Why Are My Sweetcorn Leaves Curling?

Drought

Drought is by far the most common cause of leaf rolling on sweetcorn in UK gardens. Sweetcorn has one of the highest water requirements of any vegetable crop and the long, narrow leaves roll inward along their length when the plant is water-stressed, reducing the leaf surface area exposed to sunlight and lowering transpiration. This roll typically begins at the leaf tips and progresses toward the stem as stress continues. A sweetcorn plant with rolled leaves that recovers and unfurls by the following morning after evening watering was suffering from short-term heat stress; a plant whose leaves remain rolled the following morning needs immediate watering. Drought during the period when the silks are emerging (usually August in the UK) is particularly damaging, as it prevents effective pollination and results in poorly filled cobs.

What to do

• Water sweetcorn deeply and consistently from July onward, giving particular priority to the silk-emergence period when the female silks first appear on each cob. A thorough soaking every 5 to 7 days in dry conditions is more beneficial than frequent shallow watering.
• Apply a generous mulch of compost or straw around the base of the plants to conserve soil moisture and reduce the frequency of watering needed in summer.
• Sweetcorn planted in light, sandy soil benefits most from irrigation: plants in heavier, moisture-retentive clay soil are significantly more drought-tolerant.

Corn leaf aphid

Corn leaf aphid (Rhopalosiphum maidis) is a blue-green aphid that colonises sweetcorn plants from summer onward, feeding on the leaves and particularly inside the developing tassels at the top of the plant. Heavy infestations produce large quantities of sticky honeydew, which coats the leaves and promotes sooty mould growth, and can impair tassel function by gumming up the pollen-producing anthers, potentially reducing cob pollination. Infested leaves may curl around dense colonies near the growing tip.

What to do

• Inspect sweetcorn plants weekly from June for aphid colonies, paying particular attention to the leaf axils and the tassels when they emerge. Small colonies can be squashed by hand or removed with a strong jet of water.
• Apply insecticidal soap spray to larger colonies, covering the underside of affected leaves and the tassel area. Repeat every 5 to 7 days.
• Natural predators including ladybirds and lacewings provide effective aphid control on sweetcorn if they are present in the garden; avoid broad-spectrum insecticides that disrupt predator populations.

Corn smut

Corn smut (Ustilago maydis) is a fungal disease that produces large, grotesque, silver-grey galls filled with black spores on the cobs, tassels, and occasionally on the leaves and stems of sweetcorn plants. It is not common in the UK but occurs in hot summers and is more prevalent in areas where sweetcorn is grown repeatedly on the same ground. The galls are initially silver and firm, then rupture to release masses of black spores. Plants with smut galls are often stunted and produce reduced yields.

What to do

• Remove and destroy smut galls before they rupture and release spores. Do not compost infected material.
• Rotate sweetcorn to a new position each year and avoid growing on land that had smut the previous season.

Wind damage

Sweetcorn is tall and top-heavy and is highly susceptible to wind damage, which can flatten plants, snap stems, or lodge (push the stems at an angle) the entire block. Wind-damaged plants develop kinked or leaning stems and the leaves may be torn or curl around wind-damaged sections. Even without visible breakage, wind rock loosens the roots and impairs water and nutrient uptake, causing symptoms similar to drought stress.

What to do

• Plant sweetcorn in a sheltered, sunny position away from prevailing winds. A block planting arrangement provides mutual shelter: the central plants in a block are protected by the surrounding plants.
• Earth up the base of sweetcorn stems to the first node when the plants are 30 to 40 centimetres tall. Aerial roots develop from the stem nodes and anchor the plant, providing significantly better wind resistance than un-earthed plants.

Slugs

Slugs attack sweetcorn seedlings and young transplants, rasping holes in the emerging leaves that cause them to unfurl with ragged, curled edges. A seedling whose growing tip is attacked by slugs may produce deformed or absent subsequent leaves. Slug damage is most severe in cool, wet spring conditions immediately after transplanting.

What to do

• Apply nematode slug control (Phasmarhabditis hermaphrodita) to the soil around transplanted sweetcorn immediately after planting. This provides protection through the most vulnerable early weeks.
• Start sweetcorn under cover in individual pots and transplant as robust young plants rather than sowing direct: transplanted plants with established root systems tolerate slug damage far better than newly germinated seedlings.

Frequently asked questions

Why are my sweetcorn leaves curling and rolling?

Sweetcorn leaves curl and roll most often from drought or corn leaf aphid infestation. Sweetcorn is a high-water-demand crop with long, strap-like leaves that roll upward along their length in response to water stress: this leaf rolling is the plant's mechanism for reducing transpiration and is a reliable early warning sign that the plant needs water. Corn leaf aphid (Rhopalosiphum maidis) colonises the leaves and inside the developing tassels, causing sticky honeydew deposition and sooty mould growth, and infested leaves may curl around the colonies. The leaves of sweetcorn also curl naturally in hot, sunny conditions even when well-watered: this physiological roll reverses in the evening.

How do I get sweetcorn to fill the cob properly?

Poor cob fill (missing or shrivelled grains on the cob) is one of the most common problems with sweetcorn in UK gardens. The most important factor is good pollination: sweetcorn is wind-pollinated and relies on pollen from the male tassels at the top of the plant falling onto the female silks that emerge from each cob. Plant sweetcorn in a block of at least 4 rows of 4 plants (16 plants minimum) rather than in a single row, as this maximises the chance of pollen from one plant landing on the silks of adjacent plants. Dry, hot weather during silking (when the silks are first emerging) can desiccate the silks and prevent pollen adhesion; water consistently during this period.

When is sweetcorn ready to harvest in the UK?

Sweetcorn is ready to harvest in the UK from August to October, approximately 3 to 4 weeks after the silks first emerge on the cob. The silks brown and dry from the tip downward as the cob matures. To check ripeness, peel back part of the outer husk and pierce a grain with a fingernail: the liquid inside should be milky and creamy (perfect ripeness). Clear, watery liquid means the cob is unripe; thick, paste-like liquid means it is overripe. Ripe sweetcorn deteriorates very quickly after harvest as the sugars convert to starch: pick just before cooking and eat the same day for the best flavour.

Can I grow sweetcorn in the UK?

Sweetcorn can be grown successfully throughout most of the UK but requires a warm, sheltered position and a reasonably long, warm summer to ripen its cobs. It is frost-tender and cannot be sown outside until late May after the last frost. To maximise the growing season in the UK, start sweetcorn under cover in pots from late April and transplant outside in late May or early June. Modern F1 varieties bred for early ripening and cooler climates, such as 'Earlibird', 'Lark', and 'Swift', are much better suited to UK conditions than older varieties. Sweetcorn needs a warm, sheltered, sunny position and does poorly in exposed, windy gardens.