Brunnera macrophylla, Siberian bugloss, is one of the most valuable ground-covering perennials for shaded gardens. Its heart-shaped leaves, which can reach 20 cm or more across in well-grown specimens, provide lush foliage coverage from spring into autumn. Variegated cultivars such as 'Jack Frost', 'Looking Glass', and 'Alexander's Great' add striking silver or white markings that light up dark corners. The plant begins the season with clouds of small, forget-me-not blue flowers above emerging foliage, and with the right conditions the foliage remains the main attraction for the rest of the year. Leaf curl is the most common indication that something is wrong, and it nearly always points to a moisture or light problem rather than a serious disease.
1. Drought stress
Brunnera evolved in moist woodland and riparian habitats in the Caucasus and western Siberia. It has large, transpiring leaves and a relatively shallow, fibrous root system that exhausts available moisture quickly in dry conditions. Drought is the most common reason for leaf curl on brunnera in summer, particularly in gardens that rely on rainfall alone or in plantings under large trees where competing roots absorb all available water. Variegated varieties with thin, pale leaf tissue are especially sensitive and show drought damage more quickly than the species.
What to look for
- Leaves curling inward at the margins or cupping upward at the edges
- Dull, slightly grey-green or yellowish colour rather than fresh bright green
- Soil dry at depth when probed; surface may appear moist but deeper layers are dry
- Symptoms developing during warm spells or extended dry periods
- Partial recovery overnight when temperatures drop and transpiration slows
How to fix it
Water deeply and thoroughly at the base of the plant. The root zone needs soaking to depth, not just surface wetting. Apply a generous organic mulch, 7 to 10 cm deep, to retain moisture and prevent the rapid fluctuations in soil moisture that brunnera dislikes. Under trees, extend the mulched and watered zone to the full spread of the brunnera clump. Water consistently throughout dry spells in summer rather than waiting for the plant to show distress. If drought curl has already occurred, deep water immediately and the leaves will usually recover within a day or two.
2. Sun scorch
Brunnera placed in too much direct sun experiences leaf scorch, which causes the leaf margins to turn brown and curl. Variegated varieties are far more vulnerable than the plain green species: the pale, silver, or white areas of the leaf have less chlorophyll to moderate light absorption and scorch readily in even a few hours of direct midday sun. Spring sun, which can be intense and arrives before deciduous canopy overhead opens fully, is a surprisingly common cause of early-season scorch and curl. Plants moved from shade to sun without a transition period are particularly vulnerable.
What to look for
- Leaf margins turning brown and crispy rather than just curling
- Damage concentrated on the side of the plant receiving most direct sun
- Pale or silver leaf areas on variegated varieties scorching first and most severely
- Problems appearing after a spell of clear, sunny weather
- Healthy new growth from shaded portions of the clump while sun-exposed leaves decline
How to fix it
Move brunnera to a shadier position as soon as possible. Even dappled shade under high-canopy trees is preferable to more than two to three hours of direct sun per day. Cut back badly scorched leaves at soil level to allow fresh, shade-adapted growth to replace them. In situ, erect temporary shade netting or a garden umbrella to protect the plant while a more permanent solution is arranged. For variegated varieties, do not site them in any position with direct sun in summer, even morning sun, unless the climate is very cool.
3. Powdery mildew
Powdery mildew affects brunnera less frequently than it does closely related shade perennials like pulmonaria, but it does occur, particularly in warm, dry summers or in plantings with poor air circulation. The large, rough-textured leaves provide an ideal surface for the fungal spores to germinate, especially when plants are drought-stressed. Mildew appears as a white or grey powdery coating on leaf surfaces, alongside curling and yellowing of infected leaves. The infection spreads between leaves and plants in conditions of warm days and cool nights with low humidity.
What to look for
- White or pale grey powdery coating on upper leaf surfaces
- Leaves curling upward around infected areas
- Yellowing and browning of infected leaf sections
- Infection spreading from older, outer leaves inward toward the crown
- Problem worse in crowded plantings or after a dry spell
How to fix it
Cut the plant back hard to ground level if mildew is widespread. This removes all infected growth and stimulates a fresh flush of clean leaves, provided the season is not too advanced (mid-August is usually the latest useful time to cut back). Improve air circulation by thinning crowded plantings. Water consistently to prevent the drought stress that predisposes plants to mildew infection. Apply a potassium bicarbonate or sulfur-based fungicide as a preventive spray if conditions favour mildew before you are ready to cut the plant back.
4. Slugs and snails
Slugs are highly attracted to brunnera. The large, slightly rough, fleshy leaves are a prime target, and brunnera's preference for moist, shaded conditions places it exactly where slug populations are highest. Spring growth when the leaves are soft and rapidly expanding is particularly vulnerable. Slug damage causes irregular holes and ragged edges that then curl and deteriorate around the damaged areas. The problem can be severe enough to destroy newly emerging growth entirely in wet springs, leaving only tattered remains where the ornamental foliage should be.
What to look for
- Irregular holes in leaf surfaces and ragged, eaten leaf edges
- Leaf tissue curling around areas of damage
- Silvery slime trails on leaves, stems, and surrounding soil
- Damage appearing overnight or after rain
- New emerging growth particularly targeted in spring
How to fix it
Apply iron phosphate slug pellets around the plant at the start of the season and again after any cut-back. Iron phosphate is safe for wildlife and pets. Use copper tape around container brunnera. Check under the plant and nearby mulch after dark with a torch and remove slugs and snails by hand. Avoid over-mulching directly against the crown. Encourage frogs, toads, and hedgehogs in the garden as effective natural predators. A nematode soil drench (Phasmarhabditis hermaphrodita) applied in early spring provides good biological control in moist soil conditions.
