Why Is My Thyme Turning Brown and Curling?

Waterlogging and root rot

All commonly grown UK garden thymes are extremely intolerant of persistently wet, poorly draining, cold soil. Root rot from Pythium and Phytophthora progresses very rapidly in thyme because of the plant's small size and limited root biomass. Leaves turn from grey-green to dull greyish then brown; stems die back from the outer branch tips inward; the plant collapses. Root rot-driven decline is significantly faster in thyme than in lavender or rosemary.

What to do

• Improve drainage radically: incorporate very large quantities of coarse horticultural grit (at least 50% grit by volume in the planting hole); or grow in a raised bed of lean, gritty mix; or in terracotta containers of gritty free-draining compost with large drainage holes; apply a top dressing of horticultural grit around the crown of the plant rather than organic mulch to keep the crown dry and improve drainage at the critical soil-surface level; choose a south or west-facing, fully open, sunny position.

Old age and woody decline

Thyme is shorter-lived than lavender or rosemary: most culinary thyme plants have a productive lifespan of only three to five years before the centre becomes increasingly woody, bare, and non-productive. Regular annual post-flowering pruning delays this but does not prevent it. Eventually replacement is the correct response.

What to do

• Take softwood cuttings from healthy plants in June to July (7 to 10 cm stem tip cuttings below a node, stripped of lower leaves, in perlite or gritty cuttings compost; root in three to five weeks) to have fresh young replacements ready for autumn planting; prune annually immediately after flowering by cutting back the flowered growth by one-third to one-half into the current year's soft green growth; never cut into old hard bare woody stems; replace old plants every three to five years with fresh young specimens.

Rosemary beetle damage

Chrysolina americana (rosemary beetle; iridescent metallic green and purple striped adult, 6 to 8 mm) attacks thyme alongside rosemary, lavender, and sage. Both adults and larvae feed on leaves and tender stems from late summer through winter and into spring, causing significant defoliation and die-back in heavy infestations on thyme plants.

What to do

• Check plants regularly from late summer to spring; hold a sheet of paper or tray under the plant and shake so beetles fall onto the sheet; collect and dispose of in soapy water; repeat regularly; the metallic green and purple striped adults are unmistakable once seen; apply pyrethrin-based insecticide spray to coat leaves and stems in the evening if needed; repeat applications necessary.

Drought stress in containers

Thyme is very drought-tolerant in the ground in free-draining soil. In a very small container with limited compost volume or in a very hot, exposed position in the hottest part of a UK summer, the leaves may curl, dry, and become crisp from drought stress. Much less common than waterlogging as a cause of problems but occasionally encountered in container thyme.

What to do

• Use a sufficiently large container; water when the top few centimetres of compost feel completely dry in hot summer weather; do not allow to dry to the point that the compost shrinks away from the container sides; reduce watering significantly in autumn and eliminate it in winter; do not sit containers in saucers of water at any time.

Frequently asked questions

Why are my thyme leaves curling?

Thyme leaves curl and turn brown most commonly because of waterlogging and root rot (all commonly grown thymes extremely intolerant of persistently wet poorly draining cold soil; Pythium and Phytophthora root rot progresses very rapidly in thyme given the plant's small size; leaves turn grey-green then dull greyish then brown; stems die back from outer branch tips inward; improve drainage radically with 50% grit by volume; or raised bed; or terracotta container with gritty free-draining compost; grit top-dressing around crown rather than organic mulch; south or west-facing fully open sunny position), old age and woody decline (productive lifespan only 3 to 5 years; centre becomes woody bare and non-productive; take softwood cuttings from healthy plants in June to July; replace old plants every 3 to 5 years; prune annually after flowering into soft green current-year growth only; never into old hard bare woody stems), rosemary beetle damage (Chrysolina americana; iridescent metallic green and purple striped adult beetles 6 to 8 mm; adults and larvae feed on leaves and stems from late summer through winter and spring; check and shake onto paper to collect; dispose of in soapy water; pyrethrin spray in the evening), or drought stress in containers (very small container or very hot exposed position; much less common than waterlogging; water when top few centimetres of compost feel completely dry in hot weather; reduce watering significantly in autumn; eliminate in winter; do not sit in saucers of water).

How do I grow thyme successfully in UK conditions?

Perfect drainage: must have excellent free-draining soil; one of the most drainage-sensitive plants commonly grown in UK gardens; on heavy UK clay soil incorporate very large quantities of coarse horticultural grit (at least 50% grit by volume); or grow in raised bed of lean gritty mix; or in terracotta containers with large drainage holes; grit top-dressing around crown (not organic mulch) keeps crown dry and improves drainage at the critical soil-surface level. Full sun: requires full sun for as much of the day as possible; in shade plants become drawn and weak; leaves lose characteristic intense aroma and flavour; more susceptible to disease and rot. Lean soil: poor to moderate fertility; do not feed with high-nitrogen fertiliser; feeding promotes leaf growth but reduces aromatic oil concentration. Annual post-flowering pruning: prune annually immediately after flowering (typically June to July) by cutting back flowered growth by approximately one-third to one-half into current year's soft green growth; never cut into old hard woody bare stems; delays onset of inevitable woodiness and extends useful life.

Which thyme varieties are best for UK gardens?

For culinary use: Thymus vulgaris (common thyme, garden thyme); the standard culinary thyme for UK kitchens; most intense classic thyme flavour and aroma; cultivars include 'Compactus' (more compact), 'Erectus' (more upright), 'Silver Posie' (white-edged variegated leaves; good ornamental and culinary). For lemon flavour: Thymus citriodorus (lemon thyme); cultivars 'Aureus' (golden-variegated; very ornamental) and 'Bertram Anderson' (gold-margined). For ornamental ground cover: Thymus serpyllum (wild thyme, creeping thyme); forms prostrate mat-forming spreading carpet; cultivars include 'Coccineus' (rich magenta-crimson flowers), 'Pink Chintz' (pale pink; hairy grey leaves), 'Annie Hall' (pale lilac-pink), 'White Magic' (white). For gravel or dry garden: T. serpyllum and T. pseudolanuginosus (woolly thyme; very low-growing pale silvery-grey woolly leaves); ideal in gaps in gravel path or between paving; tolerates light foot traffic; releases fragrance when brushed. Variegated and coloured-leaf cultivars: slightly less vigorous and robust than plain-leaved species; require sheltered warm sunny position with excellent drainage.

How often should I replace thyme plants?

Typical productive lifespan: 3 to 5 years under ideal UK growing conditions (full sun, excellent drainage, lean soil, annual post-flowering pruning); in less than ideal conditions (heavy soil, not perfectly drained, less than full sun) often only 2 to 3 years. Signs replacement is needed: centre of plant increasingly bare woody and dead or dying; only outer stem tips producing vigorous new growth; overall habit open sprawling and hollow-centred; plant fails to respond to pruning with vigorous new growth from lower down the stems; total quantity of harvestable leaf has declined significantly. When to propagate replacements: early summer (June to July) from current year's soft non-flowering green stem tips; softwood cuttings 7 to 10 cm long just below a node; stripped of lower leaves; in perlite sharp sand or very gritty cuttings compost; root in 3 to 5 weeks; pot on into small pot of gritty compost; plant out in autumn or the following spring; 3 to 6 cuttings from a healthy plant provides reliable supply of fresh young replacement plants.