Plant problems

Why Is My Rosemary Turning Grey and Curling?

Q: Should I prune rosemary and when?

Pruning rosemary correctly and at the right time helps maintain a compact, well-shaped, productive plant and extends its ornamental and culinary usefulness for as long as possible in UK gardens. When to prune: the best time to prune rosemary in the UK is immediately after flowering, typically in April to May for the main spring-flowering period; a second, lighter trim can be made in early autumn (late August to September) if needed to maintain shape; avoid pruning in late autumn or winter as this stimulates vulnerable new growth at a time when cold and wet conditions can cause frost and disease damage to the fresh growth; avoid pruning during periods of hard frost. How to prune: cut back the flowered growth by approximately one-third to one-half of the current year's soft green growth; cut back into the current year's green growth; never cut into the old, hard, grey, woody stems at the base of the plant below the level of any visible green leaf growth; cutting into old, grey, leafless woody stems almost always results in that section of the plant failing to regrow; rosemary has very limited capacity to regenerate from old, hard, leafless woody wood; this is the same rule as for lavender pruning. Harvesting for culinary use: light, regular harvesting of shoot tips for culinary use throughout the growing season (March to September) is beneficial and acts as a mild, continuous form of pruning that helps maintain the bushy shape of the plant; harvesting should be confined to the soft green current-year growth; do not strip whole branches or cut back into woody material during culinary harvesting. Shaping and training: upright rosemary cultivars can be trained into a compact, rounded, dome-shaped bush by regular clipping after flowering; standard (lollipop) forms can be trained by removing all side shoots on a single leading stem to the desired trunk height; prostrate cultivars are best left to cascade naturally over a wall or bank without heavy pruning.

Rosmarinus officinalis (Salvia rosmarinus; rosemary) is a Mediterranean shrub that needs perfectly drained, dry, sunny conditions to thrive in the UK. Leaves curl, turn grey-brown, and growth dies back most often from waterlogging and root rot in wet UK soils (by far the most common cause of rosemary death), from frost and cold stress in severe UK winters, from rosemary beetle feeding, or from drought stress in containers.

Waterlogging and root rot

The primary cause of rosemary death in UK gardens; rosemary is native to dry, rocky, freely draining Mediterranean hillsides and is extraordinarily intolerant of persistently wet, cold, oxygen-depleted soil. Leaves turn from normal dark green to dull grey-green, then silvery-grey or pale brown; needles become dry and sparse; the plant progressively loses vigour. Roots are typically dark brown to black and mushy by the time foliar symptoms are very obvious, at which point recovery is rarely possible.

What to do

On heavy clay or loamy UK soils, improve drainage radically with very large quantities of coarse horticultural grit (40 to 50% grit by volume in the planting hole) or grow in a raised bed or large terracotta container of gritty, free-draining compost; choose a south or west-facing, sunny, open position; do not water established in-ground plants in autumn, winter, or spring; keep mulch away from the main stems and crown to prevent persistent moisture at the stem base.

Frost and cold stress

Hardy to approximately -10 to -15°C in a sheltered, dry, freely draining position; the combination of wet soil and frost is significantly more damaging than dry soil and the same temperature. After hard frost, leaves may turn grey-brown and dry, growing tips die back, and in severe cases all above-ground growth may be killed. If the root system survived, new growth may emerge from the base in spring.

What to do

Position in the most sheltered, warmest, south or west-facing position available; in colder UK regions, choose the cultivar 'Miss Jessopp's Upright' or other upright forms which tend to be slightly hardier than prostrate trailing cultivars; do not prune in autumn or winter; wait until after the risk of hard frosts has passed in April before cutting back frost-damaged growth to the first visible live green growth; avoid pruning into old wood with no visible green growth.

Rosemary beetle

Chrysolina americana (rosemary beetle) is a strikingly beautiful, iridescent metallic green and purple striped beetle (6 to 8 mm) that has spread progressively through the UK since the 1990s. Both adults and larvae feed on leaves and tender stems from late summer through winter and into spring, causing significant defoliation and shoot tip die-back in heavy infestations. Also attacks lavender, thyme, and sage.

What to do

Check plants regularly from late summer to spring; hold a sheet of paper or tray under each branch and shake it gently so the adult beetles fall onto the sheet; collect and dispose of in soapy water; repeat regularly as new adults arrive; the metallic green and purple striped adults are unmistakable once seen; pyrethrin-based insecticide sprays applied to coat leaves and stems in the evening will kill contacted beetles; repeat applications necessary.

