Acanthus, or bear's breeches, is grown for its bold, deeply lobed leaves and dramatic flower spikes, so curling foliage is particularly noticeable on a plant that is supposed to look architectural and imposing. Drought and powdery mildew are the most common causes, though slugs targeting young emerging leaves, vine weevil, sun scorch on exposed sites, and crown rot in wet soils all produce similar symptoms. Identifying the cause correctly saves the plant and prevents a recurrence.
1. Drought stress
Drought is the primary cause of acanthus leaf curling, particularly in plants that have been in the ground for fewer than two growing seasons. Established acanthus develops a very deep, fleshy taproot that reaches moisture far below the surface and makes the plant remarkably drought-tolerant. But newly planted specimens and those grown in containers are completely dependent on surface irrigation, and their large leaf surfaces lose water rapidly. The leaves curl inward along their length, the deeply cut lobes droop, and the whole plant looks deflated compared to its usual bold presence.
Even established plants can curl during extreme heat waves when evapotranspiration from the big leaves outstrips what even a deep taproot can supply.
What to look for
- Leaves curling inward, lobes drooping toward the midrib
- Soil dry below 5 cm near the plant
- Symptoms worst in afternoon heat or after prolonged dry spells
- New plantings affected first; established plants more resistant
- Leaves recover quickly after watering
How to fix it
Water deeply and infrequently, allowing a full bucket of water to soak slowly into the root zone rather than a quick surface wetting. Deep watering encourages the taproot to grow down rather than staying near the surface. Apply a thick mulch of bark chip or gravel over the root zone, extending 50 cm out from the crown. Keep the mulch away from the crown itself to prevent moisture sitting against the base of the stems. During the establishment period in the first two years, water every week in dry spells. Once established, acanthus rarely needs supplementary watering except in extreme drought.
2. Powdery mildew
Powdery mildew is extremely common on acanthus and one of the most visually damaging problems the plant faces. The fungal infection covers the leaf surface with a white powdery coating and simultaneously causes the leaf margins and lobes to curl upward, the tissue between the veins to pucker, and the overall leaf to look tatty and shrunken compared to healthy foliage. Mildew on acanthus typically becomes visible in mid to late summer when nights cool and humidity rises, but warm dry days with poor air circulation between the leaves is all the fungus needs to establish.
What to look for
- White or pale grey powdery coating on upper and lower leaf surfaces
- Leaf margins curling upward and lobes cupping
- Leaves puckering or looking distorted and smaller than normal
- Affected tissue eventually turns yellow or brown
- Worse on crowded plantings or against walls with poor airflow
How to fix it
Cut back badly affected leaves at the base of their stem. Where only the margins are affected, remove those leaves and improve conditions to limit spread. Applying a potassium bicarbonate spray to unaffected leaves reduces spore germination. Water at the base, not overhead, and water in the morning so foliage stays dry through the night. Acanthus responds well to being cut back entirely after the flower spikes finish in late summer: new growth that emerges in autumn and the following spring will be clean and undamaged. Avoid feeding with high-nitrogen fertilisers which produce soft, susceptible growth.
3. Slugs
Acanthus produces its new leaves as tightly rolled scrolls that unfurl from the crown in spring. These emerging leaves are among the most vulnerable in the garden, combining soft, succulent tissue with the slugs' attraction to the lush new growth. Slug feeding on a leaf that has not yet unfurled causes the tissue to develop unevenly as cells on the damaged side expand at a different rate from those on the intact side, and the leaf emerges permanently curled or twisted rather than flat. Mature leaves can also be attacked, producing ragged holes within the lobes.
What to look for
- Irregular holes within the leaf lobes and between veins
- Young emerging leaves curled or twisted in an irregular pattern
- Silvery slime trails on leaves and surrounding soil in the morning
- Damage worst in spring when new growth is most vulnerable
- Slugs found under debris or mulch near the crown at night
How to fix it
Patrol around the crown after dark with a torch in spring and collect slugs by hand. Remove any debris, fallen leaves, or loose mulch immediately around the crown that could shelter slugs during the day. Apply iron phosphate pellets around emerging growth in spring, renewing after heavy rain. Biological control with Phasmarhabditis hermaphrodita nematodes works well in moist conditions when soil temperature is above 5 degrees C. A collar of coarse grit or horticultural sharp sand around individual crowns deters slugs but needs refreshing after rain.
4. Vine weevil
Adult vine weevils notch the characteristic lobed margins of acanthus leaves on summer evenings, leaving a distinctive scalloped edge. This is irritating but not serious. The real threat comes from the larvae: fat white grubs that hatch from eggs laid in the soil around the plant and spend autumn and winter eating the fleshy taproot. Because acanthus has a substantial root, even heavy larval feeding may not kill the plant outright, but it will cause leaves to curl and wilt as the damaged root system struggles to supply water.