5. Vine weevil
Vine weevil larvae feed on the roots of brunnera, chewing through the fibrous root system and weakening the plant's ability to take up water and nutrients. Affected plants show a characteristic drought-like wilt and leaf curl despite adequate soil moisture, since the damaged roots cannot function properly. The adult beetles feed on leaf margins at night, causing distinctive scalloped notches around leaf edges. Larval damage is worst in late summer and autumn, when the larvae are large and most destructive. Container brunnera is more vulnerable than border plants.
What to look for
- Scalloped notches around leaf margins, particularly on the lower leaves
- Wilt and leaf curl despite moist soil
- Plant loosening in the soil or lifting easily when gently tugged
- Curved, white C-shaped grubs with a brown head found in the root zone when dug
- Symptoms worsening through late summer and autumn as larvae enlarge
How to fix it
Apply a nematode soil drench containing Steinernema kraussei in late summer or early autumn when soil temperatures are above 5 degrees Celsius. This is the most effective and reliable control method. For container plants, repot into fresh compost and remove any visible larvae by hand before treating with nematodes. Physically checking border plants when digging or dividing in autumn and squashing any larvae found helps reduce populations. Adult beetles cannot fly, so physical barriers around valuable container plants can prevent females from reaching the compost to lay eggs.
6. Rust
Rust (caused by Puccinia or related fungi) is less common on brunnera than the other problems listed here, but it does occur. It produces characteristic orange or rust-coloured pustules on leaf undersides, alongside yellowing of the upper surface and eventual curling and browning of affected leaves. Rust spreads by airborne spores and is favoured by cool, damp conditions in spring and autumn. It is generally less devastating than powdery mildew on brunnera but can disfigure the foliage and weaken the plant if allowed to spread unchecked.
What to look for
- Orange, red, or brown pustules on the undersides of leaves
- Yellow or pale spots on the upper leaf surface directly above pustules
- Leaves curling and distorting as infection progresses
- Infection worst in cool, damp conditions in spring or autumn
- Spores rubbing off easily on a tissue held against the pustules
How to fix it
Remove and destroy all infected leaves promptly to prevent spore spread. Do not compost infected material. Apply a copper-based fungicide or a triazole-class fungicide (such as tebuconazole) to protect remaining healthy leaves. Improve air circulation in the planting by thinning and spacing plants adequately. Cut the entire plant back to ground level if infection is widespread and the season is not too advanced. Clear all plant debris from the site in autumn to remove overwintering spore sources.
Quick diagnosis checklist
| Symptoms | Most likely cause | First action |
|---|---|---|
| Margin curl, dull colour, dry soil | Drought stress | Deep water + generous mulch |
| Brown crispy margins, variegated areas worst | Sun scorch | Relocate to deeper shade, cut back damaged leaves |
| White powdery coating, upward curl, dry conditions | Powdery mildew | Cut plant back hard, improve moisture |
| Ragged holes, slime trails, overnight damage | Slugs and snails | Iron phosphate pellets, night-time handpicking |
| Notched margins, wilt in moist soil, white grubs at roots | Vine weevil | Nematode soil drench in late summer |
| Orange pustules on undersides, yellow spots above | Rust | Remove infected leaves, copper fungicide |
Frequently asked questions
Why are my brunnera leaves curling?
Brunnera leaves most often curl because of drought stress in summer (it is a shade woodland plant that needs consistent moisture), powdery mildew, or leaf scorch from too much sun. Variegated varieties such as 'Jack Frost' are particularly prone to scorch and curl in direct sunlight. Check soil moisture and sun exposure first before treating for disease.
Does brunnera get powdery mildew?
Yes, brunnera can develop powdery mildew, particularly in summer when conditions are warm and dry or in crowded plantings with poor air circulation. The large, rough-textured leaves are susceptible once stressed by drought. Keeping the soil consistently moist and thinning out crowded clumps helps prevent mildew. Species brunnera tends to be more susceptible than modern cultivars.
Can brunnera grow in full sun?
Brunnera strongly prefers shade to dappled shade and performs poorly in full sun, especially in summer. In direct sun, the large leaves scorch at the margins, curl, and develop brown, crispy edges. Variegated varieties are the most sensitive; their pale leaf areas scorch most readily. Move brunnera to a shadier position if sun exposure is the problem.
Should I cut back brunnera when the leaves look bad?
Yes. If brunnera leaves curl, scorch, or develop significant mildew in summer, cutting the entire plant back to ground level encourages a fresh flush of leaves that often looks much better than the damaged foliage. The plant regrows readily from its crown. Water and mulch well after cutting back. This approach works best in mid to late summer, giving the new growth time to establish before winter.
Why do brunnera leaves curl at the edges?
Brunnera leaves curling specifically at the edges rather than across the whole blade are most often showing drought stress or leaf scorch. Edge curl is the plant's response to water deficit: it rolls the leaf edges inward to reduce the area exposed to wind and sun. If the edges also turn brown and crispy, sun scorch is the likely cause rather than drought alone. Improve shade and soil moisture to resolve both.