Why Are My Amaranthus Leaves Curling?

Aphid infestation

Aphids are the most common pest on amaranthus. They colonise the shoot tips and the undersides of the large leaves, where their feeding causes the leaves to curl, pucker, and distort. Green peach aphid and black bean aphid are both common on amaranthus. Heavy infestations reduce the quality of the foliage, which is the main ornamental feature of decorative varieties, and can distort the developing flower tassels of Amaranthus caudatus.

What to do

• Inspect the shoot tips and the undersides of large leaves regularly. The large leaf surface makes aphid colonies easy to spot once present.
• Knock aphids off with a strong jet of water or spray with insecticidal soap every four to five days for two to three weeks.
• Encourage natural predators: avoid broad-spectrum insecticides that kill ladybirds and hoverflies as well as aphids.
• Avoid excessive high-nitrogen feeding, which produces soft, lush growth that attracts aphids.

Cold stress

Cold stress is one of the most common reasons amaranthus looks poor in UK gardens. Amaranthus is a warm-season tropical annual that needs sustained warmth to perform well: temperatures below 10 to 12 degrees Celsius at night cause the large leaves to curl, pale, develop purplish discolouration, and lose their characteristic rich colour. Plants put out too early in the season, or grown through a cool, wet British summer, often look thin and weak compared with plants grown in consistently warm conditions. Plants that survive a cold spell usually recover once warm weather returns.

What to do

• Do not plant amaranthus outside until late May or early June when nighttime temperatures are reliably above 12 degrees Celsius.
• Harden off transplants thoroughly for at least two weeks before planting out.
• Choose a warm, sheltered position in full sun for the best foliage colour and most vigorous growth.
• Cover plants with fleece on cold nights in early summer if temperatures drop unexpectedly.
• In cool UK summers, growing amaranthus in a polytunnel or greenhouse significantly improves performance.

Drought stress

Despite being a warm-season plant, amaranthus has large leaves with significant water demand, and the plant wilts and curls quickly when the soil dries out in containers or in free-draining soils. The large leaves transpire heavily in full sun and the plant can look stressed within a single hot day if the soil has dried out. However, amaranthus is more resilient to short-term drought than many annuals and usually recovers quickly once watered.

What to do

• Water amaranthus regularly during warm, dry spells: check the soil or compost daily in hot weather and water before the leaves show stress symptoms.
• Container-grown amaranthus needs watering every one to two days in warm weather due to the large leaf area and rapid water loss.
• Mulch around border plants to conserve soil moisture and reduce temperature fluctuations at the root zone.

Spider mite

Spider mite is a common problem on amaranthus in hot, dry conditions, particularly on plants grown under glass or in sheltered, warm positions. The large leaves develop fine bronzing or stippling on their upper surface, the leaf edges curl, and the characteristic foliage colour is dulled. In severe cases, fine webbing is visible between the stems and on the undersides of the leaves. The warm, dry conditions that amaranthus prefers are also ideal for spider mite development, so the two frequently coincide.

What to do

• Examine the undersides of the large leaves for the tiny mites and their stippling damage.
• Spray with insecticidal soap or neem oil every five to seven days for four to six weeks.
• Increase humidity around the plant: mist the foliage in the morning and improve ventilation around container-grown plants.
• For plants under glass, the predatory mite Phytoseiulus persimilis provides highly effective biological control.

Overwatering

Despite its large leaves and warm-weather vigour, amaranthus does not tolerate persistently waterlogged soil. In containers without adequate drainage or in heavy, poorly drained soil, the roots deteriorate and the large leaves yellow, curl, and droop even though the soil is wet. The thick, fleshy stems of some varieties are particularly prone to rotting at the base in wet conditions. This is most common when plants are kept well-watered during a cool, overcast UK spell when growth slows significantly.

What to do

• Plant amaranthus in well-drained soil. The roots need oxygen as well as moisture and do not tolerate saturated conditions.
• In containers, use a free-draining compost and ensure excellent drainage. Reduce watering frequency during cool or overcast periods.
• Allow the surface of the compost to dry slightly between waterings rather than keeping the compost constantly moist.

Frequently asked questions

Why are my amaranthus leaves curling?

Amaranthus leaves curl most often from aphid infestation or cold stress. Aphids colonise the shoot tips and the undersides of the large leaves, causing them to curl and distort. Cold nights below 10 degrees Celsius cause the large, heat-loving leaves to curl, pale, and look bleached, as amaranthus is a warm-season plant that performs poorly in cool conditions.

Why are my amaranthus leaves turning pale and curling?

Pale, curling leaves on amaranthus most often indicate cold stress, nitrogen deficiency, or spider mite infestation. Amaranthus is a warm-season annual that needs temperatures above 15 degrees Celsius to thrive: cold nights below 10 degrees cause the leaves to pale and curl. Check the undersides for the stippling and fine webbing of spider mite. Feed with a nitrogen-rich fertiliser if the plant is growing in poor soil.

How do I get rich colour in amaranthus foliage?

The richest foliage colours in decorative amaranthus varieties develop in full sun, warm temperatures, and slightly lean soil. Avoid high-nitrogen feeds which encourage lush green growth at the expense of the characteristic deep red, burgundy, and bronze pigmentation. Phosphorus-rich feeds and adequate sunlight intensify the foliage colour.

When should I sow amaranthus?

Sow amaranthus under glass in April at 20 to 22 degrees Celsius. The seeds are tiny and should be surface-sown or covered only lightly with vermiculite. Germination takes seven to fourteen days at the right temperature. Plant out only after all risk of frost has passed and nighttime temperatures are reliably above 12 degrees Celsius, typically late May to early June in most UK regions.