Why are my nasturtium leaves curling?

Nasturtium (Tropaeolum majus) leaves curl most commonly due to black bean aphids, whitefly, or overwatering. The black bean aphid (Aphis fabae) is one of the most notorious pests of nasturtium: dense colonies of shiny black aphids crowd the stems, leaf undersides, and growing tips in late spring and summer, causing the leaves to curl, cup downward, and become coated in sticky honeydew. Nasturtium is so attractive to black bean aphids that it is commonly planted as a trap crop to lure aphids away from vegetables. Whitefly also infest nasturtium, particularly in sheltered garden positions and greenhouses; the adults are white and fly up when the plant is disturbed, while the scale-like larvae on the leaf undersides cause the leaves to curl and turn yellow. Overwatering or waterlogged soil causes the roots to rot and the leaves to curl and wilt even though the soil is wet.

Why are there black bugs on my nasturtium leaves?

Black insects on nasturtium leaves are almost certainly black bean aphids (Aphis fabae), one of the most common and prolific aphid species on nasturtiums. These are small, shiny, coal-black aphids that form dense colonies on the stems, growing tips, and leaf undersides of nasturtium from late spring onward. They feed by extracting sap from the plant tissue, causing the leaves to curl, cup downward, and become deformed. The honeydew they excrete makes the leaves and stems sticky, and black sooty mold fungus often grows on the honeydew deposits. Nasturtium is so favored by black bean aphids that it is routinely recommended as a trap crop in vegetable gardens: planted near broad beans, beets, and other susceptible crops, it draws the aphids away from the food plants. Because nasturtium is primarily grown as an edible or ornamental annual, heavy aphid infestations on it do not usually warrant pesticide use; the plants can be sacrificed as the trap crop does its job, or simply pulled at the end of the season. For nasturtiums you want to keep productive, blast aphid colonies off with a strong stream of water, which is highly effective when repeated every few days.

Why are my nasturtium leaves curling and turning yellow?

Nasturtium leaves curling alongside yellowing most commonly indicate overwatering and root rot, whitefly infestation, or nutrient deficiency, particularly in containers. Root rot from overwatering or poor drainage causes the lower leaves to turn yellow first, with the yellowing progressing upward and the leaves curling as the plant loses its ability to take up water; the soil will be wet and the roots brown and mushy rather than white and healthy. Whitefly infestation causes a progressive yellowing of affected leaves as the larvae drain the sap; the undersides of yellowing leaves are covered in the white, scale-like nymphs and sticky honeydew deposits. Nitrogen deficiency in containers where the potting mix is exhausted causes uniform yellowing of the older leaves from the base of the plant upward; nasturtium requires very little fertilizer, however, and over-fertilizing with nitrogen is a more common problem than deficiency, since excess nitrogen causes lush leafy growth at the expense of flowers.

Nasturtium Leaves Curling — Why It Happens and How to Fix It

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At a glance

Dense black aphid colonies on stems and leaf undersides, leaves cupped downward: Black bean aphids; water blast every few days, or sacrifice as trap crop
White insects flying up when disturbed, scale-like larvae on leaf undersides: Whitefly; yellow sticky traps, insecticidal soap on undersides
White powdery coating with upward curl, worst in late summer: Powdery mildew; improve air circulation, apply neem oil or potassium bicarbonate
Leaves yellowing and curling, stems soft at base, soil wet: Root rot from overwatering; improve drainage, reduce watering
Leaves curling and plant flagging in afternoon heat: Heat stress; provide afternoon shade, water consistently

Why nasturtium leaves curl

Nasturtiums (Tropaeolum majus and related species) are easy-to-grow annuals and short-lived perennials with edible flowers and leaves, cheerful orange, red, and yellow blooms, and a reputation for thriving in poor soil with minimal care. They grow vigorously and bloom prolifically when planted in a sunny position in average to poor, well-drained soil. When nasturtium leaves curl, the most likely culprits are aphids and whitefly, for which nasturtium is notoriously attractive, or overwatering and excessive fertilizer, which are the most common cultural mistakes with this easy-going plant.

Cause 1: Black bean aphids

Signs: Dense colonies of shiny, coal-black aphids cluster on the stems, leaf undersides, and growing tips. The leaves curl and cup downward, distorted by the feeding. The stems and leaves are coated in sticky honeydew. Black sooty mold may develop on the deposits. The infestation can appear suddenly and spread rapidly through the plant in warm weather. Ants may be tending and protecting the aphid colonies.

Why it happens: Nasturtium is one of the favorite host plants of the black bean aphid (Aphis fabae), a polyphagous species that is also a major pest of broad beans, beets, spinach, and many other garden plants. The aphids are attracted to nasturtium's succulent, peppery foliage and congregate in enormous numbers. Because nasturtium is so attractive to this pest, it is deliberately planted as a trap crop to draw black bean aphids away from more valuable vegetables.

