Canna Leaves Curling

Cannas bring a genuine sense of the tropics to British gardens. The paddle-shaped leaves on varieties like 'Tropicanna', with its spectacular striped foliage, or the near-black of 'Black Knight', are as ornamental as the showy flowers themselves. So when those leaves start curling, rolling, or dying back, it is worth diagnosing the cause quickly. Most problems are fixable if you catch them early.

The canna leafroller caterpillar

If you see a canna leaf that has been rolled into a tight cigar shape and bound with visible silk threads, you are looking at the work of the canna leafroller caterpillar, Calpodes ethlius. This is the most distinctive pest you are likely to encounter on cannas in the UK, and the damage is hard to confuse with anything else.

The caterpillar wraps itself inside a single leaf, securing the roll with silk, and feeds safely within its shelter. You will notice the rolled leaf does not respond normally to water or wind, and if you gently unroll it you will find a green caterpillar inside.

Chemical sprays are useless here. The rolled leaf acts as a physical barrier and no contact insecticide can penetrate it. The only effective approach is to open the roll carefully by hand, remove the caterpillar, and destroy it. Go through your plants every few days during summer, as new rolls can appear quickly.

In recent mild UK winters, particularly in the south-west, this pest has been known to survive outdoors on cannas left in the ground. Lifting and storing rhizomes (which you should be doing anyway in most parts of the UK) removes the overwintering habitat and helps keep numbers down year on year.

Canna rust

The most damaging disease affecting cannas in the UK is canna rust, caused by the fungus Puccinia thaliae. Look for orange-brown powdery pustules on the undersides of leaves, with corresponding yellow-orange spots or patches on the upper surface. Affected leaves curl, yellow from the tips downward, and eventually die. In wet, warm summers the disease can spread with alarming speed from plant to plant.

Act immediately when you spot rust. Remove and bin every affected leaf, and do not put them in the compost heap as spores can persist. Improve air circulation around your plants by giving them more space, and switch to watering at the base rather than overhead. There is no fully effective fungicide treatment available to home gardeners in the UK for this disease, so physical removal is your main tool.

If the infection is severe and has spread through most of the plant, it is better to destroy the whole plant rather than battle on. Critically, do not lift and store rhizomes from heavily rusted plants. Rust spores can survive on the dormant rhizome over winter and the disease will re-emerge the following season, potentially spreading to the rest of your collection.

Other causes of curling and distortion

Canna mosaic virus. If your canna leaves show yellow streaking, mosaic-patterned discolouration, or distorted growth, the plant is likely infected with canna mosaic virus (CaMMV or CMV). This is spread by aphids moving between plants. There is no cure and no treatment. Remove and destroy the entire affected plant as soon as you suspect infection, and put aphid controls in place on the remaining plants to limit the spread of the virus.

Slugs and snails on emerging shoots. In spring, cannas push up new shoots that are tightly furled into a rolled tube before they open. That roll is an ideal hiding place for slugs. Check inside every emerging shoot at dusk or after wet weather and remove any slugs you find. Damage to the shoot at this stage results in ragged, torn leaves once they unfurl, which can look like caterpillar damage but lacks the silk binding.

Aphids. Aphids are less common on cannas than on many garden plants, but shoot tips can occasionally become colonised, leading to distorted, curled growth at the growing point. Check shoot tips regularly and use an appropriate aphid control if populations are building.

Rhizome rot from waterlogging. Cannas need moisture-retentive soil, but they are not tolerant of waterlogging, particularly when the rhizomes are dormant in winter. A rhizome that has sat in cold wet ground will rot, and the plant that emerges the following spring, if it emerges at all, will be weak and prone to poor growth and leaf problems. Good drainage is essential, and in wetter parts of the UK, lifting and storing rhizomes over winter protects them from this risk entirely.

Growing cannas well in the UK

Cannas are among the most rewarding plants you can grow for a tropical effect in British gardens. Paired with bananas, dahlias, and salvias they create genuinely dramatic plantings. They reward the effort you put in: a warm, sunny, sheltered position (ideally in front of a south or west-facing wall or fence), moisture-retentive but well-drained soil, generous feeding through summer, and the right approach to winter storage.

In mild areas including Cornwall, sheltered coastal gardens, and the warmest parts of London, rhizomes can sometimes be left in the ground beneath a deep, dry mulch of bark or straw applied after the first autumn frost. Everywhere else, lift the rhizomes once the first frost has blackened the foliage in October or November. Shake off loose soil, allow them to dry briefly, and pack them into boxes of barely moist compost or bark. Store in a frost-free greenhouse, garage, or shed. In March, pot them up and bring them into warmth to start them into growth, or wait until after the last frost in May before planting them directly outside.

Cannas that are well sited, well fed, and lifted and stored correctly will reward you with bolder foliage and more flowers every year.

Frequently asked questions

Why is my canna leaf rolled into a tight cigar shape bound with silk?

That is the unmistakable work of the canna leafroller caterpillar (Calpodes ethlius). The caterpillar wraps a single leaf into a tube, secures it with silk thread, and feeds inside. Chemical sprays cannot penetrate the rolled leaf, so the only effective method is to unroll the leaf by hand, remove the caterpillar, and destroy it. Check your plants every few days through summer, as new rolls appear quickly.

What are the orange powdery spots on the underside of my canna leaves?

Orange-brown powdery pustules on leaf undersides, paired with yellow-orange spots on the upper surface, indicate canna rust (Puccinia thaliae). Remove and bin all affected leaves immediately. Do not compost them. Improve air circulation, avoid overhead watering, and do not store rhizomes from severely infected plants as rust spores can survive on them through winter.

My canna leaves have yellow streaks and a mosaic pattern. What is wrong?

Yellow mosaic patterns, streaking, and distorted growth point to canna mosaic virus (CaMMV or CMV), spread by aphids. There is no cure. Remove and destroy the entire plant to prevent the virus spreading to healthy cannas nearby, and put appropriate aphid controls in place on the rest of your collection.

Can I leave canna rhizomes in the ground over winter in the UK?

In mild areas such as Cornwall, coastal south Wales, sheltered London gardens, and the Isles of Scilly, cannas sometimes survive outdoors beneath a very deep mulch of dry bark or straw applied after the first frost. Elsewhere in the UK it is far safer to lift rhizomes in October or November, dry them briefly, and store them in barely moist compost or bark in a frost-free greenhouse. Replant outside after the last frost, usually from mid-May.

Why are the new rolled shoots on my canna being damaged in spring?

Emerging canna shoots unfurl from a tightly rolled tube of leaf, which makes a perfect hiding place for slugs and snails. Check inside every rolled shoot at dusk or after rain, remove any slugs you find, and use slug controls around the base of the plant. The same rolled shoot can also attract aphids, so inspect shoot tips regularly as growth gets underway.