Ajuga, or bugleweed, is one of the most widely planted ground covers for shade and partial sun, valued for its colourful foliage and spring flower spikes. When the leaves curl it is a clear sign that something is wrong. Drought is the most common cause, closely followed by powdery mildew, which thrives in the humid microclimate that builds up within a dense ajuga mat. Vine weevil, aphids, crown rot, and frost damage each produce characteristic symptoms that help narrow down the cause quickly.
1. Drought stress
Drought is the most common cause of curling leaves on ajuga. The plant has a shallow, fibrous root system that sits near the soil surface and dries out quickly when rainfall is scarce. Ajuga planted under trees faces an additional challenge: the canopy intercepts a large proportion of rainfall and the tree roots compete aggressively for any moisture that does reach the soil. The rosette leaves curl inward along their length and the plant takes on a wilted, flattened look. Coloured-leaf cultivars such as 'Black Scallop' and 'Burgundy Glow' show drought symptoms more dramatically than plain green forms because their pigmentation is more affected by cellular water stress.
What to look for
- Leaves curling inward along their length across the whole mat
- Soil dry below the surface near the plants
- Plants under tree canopies affected more severely
- Leaf colour looking dull or faded alongside the curling
- Recovery within hours of deep watering
How to fix it
Water the whole mat deeply, allowing moisture to soak down to root depth. Apply a light mulch of leaf mould between the rosettes to retain moisture without smothering them. In positions under dense tree canopies where rainfall is regularly intercepted, water ajuga every week during dry spells. If the position is consistently too dry for ajuga to perform well, consider replacing it with a more drought-tolerant ground cover and relocating the ajuga to a position that receives natural rainfall.
2. Powdery mildew
Powdery mildew is extremely common on ajuga, particularly on dense established mats where the rosettes press tightly together and air cannot circulate freely between the leaves. The fungal infection coats the leaf surface with a white powdery layer and causes the margins to curl upward and the tissue between the veins to pucker. Warm dry days and cool humid nights are the ideal conditions. Once established in a mat, mildew spreads rapidly from rosette to rosette, and by late summer a badly infected planting can look completely white and tatty.
What to look for
- White or pale grey powdery coating on the upper leaf surface
- Leaf margins curling upward across multiple rosettes
- Leaves puckering or appearing distorted between the veins
- Worst on the densest sections of the mat in still conditions
- Rapid spread through the mat during warm dry spells
How to fix it
Remove badly infected rosettes entirely and dispose of them in the bin. Thin the mat by pulling out excess rosettes to create space between plants and allow air to move through. Divide very dense established patches by lifting sections, removing the excess, and replanting with generous spacing of 20 to 30 cm between rosettes. Water at the base only and in the morning. A potassium bicarbonate spray applied to unaffected leaves slows further spread. Dividing every two to three years to prevent congestion is the single most effective long-term prevention.
3. Vine weevil
Vine weevil is one of the most serious pests of ajuga, particularly for plants growing in containers or in the confined root zones of raised beds. The larvae hatch from eggs laid in the soil in late summer and feed on ajuga roots through autumn and winter. Because ajuga has a shallow fibrous root system, larvae can destroy it relatively quickly, and plants lose their ability to take up water. The leaves curl and wilt even when the surrounding soil is moist, and affected rosettes can be lifted out of the ground with little resistance because the roots have been consumed.
What to look for
- Rosettes curling and wilting despite moist soil
- Individual rosettes lifting out of the soil with no roots attached
- White C-shaped grubs found in the soil beneath affected rosettes
- Notched leaf margins from adult beetle feeding in summer
- Symptoms appearing from August through spring
How to fix it
Lift affected sections of the mat and examine the soil for grubs. Remove every grub by hand and drench the soil with a nematode treatment containing Steinernema kraussei, applied when soil temperature is above 5 degrees C. This is most effective in August and September before the larvae grow large enough to cause serious damage. Replant healthy rosettes in fresh soil. Apply nematode drenches preventatively each August to stop populations building up year on year. Ajuga in containers is especially vulnerable: check container-grown plants for grubs every autumn by tipping them out.
4. Aphids
Aphids can infest ajuga at the growing tips and in the crown of individual rosettes, feeding on the softest tissue and causing the innermost leaves to curl and fail to open properly. Root aphids are also possible, colonising the root base invisibly and causing wilting and curling symptoms similar to vine weevil damage but without the obvious root destruction. Above-ground aphids are easier to spot as they are visible when the rosette is inspected at close range.
