Verbascum, or mullein, is a striking architectural plant grown for its large woolly rosettes and tall flower spikes. Curling leaves on a verbascum can look dramatic given the scale of the plant, and the cause is often one of two things: mullein moth caterpillars feeding openly on the rosette, or powdery mildew coating the leaf surface. Drought, aphids on flower stems, virus, and crown rot round out the causes, each producing recognisable symptoms.
1. Mullein moth
The mullein moth (Cucullia verbasci) is the most distinctive and damaging pest of verbascum in UK and European gardens. The caterpillars are unmistakable: boldly patterned in yellow, white, and black with a striking spotted pattern running along their length. They feed openly on the rosette leaves from late May through July, consuming large sections of leaf and leaving the remaining tissue to curl and dry around the damage. A heavy infestation can strip a large verbascum rosette within a week or two. They also feed on buddleja and figwort (Scrophularia).
What to look for
- Boldly striped yellow, white, and black caterpillars on the leaves
- Large sections of leaf consumed, with remaining tissue curling and drying
- Frass (droppings) visible on leaves and below the plant
- Damage appearing from late May through July
- Caterpillars feeding openly, not hidden: they rely on conspicuous colouring as a warning
How to fix it
Remove caterpillars by hand and drop them into a bucket of soapy water. They are large, conspicuous, and easy to spot in daylight. Check the plant every few days through late May and June when the adults are laying eggs. Because the caterpillars are so visible, chemical control is rarely necessary: hand-picking is reliable and effective. The plant recovers once the caterpillars have finished feeding and pupated in the soil. Perennial verbascum usually pushes up new rosette growth; biennial forms in their first year may be compromised in their ability to flower the following season if the attack is severe.
2. Powdery mildew
Powdery mildew is very common on verbascum and can be difficult to distinguish from the plant's natural woolly surface at first glance. The fungal coating appears as a white or pale grey powdery layer over the leaf, and the margins curl upward and the tissue between the veins puckers as the infection takes hold. The key distinction is that verbascum's natural feltiness has a consistent texture across the whole leaf surface, while mildew has a patchy, floury quality that differs from the natural felt. Warm dry summers with cool nights favour infection.
What to look for
- Patchy white or grey powdery coating that differs from the natural leaf texture
- Leaf margins curling upward around the infected patches
- Tissue puckering between the veins
- Worst on sheltered plants with poor airflow in warm dry conditions
- Spreading from leaf to leaf within the rosette
How to fix it
Remove badly infected leaves at the base of the rosette. Improve airflow around the plant by removing crowding neighbours. Water at the base in the morning only: overhead irrigation spreads spores and keeps the feltlike leaves wet, which is where the spores germinate. A potassium bicarbonate spray applied to unaffected leaves reduces further spread without harming beneficial insects. Verbascum grown in open, sunny positions with good airflow rarely develops severe mildew. Spacing plants generously allows air to circulate freely between the large rosettes.
3. Drought stress
Most verbascum species are adapted to dry, well-drained soils and are genuinely drought-tolerant once established. Their woolly leaves reflect light and reduce water loss, making them excellent plants for hot, dry spots. However, newly planted verbascum and young seedlings have not yet developed the deep taproot that makes mature plants so resistant. In their first season, prolonged dry spells can cause the rosette leaves to curl inward and the outermost leaves to shrivel, particularly if the plant was installed in summer.
What to look for
- Rosette leaves curling inward, starting with the outer and lower leaves
- Soil dry below the surface near the root zone
- Symptoms on newly planted specimens or young seedlings
- Established plants unaffected while new plantings show stress
- Recovery after deep watering on young plants
How to fix it
Water newly planted verbascum deeply every week for the first season to encourage the taproot to develop. Once the taproot is established, typically by the end of the first full growing season, the plant rarely needs supplementary watering. Apply a light gravel mulch around the plant rather than an organic mulch: verbascum is susceptible to crown rot in wet conditions, and gravel reflects heat back up to the plant while allowing moisture to drain away from the crown efficiently.
4. Aphids
Aphids are less common on verbascum rosette leaves than on many other perennials, but they frequently attack the elongating flower spikes in early summer. Colonies of small green or grey aphids cluster on the tender new growth below the flower buds, and the leaves immediately below the infestation curl and pucker as a result of the sap removal and toxin injection. The aphids are often found together with ants, which farm them for honeydew.
What to look for
- Aphids clustered on the flower spike below the buds
- Stem leaves immediately below the colony curling or puckering
- Sticky honeydew and sooty mould on stems below the infestation
- Ants moving up and down the flower spike
- Damage concentrated on the flowering stem, not the basal rosette
How to fix it
Apply insecticidal soap or a jet of water directly to the colony on the flower spike. Natural predators including ladybirds and lacewings usually control aphid populations on verbascum spikes effectively without intervention, since the tall, open flower spikes are easy for predators to access. Heavy infestations that distort the spike significantly can be removed entirely: cutting off and binning the infested section of stem removes the colony and leaves the lower, undamaged portion of the spike to continue flowering.
