender young growth is still susceptible; check shoot tips regularly from April to June.

Old age and woody decline

Like lavender and thyme, origanum becomes increasingly woody, sprawling, and less productive at the base over three to five years. The leaves become sparse and the plant has a leggy, untidy appearance. Annual post-flowering pruning delays this but does not prevent it.

What to do

• Prune annually after flowering (August to September) by cutting back the flowered growth by one-third to one-half into current year's soft green growth; never cut into old, hard, bare, woody stems below visible new green growth; divide clump-forming varieties in spring every two to three years to refresh vigour; take softwood cuttings in June to July for a supply of fresh young replacement plants; replace old, woody plants with young specimens every three to five years.

Drought stress in containers

Particularly common with supermarket herb tray plants that come in very small, undersized pots; these need immediate repotting into a larger, more gritty container. In a very hot, exposed position in a UK summer the leaves may curl inward, dry, and the plant looks parched. Much less common than waterlogging as a cause of problems in garden or border positions.

What to do

• Repot supermarket herb tray plants immediately into a larger pot (at least 15 to 20 cm diameter) with a half-compost, half-grit mix; water when the top few centimetres of compost feel dry; do not allow to dry out completely in very hot weather; do not over-water; reduce watering significantly in autumn and eliminate it in winter.

Frequently asked questions

Why are my oregano leaves curling?

Oregano leaves curl most commonly because of waterlogging and root rot (primary cause in heavy UK soils; like lavender thyme and rosemary oregano is native to dry free-draining calcareous Mediterranean hillsides and extremely intolerant of persistently wet cold soil; leaves turn dull yellowish-green then yellow or pale brown; improve drainage radically with very large quantities of coarse horticultural grit or raised bed or terracotta containers; south-facing open fully sunny position; do not water established in-ground plants in autumn winter or spring), aphid infestations on young shoot tips (aphids attack young tender shoot tips from April to June; leaves curl pucker distort and become stunted; aphid colonies visible on shoot tips and undersides of curled leaves; squash by hand or knock off with water jet; hand control preferred for culinary oregano), old age and woody decline (productive lifespan 3 to 5 years; becomes increasingly woody sprawling and less productive; prune annually after flowering into current year's soft green growth only; divide clump-forming varieties in spring every 2 to 3 years; take softwood cuttings in June to July; replace old plants every 3 to 5 years), or drought stress in containers (particularly common with supermarket herb tray plants in very small undersized pots; repot immediately into larger pot with half-compost half-grit mix).

What is the difference between oregano and marjoram?

Origanum vulgare (common oregano, wild marjoram): most widely grown and most cold-hardy species for UK gardens; fully hardy throughout the UK; relatively mild aroma in the species plant grown in UK climate; most intensely flavoured oregano for culinary use is Origanum vulgare subsp. hirtum (Greek oregano, pizza oregano, rigani) or O. x majoricum (hardy marjoram, Italian oregano); both more intense than plain species plant; fully hardy for UK outdoor growing. Origanum majorana (sweet marjoram, knotted marjoram): more tender species; milder sweeter more delicate flavour and aroma; not reliably hardy in the UK; typically grown as a half-hardy annual in UK gardens (sown from seed in spring indoors, planted out after last frost; dies in UK winter frosts in most regions). Origanum x majoricum (hardy marjoram, compact oregano, Italian oregano): hybrid between O. majorana and O. vulgare; excellent culinary flavour closer to sweet marjoram; better cold-hardiness than O. majorana; may survive mild UK winters in sheltered free-draining position but not as reliably hardy as O. vulgare; more compact and neat-growing. Ornamental origanum: Origanum laevigatum 'Herrenhausen', O. 'Kent Beauty', O. 'Rosenkuppel' popular UK ornamental varieties with attractive pendant hop-like pink or purple bracts around flowers.

How do I grow oregano successfully in UK gardens?

Free-draining soil: same drainage requirements as lavender and thyme; in heavy UK clay soil incorporate very large quantities of coarse horticultural grit (40 to 50% grit by volume); or grow in a raised bed of gritty lean compost; or in terracotta containers with large drainage holes. Full sun: requires full sun for as much of the day as possible for most intensely aromatic and flavourful leaves; in shade growth is leggy and weak; south-facing open sunny position is ideal. Lean soil: poor to average fertility lean soil; in very rich heavily composted high-nitrogen soil growth is very lush but flavour and aroma significantly diluted; do not fertilise with high-nitrogen fertiliser. Annual pruning: prune after flowering (August to September) by cutting back flowered growth by one-third to one-half into current year's soft green growth; NEVER cut into old hard bare woody stems below visible new green growth; light tidying cut in spring. Division: divide clump-forming varieties in spring every 2 to 3 years to maintain vigour.

How do I harvest oregano for cooking?

Best time to harvest: flavour and aroma most intense just before or at flowering when essential oil content of leaves is at peak; in UK gardens this is typically June to August; this is the best period for flavour and also for drying. How to harvest: cut 5 to 10 cm lengths of soft green current-year shoot growth; cut just above a pair of leaves or a leaf node; regular harvesting of shoot tips throughout the growing season (April to September) keeps plant compact and productive; harvest little and often rather than stripping a plant bare. For drying: harvest shoots of 15 to 20 cm just before flowering when flower buds beginning to form; tie into small bunches and hang upside down in a warm dry well-ventilated position out of direct sun; leaves dry in approximately 2 weeks; strip dried leaves from stems; store in a sealed jar out of direct sunlight; retains good flavour for 6 to 12 months. Quantity: never strip more than one-third of the total shoot growth in any single harvest; leaving at least two-thirds of the green leafy growth allows rapid regrowth.