What to look for
- Innermost rosette leaves curled or failing to open
- Small green or grey aphids visible in the crown of the rosette
- Sticky honeydew and sooty mould on leaves and stems
- Wilting despite moist soil with no grubs found (check root base for root aphids)
- Ants moving in and out of the mat farming the colony
How to fix it
Apply insecticidal soap directly into the crown of infected rosettes, covering the aphids thoroughly. A jet of water directed into the rosette dislodges colonies effectively for lighter infestations. Ladybirds and other predators naturally control aphid populations in ajuga mats if not disturbed by insecticide use. For suspected root aphids, lift the affected rosette, wash the root base under running water, and replant in fresh soil or treat the root zone with an insecticidal drench.
5. Crown rot
Ajuga crowns can rot in very wet or waterlogged conditions, particularly when the dense mat traps moisture against the base of the rosettes over winter or in prolonged wet spells. Phytophthora and other pathogens colonise the stressed crown tissue and spread outward. Infected rosettes turn brown and collapse, with the crown tissue becoming soft and dark. The leaves curl and die as the rosette loses its ability to supply water upward from the roots.
What to look for
- Individual rosettes turning brown, collapsing, and dying
- Crown tissue soft, dark, or foul-smelling at the base of the rosette
- Problem appearing after prolonged wet weather or in heavy clay soil
- Bare patches in the mat where rosettes have died
- Healthy rosettes adjacent to dead ones with clear boundaries between them
How to fix it
Remove every affected rosette completely, including the rotted crown and roots. Improve drainage in the area by incorporating coarse grit into the soil. Avoid mulching over the crowns of the rosettes: mulch between them is fine, but covering the crown itself traps moisture and promotes rot. In heavy clay or low-lying spots, raise the bed level before replanting. Ajuga grows best in moist but freely draining soil: standing water around the crown over winter is the primary risk factor for crown rot.
6. Frost damage
Ajuga is generally hardy, but new growth emerging in late winter or early spring is vulnerable to unexpected late frosts. A sharp frost after new leaves have begun to unfurl can freeze the soft tissue, causing it to curl, darken, and appear scorched. The damage is usually confined to the newest innermost leaves of each rosette while the older outer leaves remain unharmed. The plant recovers quickly once frost risk has passed and new growth replaces the damaged leaves within two to three weeks.
What to look for
- Innermost young leaves darkened, curled, and appearing water-soaked
- Damage appearing suddenly after a cold night
- Outer, older leaves of each rosette unaffected
- Timing coinciding with a late frost event
- Recovery within two to three weeks as temperatures stabilise
How to fix it
Leave frost-damaged foliage in place until temperatures stabilise and all frost risk has passed. The damaged leaves provide some protection to the developing crown below. Once the risk has gone, remove the blackened leaves to tidy the plant and allow the fresh growth developing beneath to emerge unimpeded. Covering ajuga with horticultural fleece on forecast frost nights protects the soft emerging growth without creating conditions that would encourage mould.
Quick diagnosis checklist
| What you see | Most likely cause | First action |
|---|---|---|
| Whole mat curling, soil dry, under tree canopy | Drought | Water deeply, mulch between rosettes |
| White powder on leaves, margins curling across mat | Powdery mildew | Thin the mat, remove affected rosettes, improve airflow |
| Rosettes wilting in moist soil, lift out easily, grubs found | Vine weevil | Remove grubs, drench with nematodes, replant |
| Innermost leaves curled, aphids visible in crown | Aphids | Insecticidal soap into crown, water jet |
| Rosettes collapsing and dying, crown soft and dark | Crown rot | Remove infected rosettes, improve drainage |
| Innermost leaves dark and curled after cold night | Frost damage | Leave until frost risk passes, then remove damaged leaves |
Frequently asked questions
Why are my ajuga leaves curling and going brown?
Curling combined with browning on ajuga most often signals drought stress. Ajuga has shallow fibrous roots that dry out quickly, especially when growing under trees where the canopy intercepts rainfall. Water deeply, apply a mulch around the plants, and if they are under a dry tree canopy consider moving them to a position where they receive natural rainfall.
Does ajuga get powdery mildew?
Yes, powdery mildew is very common on ajuga, particularly in warm dry summers with poor air circulation between the dense mat of rosettes. A white powdery coating appears on the leaves and the margins curl upward. Thin out the densest patches by dividing the mat, water at the base, and remove badly affected leaves. Ajuga recovers well once conditions improve.
Why is my ajuga wilting and curling even in moist soil?
Ajuga wilting and curling despite moist soil usually means vine weevil has destroyed the roots. The larvae eat through the fibrous root system from late summer through winter, and the plant loses its ability to take up water. Lift the plant and check the root zone for white C-shaped grubs. Treat with Steinernema kraussei nematodes and replant in fresh soil.
How do I stop ajuga from getting powdery mildew every year?
The most effective prevention is dividing the mat every two to three years. Dense, congested ajuga mats trap humid air between the rosettes and create ideal mildew conditions. Divide in autumn or early spring, replant with generous spacing, and remove any debris from the previous year's growth. Water at the base in the morning only.