Verbena bonariensis, known as purple top or tall verbena, is one of the most beloved plants in UK cottage and naturalistic gardens. Its tall, branching stems reach 1.5 to 1.8 metres and carry loose clusters of tiny purple flowers from July right through to October. The near-transparent structure lets it weave through other plants without blocking them, and its flowers are irresistible to painted lady, red admiral, and small tortoiseshell butterflies as well as bumblebees. It self-seeds prolifically once established, and those self-sown plants tend to be more vigorous than anything you buy in a pot.
For all its robustness, verbena bonariensis does occasionally show curling or distorted leaves, particularly on young growth in spring and early summer. The cause is almost always one of four things: aphids, powdery mildew, red spider mite, or drought stress. Identifying which one you are dealing with takes about a minute once you know what to look for.
Aphids: the most likely cause on young growth
If the curling is concentrated at the shoot tips, with new leaves cupping downward or inward, aphids are the most probable culprit. The peach-potato aphid (Myzus persicae) and verbena-specific aphids both colonise the soft growing points of young verbena plants, feeding on sap and causing the tissue to distort as it expands. Look closely at the undersides of the youngest leaves and along the stems just below the flower buds. The insects are small, soft, and usually pale green or yellowish, sometimes in dense colonies.
You will often also see sticky honeydew deposits on the leaves below an infestation, and possibly sooty mould starting to develop on that honeydew. Young seedlings and plants that have not yet developed woody stems are most vulnerable, typically from April through to early July.
The best response on an established verbena bonariensis is usually to do nothing dramatic. The plant attracts parasitic wasps, hoverflies, and ladybirds throughout the season, and these natural predators typically bring aphid numbers down on their own by midsummer. Spraying an insecticide kills those beneficial insects along with the aphids and often makes the situation worse in the medium term. If the infestation is severe and the plant is struggling, a firm jet of water to dislodge the colonies, or a single application of a fatty acid or soap-based spray, is enough to shift the balance back toward the predators without significant collateral damage.
Powdery mildew: white coating and leaf curl in dry spells
Powdery mildew on verbena bonariensis looks exactly as the name suggests: a floury white coating on the leaf surfaces and sometimes the stems. Affected leaves may yellow, curl, and eventually drop, but the plant usually continues growing and flowering around the infected areas. Established plants rarely suffer serious long-term damage.
The fungus (most commonly Erysiphe species or related genera) thrives in the combination of dry roots and humid air, which is exactly what a warm UK summer often delivers. Plants in sheltered spots with poor airflow, or those growing in compacted or sandy soil that dries out quickly, are most at risk. Overhead watering in the evenings, which leaves the foliage wet overnight, also encourages infection.
Prevention is straightforward: grow verbena bonariensis in an open position where air can move freely around it, avoid wetting the foliage, and keep the soil reasonably moisture-retentive at root level. Removing the most heavily affected leaves slows the spread. Established plants generally outgrow a moderate infection as the season progresses.
Red spider mite: a hot dry summer problem
In hot, dry summers, red spider mite can become an issue on verbena bonariensis, particularly on plants in sheltered spots or containers. The mites themselves are tiny and hard to see without a hand lens, but the damage is distinctive: a fine, pale stippling across the upper leaf surface, a bronzed or washed-out colour to the foliage, and in heavier infestations, fine silky webbing across the stem tips and leaf undersides. Affected leaves curl under at the edges and eventually drop.
Misting the foliage regularly in dry weather reduces mite populations, as they dislike humidity. Plants growing in the open garden in UK conditions rarely develop serious infestations because the climate is generally too cool and damp to let mite numbers build to damaging levels. Containers are more vulnerable because the microclimate around them can become very warm and dry.
Slugs, snails, and overwinter damage
Verbena bonariensis is a South American native and a short-lived perennial that survives UK winters as a basal rosette rather than dying back completely. Those low-growing rosettes and the young seedlings that emerge in spring are soft and highly attractive to slugs and snails. The damage is less likely to look like leaf curling and more likely to show as irregular holes, shredded leaf margins, or whole seedlings collapsing at ground level, but rosette growth that has been partially eaten can emerge distorted and curled in spring.