Fix: If the nasturtium is serving as a trap crop, leave the aphids in place and remove the infested plants entirely when populations peak to destroy the colony. For nasturtiums you want to keep productive, blast aphid colonies with a strong stream of water every few days, which is highly effective. Apply insecticidal soap or neem oil for persistent infestations. Avoid broad-spectrum insecticides that harm natural predators, and avoid over-fertilizing, which promotes the lush, soft growth aphids prefer.

Cause 2: Whitefly

Signs: Tiny white moth-like insects fly up in a cloud when the plant is disturbed. The undersides of the leaves are coated with tiny, flat, scale-like nymphs and waxy white egg cases. The affected leaves curl, turn yellow, and eventually drop. Sticky honeydew coats the foliage and black sooty mold may follow. Whitefly is most problematic in sheltered gardens, against warm walls, and in greenhouses where air circulation is limited.

Why it happens: Glasshouse whitefly (Trialeurodes vaporariorum) and silverleaf whitefly (Bemisia tabaci) are both attracted to nasturtium. The adults lay eggs on the leaf undersides and the flattened scale-like nymphs that hatch from them are difficult to dislodge once established. They feed by extracting phloem sap, weakening the plant and producing the copious honeydew that promotes sooty mold. Warm temperatures and still air favor rapid population growth.

Fix: Hang yellow sticky traps near plants to catch adult whiteflies and monitor population levels. Apply insecticidal soap or neem oil to the undersides of all leaves, where the nymphs are concentrated; repeat every five to seven days for three or more applications, as the treatments do not kill eggs and newly hatched nymphs must be targeted as they emerge. Introduce parasitic wasps (Encarsia formosa) in greenhouse settings. Improve air circulation around plants.

Cause 3: Powdery mildew

Signs: A white to grayish powdery coating develops on the upper leaf surfaces in mid to late summer. The affected leaves curl upward and may become distorted. The mildew is worst in warm, humid weather with poor air circulation, particularly as the plants become crowded and the lower leaves are shaded. Powdery mildew on nasturtium tends to be a late-season problem and rarely prevents the plants from flowering through their natural season.

Why it happens: Powdery mildew fungi infect nasturtium leaves under warm, still, humid conditions. Trailing and climbing nasturtiums that become congested and tangled create the dense, poorly ventilated microclimate the disease prefers. The mildew in late summer is largely cosmetic and rarely kills the plant before autumn frosts do so naturally.

Fix: Remove the most affected leaves. Apply neem oil or potassium bicarbonate at the first sign of infection. Thin overcrowded plants to improve airflow. As a cool-season annual, nasturtium often naturally declines as summer heat peaks, so late-season mildew may not be worth treating if the plants are approaching the end of their season anyway. Resow in late summer for an autumn flush in mild climates.

Cause 4: Overwatering and root rot

Signs: The lower leaves turn yellow and the plant wilts and curls despite the soil being wet. The stems may be soft or slimy at the base. In containers, the potting mix stays saturated and the roots are brown and mushy when checked. Nasturtiums are notably intolerant of waterlogged conditions; root rot can develop rapidly in pots without drainage holes or in heavy clay soil.

Why it happens: Nasturtium grows best in average to poor, well-drained soil with moderate watering. It is one of the few garden annuals that performs better in lean, dry conditions than in rich, fertile, moist ones. Overwatering deprives the roots of oxygen and quickly leads to root rot pathogens. Container nasturtiums planted in standard potting compost that retains moisture well are particularly vulnerable when watered frequently.

Fix: Reduce watering frequency significantly and ensure containers have large drainage holes. Allow the soil surface to dry between waterings. Use a gritty, free-draining compost or mix in 20 to 30 percent perlite for container plants. Do not fertilize with rich compost or high-nitrogen feeds; nasturtium performs poorly and becomes all leaf with little flower when overfed. In waterlogged garden beds, improve drainage by incorporating coarse grit before replanting.

Cause 5: Heat stress

Signs: The leaves curl and wilt in the heat of the afternoon, recovering partially overnight. The plant stops flowering in midsummer. Nasturtium is a cool-season plant that grows most vigorously in spring and autumn; sustained heat above 85 to 90 degrees Fahrenheit causes it to slow down, stop blooming, and look stressed even when adequately watered.

Why it happens: Like many cool-season plants, nasturtium is not adapted to sustained summer heat. In hot climates it acts as a cool-season annual, performing well in spring, declining in summer heat, and often recovering in autumn. The flowers close or stop opening in very high temperatures, and the foliage curls to reduce water loss.

Fix: In warm climates, plant nasturtiums in a position that receives morning sun and afternoon shade. Provide consistent moisture during the hottest months. Cut plants back by half in early summer and mulch around them to encourage an autumn recovery when temperatures drop. In very hot climates, treat nasturtiums as spring and autumn annuals and replace with heat-tolerant plants for summer.

Nasturtium Leaves Curling