Tropaeolum speciosum, the flame nasturtium or Scottish flame flower, is a plant that provokes strong feelings in UK gardeners. Those who have seen it in full stride, scrambling through a dark yew hedge with its vivid scarlet flowers and startling electric blue berries, tend to want it immediately. Those who have tried and failed to grow it also have strong feelings. The plant has a reputation for being capricious, and curling leaves are one of the first signs that something has gone wrong.
The good news is that most of the causes are diagnosable at a glance, and the plant's deep rhizomatous root system is often more resilient than the distressed foliage suggests. Here is how to work out what is happening.
Aphids: the most common cause of curling leaves
Nasturtium blackfly (Aphis nasturtii) and black bean aphid (Aphis fabae) are both drawn to Tropaeolum speciosum, and infestations tend to establish on the tender tips and growing shoots first. The characteristic symptom is downward leaf curl, where the leaf edges roll under toward the underside, often cupping around dense colonies of the insects themselves.
Because T. speciosum has much slimmer stems and more delicate foliage than, say, a climbing rose or a Hydrangea, aphid damage here looks more dramatic than it would on a bulkier host. A colony that would barely register on a vigorous shrub can distort entire shoot tips on this plant, making the damage appear severe even when the underlying infestation is modest.
Check the leaf undersides and the soft tissue immediately behind the growing bud. If you see clusters of small dark or greenish insects, aphids are the culprit. A strong jet of water from a hose removes most colonies without chemicals. Repeat every few days until the plant recovers. For heavier infestations, insecticidal soap sprays are effective and low in impact on beneficial insects when used carefully. Avoid broad-spectrum pyrethroids, which will also kill ladybirds and lacewings, two of the most effective natural checks on aphid populations.
Healthy, well-established plants recover quickly from aphid damage. Young plants in their first or second season are more vulnerable and may struggle to push on if colonies are not controlled early.
Late frost damage on emerging spring shoots
In UK conditions, late spring frost is one of the most damaging events for T. speciosum. The plant breaks dormancy and pushes up slender new shoots in spring, and in England and Wales this often coincides with the period when frosts are still possible in April and into May. A single cold night can blacken and curl the emerging growth, and affected shoots typically shrivel back to soil level within a few days.
The symptom looks different from aphid damage: rather than curling inward and downward around insect colonies, frost-damaged shoots curl and distort in multiple directions, often going limp and dark before dying back entirely. The damage usually affects the topmost growth first, and in a sharp frost it can take out everything above ground in one night.
This is partly why T. speciosum performs noticeably better in Scotland and the north of England than in southern England. In Scotland, the plant often starts into growth later, after the main frost risk has passed. The climate is also cooler and moister through the growing season, which suits the plant perfectly.
If frost damage has occurred, resist the urge to cut back or disturb the root zone. The rhizomes are almost certainly alive and will push up replacement shoots. Cover the planting area with a thick layer of dry straw or horticultural fleece before forecast frosts in future years, removing it during the day so the soil can warm up. For plants in their first couple of seasons, a cloche over the emerging shoots in March and April offers useful protection in most parts of England.
Slugs and snails on young shoots
The moist, shaded root conditions that T. speciosum needs are exactly the conditions that slugs and snails prefer. In spring, when the young shoots are slender and succulent, slug damage can remove emerging growth almost as fast as it appears. The symptom here is not so much curling as irregular notching and complete removal of shoot tips, sometimes with a silvery slime trail nearby.
During establishment, this can be genuinely devastating. A plant that has spent its first winter building up root reserves and is finally pushing up good growth in April can be set back by weeks or months if slugs are active in the area.
Copper tape around the planting area, coarse horticultural grit as a mulch over the root zone, and ferric phosphate pellets placed in the area at bud burst all help. Check by torchlight on damp nights in spring and remove slugs by hand. The effort is worth it: once a plant reaches a size where it is producing multiple stems, slug damage becomes proportionally less significant.
Other causes worth checking
Drought stress causes wilting and inward leaf curl, particularly on established plants in warmer or drier parts of the UK. T. speciosum's roots need consistent moisture and will not tolerate drying out. A deep organic mulch over the root zone, combined with regular watering during dry spells, reduces this risk considerably.
Powdery mildew can appear on the foliage in warm, dry conditions, causing a white coating and some distortion of younger leaves. Improving air circulation around the plant and avoiding overhead watering in the evening reduces the likelihood of infection. The plant rarely suffers severely from mildew in cooler, moister Scottish conditions.
Wind scorch on exposed sites causes leaf margin browning and curl. T. speciosum climbs naturally through hedges and shrubs, which provide shelter as well as support. On very exposed sites, providing an artificial windbreak or training the plant into a sheltered hedgerow section reduces the problem significantly.
Getting T. speciosum to thrive: the fundamentals
Many cases of curling leaves on T. speciosum come back to establishment conditions. The plant has very specific preferences, and meeting them consistently is the difference between a plant that sulks for years and one that spreads freely and flowers every summer.
Plant the rhizome deeply, in soil that is acid to neutral, cool, and reliably moist. The roots must be in shade: under a hedge or shrub, or on the north side of a wall or bank, with the top growth able to climb up toward sun. Autumn or early spring planting works best; late spring planting in dry weather is difficult.
Allow two to three years before expecting free flowering. Many gardeners see almost no top growth in the first season and worry the plant has failed. Often it has not. It is investing in root development. Mark the spot carefully to avoid disturbing the rhizomes, and in subsequent springs protect emerging shoots from slugs and late frosts.
Once established, T. speciosum is spectacular. In gardens like Crarae and Brodick Castle in Scotland, where it scrambles through Taxus hedges and Lonicera, it produces exactly the vivid display it is capable of. That result is achievable in many UK gardens, given the right conditions and patience.
Frequently asked questions
Why are my Tropaeolum speciosum leaves curling downward?
Downward leaf curl on Tropaeolum speciosum is most often caused by aphids, particularly nasturtium blackfly (Aphis nasturtii) or black bean aphid (Aphis fabae). Check the undersides of leaves and along tender growing tips for dense colonies. The plant's slender stems and delicate foliage make even moderate infestations look severe.
Can late frosts damage Tropaeolum speciosum?
Yes. Late spring frosts are one of the most damaging events for emerging shoots, especially in England and Wales where growth often starts earlier than in Scotland. Blackened, curled tips that shrivel back to the base are the typical symptom. The plant usually regrows from the root, but the lost growth delays flowering significantly.
Why is my flame nasturtium failing to thrive in England?
Tropaeolum speciosum genuinely struggles in warmer, drier parts of England. It needs cool, consistently moist, acid to neutral soil, with the roots shaded and the top growth in sun. Warmer summers, alkaline or free-draining soil, and dry spells at the root combine to cause stress and prevent establishment. It often performs far better in Scotland and the north of England.
How long does Tropaeolum speciosum take to establish?
One to three years is typical. Many gardeners see little or no top growth in the first season as the plant builds its underground rhizome network. Patience is essential. Once established, it can spread vigorously and produce the vivid scarlet flowers and electric blue berries it is famous for.
What is the best way to protect flame nasturtium from slugs?
Lay copper tape or coarse grit around the planting area before shoots emerge in spring. Ferric phosphate pellets are safe around wildlife and pets and effective against slugs. Check the area at night in damp weather, especially during the first few years when shoots are slender and easily destroyed. The moist soil conditions this plant needs are exactly what slugs prefer.