Nasturtiums (Tropaeolum majus) are one of the easiest and most rewarding UK annuals: sow direct from May, water once or twice, and they romp away to produce months of orange, yellow, and red flowers with edible peppery leaves and petals. Climbing varieties will scramble up fences and trellises; compact bush forms fill gaps at the front of borders or tumble from containers. They self-seed with cheerful abandon and are a classic companion plant, drawing pests away from beans and brassicas. That companion-plant role means nasturtiums are designed to attract insects, and the same qualities that make them effective trap crops make them magnets for certain pests. When the foliage starts to curl, pucker, or collapse, two culprits account for the great majority of cases.
What causes nasturtium leaves to curl?
Nasturtium leaves curl as a physical response to damage at or below the leaf surface. When tissue on one side of a leaf is destroyed by insect feeding, the intact side continues to grow and the leaf bends toward the damage. Dense colonies of sap-feeding insects on the growing tips remove turgor from the cells at the shoot tip, and the young leaves that unfurl in that disrupted tissue emerge already curled and misshapen. Drought causes a different kind of curling: the whole leaf rolls inward along its length as the plant reduces its surface area to limit water loss. Viral infection distorts leaf development from within and produces irregular mottling alongside the curl. Running through the visual signs in order makes diagnosis straightforward in most cases.
1. Cabbage white caterpillars (Pieris brassicae)
Cabbage white caterpillars are among the most damaging pests on nasturtiums and the most likely reason for leaves that look skeletonised, ragged, or collapsed inward. Despite its name, the large cabbage white butterfly (Pieris brassicae) lays eggs enthusiastically on nasturtiums as well as on brassicas; the plants share the glucosinolate compounds that the caterpillars can tolerate and most other insects avoid. The adult butterfly lays batches of yellow, ribbed, upright eggs in neat rows on the undersides of nasturtium leaves. Each batch contains 20 to 100 eggs, and a single female lays multiple batches across her short adult life. The eggs hatch into small, yellowish caterpillars that feed initially on the leaf underside in a group, scraping away the lower epidermis and leaving the upper surface intact and translucent, a pattern called window-feeding. As the caterpillars grow they become the familiar yellow-and-black-striped larvae and begin feeding through the whole leaf, moving outward from the midrib and leaving only the main veins behind in a pattern called skeletonisation. A substantial infestation of medium to large caterpillars can strip a nasturtium plant almost completely within a week. The leaves that remain curl and collapse as the supporting tissue between the veins is consumed. The caterpillars are social in their early instars, clustering together on the underside of a leaf and feeding outward; later instars disperse across the plant and are harder to catch all at once.
How to fix it
Inspect nasturtium leaves daily from early June, concentrating on the undersides where both eggs and early-instar caterpillars feed. Egg batches are highly visible: a cluster of neat yellow upright cylinders arranged in rows on the underside of a leaf is unmistakable. Crush egg batches on sight between your fingers; this is the single most effective intervention and costs nothing. Pick off small caterpillars by hand and drop them into a bucket of soapy water. For larger infestations, spray with a product containing Bacillus thuringiensis var. kurstaki, a naturally occurring soil bacterium that produces proteins toxic to caterpillars after ingestion but is harmless to birds, mammals, bees, and other insects. Apply to both leaf surfaces, repeat after rain, and continue at five-to-seven-day intervals until the infestation clears. Ground beetles are a valuable natural predator of both eggs and young caterpillars; shelter under nearby mulch, stones, and garden debris encourages them to patrol around nasturtium plants. Avoid using broad-spectrum insecticides, which kill the beneficial insects that help keep all pest populations in check through the season.