What to look for
- Scalloped or notched edges on leaf lobes (adult beetle feeding)
- Leaves wilting and curling despite moist soil
- White C-shaped grubs found in the soil near the root
- Root surface showing eaten sections or scarring
- Symptoms from late summer through spring
How to fix it
Drench the soil around the plant with a nematode solution containing Steinernema kraussei in late August or September while soil temperature is still above 5 degrees C. This is the most effective treatment available to gardeners and kills the young larvae before they can cause significant root damage. Adult beetles can be hand-picked after dark in summer. Acanthus is less easily killed by vine weevil than smaller plants but the root damage can significantly weaken it and allow secondary infections to take hold.
5. Sun scorch
Acanthus grows best in partial shade or a position with sun in the morning and shade in the hottest afternoon hours. In full exposure to direct sun, particularly on a south-facing slope or against a light-coloured wall that reflects heat, the large leaves can lose moisture faster than the roots can replace it and the margins scorch and curl. The problem is worse when the plant is also drought-stressed, as the two effects compound each other.
What to look for
- Leaf margins going brown, papery, and curling, starting on the outer lobes
- Damage appearing on the side facing the strongest sun
- Symptoms developing during or after hot sunny spells
- Tips of lobes browning first, then the damage spreading inward
- No pest, fungal coating, or wet soil to explain the damage
How to fix it
Erect a temporary shade cloth on the sunniest side of the plant during extreme heat events. Water deeply in the morning before temperatures rise. Scorched leaves will not recover: remove them at the base to keep the plant looking tidy and allow new growth to replace them. If the plant consistently scorches every summer, consider moving it to a position with afternoon shade, or grow it in dappled light under a high tree canopy where it will produce larger, more impressive leaves than in full exposure.
6. Crown rot
Acanthus has a fleshy crown that is vulnerable to rotting in wet winters or in heavy waterlogged soil. Phytophthora and soft rot bacteria colonise the crown when it stays wet, particularly when leaves have died back and the crown is buried or covered with wet mulch through winter. The rot spreads downward into the taproot and upward into the base of the emerging leaf stems. Affected plants push up leaves that curl, yellow, and collapse from the base upward as the crown fails to supply water and nutrients.
What to look for
- Leaves curling and collapsing from the outermost inward
- Base of leaf stems soft, brown, or with a foul smell
- Crown area sunken or discoloured
- Soil very wet or problem developing after a wet winter
- New leaf spears emerging, then wilting and turning brown at the base
How to fix it
Expose the crown by removing surrounding soil and cut away all rotted tissue back to firm, healthy growth. Dust the cut surfaces with sulphur powder. Allow the crown to dry for several hours before backfilling with gritty compost. Avoid mulching over the crown in autumn: instead, mulch around the crown but leave the growing point exposed. In heavy clay or low-lying areas, improve drainage by raising the bed or adding a generous quantity of grit before replanting. Acanthus is one of the few perennials that can survive a severe cutback of the crown if healthy tissue remains at the base.
Quick diagnosis checklist
| What you see | Most likely cause | First action |
|---|---|---|
| Leaves curling inward, lobes drooping, soil dry | Drought | Deep water, thick mulch over root zone |
| White powder, margins cupping, puckered tissue | Powdery mildew | Remove affected leaves, cut back hard if severe |
| Ragged holes, twisted young leaves, slime trails | Slugs | Night patrol, iron phosphate pellets in spring |
| Notched lobe edges, wilting despite moist soil, grubs found | Vine weevil | Drench with nematodes in August/September |
| Brown papery margins on sun-exposed side | Sun scorch | Temporary shade, morning water, relocate long-term |
| Leaves collapse from base, crown soft and foul-smelling | Crown rot | Expose crown, cut away rot, dust with sulphur |
Frequently asked questions
Why are my acanthus leaves curling and wilting?
Acanthus leaves curling and wilting together most often signal drought, especially on plants in their first two seasons before the deep taproot establishes. Water deeply and infrequently so moisture reaches the full root depth. Once established, acanthus tolerates dry conditions well, so persistent wilting on a mature plant warrants checking for crown rot or vine weevil.
Does powdery mildew make acanthus leaves curl?
Yes, powdery mildew is one of the most common problems on acanthus and causes significant leaf distortion and curling. The leaves develop a white powdery coating, the margins curl upward, and the overall look becomes tatty. Cut back affected leaves, improve air circulation, and avoid overhead watering.
Why do acanthus leaves curl in summer?
Summer curling on acanthus is usually powdery mildew or heat scorch combined with drought. Hot weather and low humidity accelerate moisture loss from the large leaf surface. Deep infrequent watering in June and July and a thick mulch over the root zone reduces summer curling significantly.
Can I cut back curling acanthus leaves?
Yes. Acanthus can be cut back to the crown if the leaves look badly curled, mildewed, or scorched. The plant will push new growth from the base, though regrowth may be slower than other perennials. Cutting back is most effective in late summer after the flower spikes have finished, allowing fresh foliage to carry the plant through autumn.