5. Virus
Verbascum can be infected by plant viruses, typically transmitted by aphids feeding on the plant after visiting infected hosts. Virus-infected plants develop leaf distortion and curling combined with mosaic or mottled discolouration on the rosette leaves. The whole plant often looks stunted, with smaller leaves than normal and an irregular, lopsided rosette. Unlike aphid damage which is confined to the growing tip, virus symptoms develop throughout the plant over time.
What to look for
- Rosette leaves curled and distorted throughout the plant
- Mosaic or mottled yellow-green patterns on the leaf surface
- Overall smaller and more irregular rosette than healthy plants
- No caterpillars, mildew, or pest to explain the distortion
- Symptoms developing gradually across the whole rosette
How to fix it
There is no cure for virus-infected verbascum. Remove and destroy the plant entirely to prevent aphids from carrying the virus to neighbouring plants. Do not compost infected material. Reduce aphid populations in the garden to limit the spread of viral diseases between plants. Source replacement verbascum from reputable nurseries. Biennial verbascum grown from seed collected from your own healthy plants rarely carries virus through the seeds.
6. Crown rot
Verbascum has a crown and taproot that are particularly vulnerable to rotting in wet, poorly drained, or heavy soil. The woolly leaves trap moisture against the crown in wet conditions, and Phytophthora or soft rot bacteria can colonise the stressed tissue. Infected plants produce leaves that curl, yellow, and collapse from the outermost inward, and the crown feels soft at the base. Crown rot is the main reason verbascum fails in heavy clay soils, and it often strikes in wet winters even after a plant has looked perfectly healthy through the previous summer.
What to look for
- Rosette leaves curling and yellowing from outside inward
- Crown soft, discoloured, or foul-smelling at the base
- Problem appearing after wet weather or in heavy clay soil
- Roots brown and soft rather than firm and white
- Woolly leaves matted with moisture against the crown
How to fix it
Verbascum with crown rot rarely recovers. Lift the plant and inspect the crown: if more than half the tissue is rotted the plant should be discarded. Where healthy tissue remains, cut away the rot to firm growth, dust with sulphur, and allow the crown to dry in a sheltered spot for several days before replanting in a completely different, better-drained position. Verbascum needs excellent drainage above all else: it thrives in gravel gardens, raised beds, and on dry slopes where winter moisture drains quickly away from the crown. Avoid mulching over the crown with organic material; a gravel mulch that keeps the crown dry is far better.
Quick diagnosis checklist
| What you see | Most likely cause | First action |
|---|---|---|
| Striped yellow-black caterpillars, large sections of leaf eaten | Mullein moth | Remove caterpillars by hand, check every few days |
| Patchy white powder (not natural felt), margins curling | Powdery mildew | Remove infected leaves, improve airflow, water at base |
| Outer rosette leaves curling inward, soil dry, young plant | Drought | Water deeply weekly until established |
| Aphids on flower spike, stem leaves below curling | Aphids | Insecticidal soap to colony, remove infested spike section |
| Mosaic mottling, whole rosette distorted, no pest visible | Virus | Remove and destroy the plant |
| Rosette collapsing from outside in, crown soft and dark | Crown rot | Lift, discard if badly rotted, replant in drained soil |
Frequently asked questions
What are the striped caterpillars eating my verbascum?
The distinctively striped yellow, white, and black caterpillars eating verbascum leaves are mullein moth caterpillars (Cucullia verbasci). They feed openly on verbascum, scrophularia, and buddleja leaves from late spring through summer and can consume an entire rosette. Remove them by hand: they are large enough to pick off easily. The plant usually recovers, though a heavy attack on a biennial verbascum in its first year can prevent it from flowering the following year.
Why are my verbascum leaves curling and going white?
Curling with a white coating on verbascum is almost always powdery mildew. The fungal infection is very common on verbascum species and causes the leaf margins to cup and curl while the surface develops a powdery white layer. Because verbascum's woolly leaf surface can look slightly powdery naturally, look closely at the actual texture: mildew coating has a distinct powdery or floury quality that the natural felt does not have.
Why are my verbascum leaves curling in summer?
Summer leaf curling on verbascum is usually mullein moth caterpillar damage, powdery mildew, or drought on young plants. Check for striped caterpillars on the leaves first as they are the most damaging. If no caterpillars are present, look for a white coating (mildew) or dry soil (drought). Established verbascum is drought-tolerant but newly planted specimens need watering during dry spells.
Does verbascum come back after caterpillar damage?
Yes, perennial verbascum species usually recover well from mullein moth caterpillar damage, pushing up new growth once the caterpillars have finished feeding and pupated. Biennial species are more vulnerable: if caterpillars strip a first-year rosette before it has stored enough energy, it may not have the resources to flower in its second year. Remove caterpillars as early as possible to limit the damage.