Drought stress in containers

Rosemary in an undersized container or in a very hot, exposed position can suffer drought stress in a UK summer; the leaves turn pale, dull, and slightly curled; much less common than waterlogging as a cause of rosemary problems. Established, in-ground rosemary in free-draining soil is one of the most drought-tolerant garden plants in UK conditions.

What to do

Use a sufficiently large container for the size of the plant; water when the top few centimetres of compost feel dry in summer; do not allow the container to dry out completely in very hot weather; terracotta containers (which dry more rapidly) may need more frequent watering than plastic or glazed ceramic containers; reduce watering significantly in autumn and through winter.

Frequently asked questions

Why are my rosemary leaves curling?

Rosemary leaves curl and turn grey most commonly because of waterlogging and root rot (primary cause of rosemary death in UK gardens; native to dry rocky freely draining Mediterranean hillsides; extraordinarily intolerant of persistently wet cold oxygen-depleted UK soil; leaves turn grey-green then silvery-grey or pale brown; needles dry and sparse; roots dark brown to black and mushy; recovery rarely possible once symptoms obvious; improve drainage radically with grit or raised bed; south or west-facing open sunny position; do not water established in-ground plants in autumn winter or spring), frost and cold stress (hardy to approximately -10 to -15°C in sheltered dry conditions; wet soil plus frost significantly more damaging than dry soil and same temperature; sheltered warmest south or west-facing position; wait until April before cutting back frost-damaged growth to first visible live green growth), rosemary beetle (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 (undersized containers or very hot exposed position; much less common than waterlogging; water when top few centimetres of compost feel dry in summer).

How do I stop my rosemary dying in the UK?

Perfect drainage is the single most important factor. Freely draining soils (light sandy loamy or chalky): plant directly in the ground in a sunny open exposed position; no supplemental water once established; can be a very long-lived shrub (10 to 20 or more years). Heavy clay or moisture-retentive UK soils: incorporate very large quantities of coarse horticultural grit (40 to 50% grit by volume); or raise the planting area in a raised bed of free-draining soil and grit; or grow in large terracotta containers with gritty free-draining compost. Position: south or west-facing aspect for maximum sun and fastest soil drying; avoid north or east-facing positions where soil stays wetter for longer. Mulching: keep mulch away from main stems and crown; heavy mulching immediately around the stem base can contribute to stem rot. Watering: never water established in-ground plants in autumn winter or spring; extremely infrequent and light in summer only if the soil is genuinely parched and the plant shows stress signs.

What is rosemary beetle and how do I control it?

Chrysolina americana (rosemary beetle): established in the UK since the 1990s, progressively spread northward; striking iridescent metallic green and purple striped adult beetles approximately 6 to 8 mm long; larvae are greyish-white with dark stripes. Host plants: rosemary, lavender (Lavandula species), thyme (Thymus species), sage (Salvia officinalis), hyssop, and other aromatic members of the mint family (Lamiaceae). Life cycle: adults present and feeding from late summer through winter and into spring; females lay eggs on plant in autumn and spring; larvae hatch and also feed on leaves; both adults and larvae feed by chewing on leaves and tender green stems causing significant defoliation and shoot tip die-back in heavy infestations. Control: hand picking is the first approach; hold a sheet of paper or tray under the branch and shake or tap the branch so the beetles fall onto the sheet; collect and dispose of in soapy water; repeat regularly from late summer to spring; pyrethrin-based insecticide sprays applied to coat leaves and stems in the evening; repeat applications necessary.

Should I prune rosemary and when?

Timing: best time is immediately after flowering in April to May; a second lighter trim in late August to September if needed to maintain shape; avoid pruning in late autumn or winter (stimulates vulnerable new growth in cold wet conditions); avoid pruning during hard frost. How to prune: cut back flowered growth by approximately one-third to one-half of current year's soft green growth; cut back into current year's green growth; NEVER cut into old hard grey woody stems at the base below the level of any visible green leaf growth; rosemary has very limited capacity to regenerate from old hard leafless woody wood; cutting into old wood below visible green growth almost always results in that section failing to regrow. Culinary harvesting: light regular harvesting of shoot tips throughout growing season (March to September) is beneficial and acts as mild continuous pruning; confine to soft green current-year growth; do not strip whole branches or cut back into woody material.