2. Black bean aphid (Aphis fabae)
Black bean aphid is the second major cause of nasturtium leaf curling and the reason nasturtiums are planted as a trap crop in the first place. Aphis fabae is a large, very dark brown to black aphid that forms dense, conspicuous colonies on the soft shoots and stems of a wide range of garden plants. Its primary summer hosts are beans (Vicia faba and Phaseolus vulgaris), courgettes, and various ornamentals, but nasturtiums are one of its most favoured alternative hosts and are widely planted near vegetable beds specifically to draw colonies away from food crops. The aphids feed by piercing plant tissue with their stylets and withdrawing phloem sap. On nasturtiums, they congregate on the young growing shoots and on the undersides of developing leaves, and their feeding causes the soft young leaves at the shoot tips to curl tightly downward and inward as they lose turgor. The curled leaves shelter the colony, making the aphids harder to reach and dislodge once the leaf has fully curled around them. Aphid colonies excrete large amounts of sticky honeydew, which coats the leaf surfaces below the colony and supports the growth of black sooty mould. Heavily infested nasturtiums produce a characteristic sticky, blackened appearance on the stems and older leaves below the colony, and the plant's overall vigour declines as the aphid population drains it of carbohydrates. In a typical UK garden the peak migration of black bean aphid from the primary host, field beans, occurs in June and July, when winged aphids seek alternative summer hosts and nasturtiums, with their scent and soft growth, are a primary target.
How to fix it
The first question to ask is whether you want to fix it at all. If the nasturtiums were planted specifically as a trap crop to protect nearby beans, courgettes, or other vegetables, the aphid colony is doing exactly what it was intended to do. Leave it in place, and allow ladybirds, lacewing larvae, and hoverfly larvae to build up and work through the population. This provides a genuinely useful reservoir of natural predators that will eventually spill over onto neighbouring plants and reduce aphid pressure across the whole plot. If the nasturtiums are a display planting rather than a sacrificial companion, act early. Check shoot tips weekly from late May. Small colonies detected early can be rubbed off by hand or dislodged with a sharp jet of water from a hose; this removes both the aphids and the sticky honeydew, and is usually sufficient when done promptly. For larger colonies, spray all affected surfaces thoroughly with insecticidal soap or a diluted neem oil solution, making sure to reach inside the curled leaves where the aphids shelter. Repeat every five to seven days until the colony collapses. Growing nasturtiums in lean, low-fertility soil rather than enriched beds makes the plant tissue firmer and less attractive to aphids; lush, over-fed nasturtiums are substantially more vulnerable to heavy infestations than plants growing in average or poor soil.
Other causes to consider
Flea beetle (Phyllotreta spp.) produces a characteristic pattern of small, round or oval holes across nasturtium leaves rather than the ragged skeletonisation of caterpillars. The tiny black beetles are rarely seen directly as they leap away when disturbed, but the shothole damage they leave is distinctive. Severe flea beetle feeding removes enough tissue to distort the leaf as it develops, producing a puckered, slightly curled appearance around each hole. Flea beetle damage on nasturtiums is generally cosmetic and does not threaten the plant; it is most common on seedlings and young transplants in dry spring conditions and typically reduces as the plant matures and the leaf tissue toughens.
Melon and cotton aphid (Aphis gossypii) affects nasturtiums grown under glass or in polytunnels. This species is smaller and paler than black bean aphid, ranging from pale yellow to dark green, and forms dense colonies on the undersides of leaves, causing similar curling and honeydew production. It is a more persistent pest under cover than Aphis fabae because the warm, sheltered conditions prevent the population crashes that outdoor colonies experience in cool, wet weather. Introduce parasitic wasps (Aphidius colemani or A. ervi) as a biological control in glasshouse situations at the first sign of colony formation.
Drought stress causes nasturtium leaves to roll inward along their length, reducing the exposed surface area to limit water loss. The whole plant may look slightly limp and the leaves take on a duller, more muted tone. Nasturtiums are reasonably drought-tolerant once established, but container-grown plants and those in free-draining sandy soil can suffer in hot UK summers. Water thoroughly at the base of the plant and the leaves will uncurl and recover within 24 hours in most cases. Avoid overhead watering, which wets the foliage without efficiently reaching the roots.
Cucumber mosaic virus produces mottled yellow and green patterning on nasturtium leaves alongside distortion and curling that does not respond to watering or pest control. The virus is spread by aphids, particularly Myzus persicae, moving from infected plants to healthy ones. Affected plants should be removed and binned immediately; do not compost them. There is no treatment. Controlling aphid populations with insecticidal soap and encouraging natural predators reduces the risk of viral spread within the garden.