Protecting seedlings with a ring of coarse grit, copper tape around pots, or a small amount of slug bait used sparingly around the base is worthwhile in spring. Once plants develop their characteristic tall, wiry stems, they are far less palatable to molluscs.
Drought stress in containers and poor soils
Verbena bonariensis has a fibrous root system that needs reasonable moisture to support its rapid upward growth in summer. Plants in containers or very free-draining soils can show curling and wilting leaves during dry spells, usually alongside a general droopiness to the whole plant that recovers after watering. The difference from pest damage is that drought stress affects the whole plant fairly evenly rather than concentrating on the newest growth.
In the open garden, verbena bonariensis is generally drought-tolerant once established and rarely needs supplementary watering. In containers it needs more attention, particularly in July and August when the plant is both flowering heavily and the weather is often at its driest. Good drainage is equally important: the plant will not tolerate waterlogged soil, and the crown rots readily in wet winters if drainage is poor. When growing in containers, use a gritty, free-draining mix and raise pots off the ground in autumn.
Prevention at a glance
Grow verbena bonariensis in an open, sunny position with free-draining soil and good air circulation around the stems. Protect seedlings and overwinter rosettes from slugs in early spring. Avoid overhead watering, especially in the evenings. Let natural predators do the work on aphids rather than reaching for the spray. In containers, water consistently through summer and ensure the pot drains freely in winter.
A note on the plant's status: verbena bonariensis is widely sold and grown across the UK and is not listed as an invasive species under UK legislation. It has been placed on invasive species watch-lists in some warmer countries where the self-seeding habit is more problematic in natural habitats, so if you are gardening outside the UK, check your local guidance before planting. In a UK garden, the prolific self-seeding is one of the plant's most appealing qualities.
Frequently asked questions
Why are the tips of my verbena bonariensis curling downward?
Downward curling at the shoot tips is the classic sign of aphid infestation. Check the undersides of the youngest leaves and along the stem tips for small, soft-bodied insects. Peach-potato aphids and verbena-specific aphids are the most frequent culprits on verbena bonariensis, especially on young plants and self-sown seedlings in spring and early summer.
Should I spray my verbena bonariensis for aphids?
In most cases, no. Verbena bonariensis is a magnet for beneficial insects including parasitic wasps and hoverflies that feed on aphids. Spraying insecticide disrupts those natural controls and is counterproductive on a plant that flowers right through summer. If the infestation is severe on seedlings, a sharp jet of water or a soap-based spray used once or twice is enough to tip the balance back in favour of the predators.
Can powdery mildew kill verbena bonariensis?
Rarely. Established verbena bonariensis plants are tough and tend to grow through a mildew infection without serious long-term damage. The condition is most likely in dry spells combined with humid nights, or in sheltered spots with poor air movement. Improving airflow around the plant and avoiding overhead watering in the evenings reduces the risk significantly. Heavily infected leaves can be removed, and the plant usually produces clean new growth as conditions change.
Is verbena bonariensis an invasive species in the UK?
It is not on the UK's Schedule 9 invasive species list and is freely sold in UK nurseries and garden centres. However, it has been flagged on invasive species watch-lists in some warmer countries, particularly in parts of Australia and the United States, where the prolific self-seeding habit causes problems in native habitats. In the UK, the self-seeding is generally regarded as a feature rather than a problem, though it is worth deadheading if you want to limit spread in a small garden.
Why are my verbena bonariensis seedlings collapsing at ground level?
Collapse at the base of young seedlings is nearly always slug and snail damage. Overwinter rosettes and spring seedlings are at particular risk because the growth is soft and close to the ground. Verbena bonariensis self-seeds so freely that losing a proportion of seedlings to slugs is rarely a serious problem, but if you want to protect them, a ring of sharp grit around each plant or copper tape around pots gives some deterrence. Encouraging ground beetles and hedgehogs in the garden also helps.