Keeping nasturtiums healthy
- Sow direct into the ground from May after the last frost, or into modules under glass from April for transplanting out in late May. Nasturtiums dislike root disturbance and establish best when sown where they are to grow.
- Grow in poor to average, well-drained soil without added fertiliser. Rich soil or regular feeding produces abundant soft, lush growth that is far more attractive to aphids than the firmer tissue produced by plants growing lean. Flowers are also reduced significantly in over-fed nasturtiums.
- Use climbing nasturtiums as a trap crop along the edge of brassica or bean beds. The plants draw cabbage white butterflies and black bean aphid away from the food crops; allow some pest pressure on the trap crop and rely on natural predators to build up.
- Inspect leaf undersides daily from late May for cabbage white egg batches. Crushing eggs as soon as they are laid is the most cost-effective form of caterpillar control and prevents the cycle before it begins.
- Encourage ground beetles by leaving some leaf litter and undisturbed soil around the plants. Ground beetles patrol the soil surface at night and consume large numbers of caterpillar eggs, small caterpillars, and aphids that fall from the plant.
- Avoid broad-spectrum pesticides. Nasturtiums attract a wide range of beneficial insects, including ladybirds, lacewings, and hoverflies, all of which feed on aphids and caterpillars. Killing these allies makes every subsequent pest problem harder to manage.
- Allow nasturtiums to self-seed at the end of the season. The seeds germinate reliably the following May and produce plants that are often more vigorous than direct-sown seedlings.
Frequently asked questions
Why are my nasturtium leaves curling?
The two most common causes of curling nasturtium leaves in UK gardens are cabbage white caterpillars (Pieris brassicae) and black bean aphid (Aphis fabae). Caterpillars skeletonise leaves from the underside, causing them to collapse and curl as the tissue dies. Dense aphid colonies on young shoots cause leaves to curl downward and inward as the aphids drain sap from the growing tips. Flea beetle, drought stress, and cucumber mosaic virus are less common additional causes.
Should I be worried about aphids on nasturtiums?
Not necessarily. Nasturtiums are widely grown as a trap crop specifically to draw black bean aphid away from beans, courgettes, and other vegetables. A heavy aphid colony on a nasturtium is doing exactly what a trap crop is supposed to do. If the nasturtiums are planted to protect other crops, allow the aphids to colonise them and rely on ladybirds and other natural predators to build up and control the population. Only treat if the nasturtiums themselves are a prized display planting rather than a sacrificial companion.
How do I get rid of caterpillars on nasturtiums?
Check the undersides of leaves daily for the yellow-and-black egg clusters laid in neat rows by cabbage white butterflies, and crush them on sight. Small caterpillars can be picked off by hand and dropped into a bucket of soapy water. For larger infestations, spray with a product containing Bacillus thuringiensis var. kurstaki (Btk), a naturally occurring bacterium that kills caterpillars but is harmless to other wildlife, birds, and pollinators. Ground beetles, which shelter under nearby mulch and debris, are an important natural predator of both eggs and young caterpillars.
Can nasturtiums get a virus?
Yes. Cucumber mosaic virus (CMV) infects nasturtiums and produces distinctive mottled yellow and green patterning on the leaves, along with distortion and curling that cannot be fixed by watering or pest control. The virus is spread by aphids feeding on infected plants and then moving to healthy ones. There is no treatment. Remove and bin affected plants immediately to prevent aphids from carrying the virus to neighbouring plants. Do not compost virus-infected material.
Why are my nasturtiums growing so poorly even without visible pests?
Nasturtiums are one of the few plants that actively perform better in poor, low-fertility soil. In rich or heavily fertilised soil they produce enormous amounts of lush leafy growth at the expense of flowers, and that soft growth is significantly more attractive to aphids than the firmer growth produced in lean conditions. If your nasturtiums look lush but are curling, pale, or covered in aphids, avoid feeding them. Grow them in average to poor, well-drained soil with no added fertiliser. Drought stress in genuinely dry conditions can also cause inward curling; a thorough watering at the base of the plant resolves this within a day.