Why Are My Lemon Balm Leaves Curling?

Aphid infestation

Aphids are the most common cause of curling leaves in lemon balm. They are particularly fond of the soft new growth at shoot tips and the undersides of young leaves, feeding on the sap and injecting compounds that cause the leaf tissue to distort and curl around them. This provides the colony with shelter while they continue to feed. A curled lemon balm leaf almost always has a cluster of small green, black, or grey aphids inside it.

Lemon balm's soft, textured leaves are especially attractive to aphids. Heavy infestations can cause significant distortion of new growth, leave sticky honeydew deposits on stems and lower leaves, and attract the sooty mould fungus that grows on the honeydew.

What to do

• Inspect the undersides of curled leaves and shoot tips. Squeeze any visible colonies between your fingers or rub them off with a gloved hand.
• Blast the plant with a strong jet of water from a hose to knock aphids off, paying attention to the undersides of leaves and within curled growth.
• Because lemon balm is an edible herb, avoid synthetic pesticides. Insecticidal soap spray is safe for edible plants and effective: spray every three to four days until the colony is eliminated.
• Encourage natural predators: ladybirds, lacewings, and parasitic wasps all prey on aphids. Avoid using broad-spectrum insecticides that kill these beneficial insects.
• Pinch out and discard the most severely affected shoot tips to remove the bulk of the population quickly.

Drought stress

Although lemon balm is more drought-tolerant than many herbs, prolonged dry spells cause the leaves to wilt, curl inward, and begin to look dull and lifeless. In containers, lemon balm can dry out surprisingly quickly in warm weather as it is a vigorous plant with a substantial root system that consumes moisture rapidly.

Drought curling is usually accompanied by the whole plant appearing to slump slightly, with older lower leaves yellowing and dropping before the newer growth shows obvious curl. The compost will be completely dry all the way through.

What to do

• Water thoroughly until water drains freely from the base of the pot, then allow the top 2 to 3 cm to dry before watering again.
• Mulch the soil around outdoor plants with a 5 to 7 cm layer of garden compost or bark chippings to retain moisture and reduce the frequency of watering needed.
• Container-grown lemon balm may need watering daily in hot weather. Consider moving pots to a slightly shadier position during the hottest part of the day to reduce moisture demand.
• The plant typically recovers quickly from mild drought: leaves that were curled from water stress usually straighten within a few hours of thorough watering.

Powdery mildew

Powdery mildew is a fungal disease that commonly affects lemon balm, particularly in late summer when warm days and cool nights create ideal conditions. Infected leaves develop a white or grey powdery coating, often accompanied by leaf distortion, puckering, and curling as the fungal hyphae interfere with the leaf tissue. The lower leaves and stems are often affected first.

Poor air circulation, overcrowding, and dry soil combined with high humidity all increase the risk of powdery mildew. Unlike many fungal diseases, powdery mildew actually thrives in relatively dry conditions on the leaf surface while the roots are also stressed by drought.

What to do

• Cut the plant back hard: lemon balm responds well to cutting back to 10 to 15 cm from the ground, which removes most of the infected growth and encourages a flush of fresh, healthy new leaves.
• Improve air circulation by thinning out crowded plants or moving containers to a less congested spot.
• Water the roots regularly rather than the leaves, as wet foliage can exacerbate mildew in some conditions.
• A spray of diluted bicarbonate of soda (1 teaspoon per litre of water) or potassium bicarbonate can help suppress mild infections on a food plant where chemical fungicides are not appropriate.

Overwatering and root rot

Lemon balm planted in heavy, poorly draining soil or kept too wet in a container without drainage holes can develop root rot. Like drought stress, root rot prevents the plant from drawing up water effectively, so the leaves paradoxically wilt and curl even though the compost is wet. The overall plant looks sickly with yellowing, soft leaves and a dark, musty smell from the root zone.

In the garden, lemon balm is most susceptible to root rot in very heavy clay soils that hold water for long periods after rain, or in low-lying spots where water pools.

What to do

• Improve drainage by incorporating grit or coarse sand into heavy clay soil before planting, or raising the planting area.
• For container plants, ensure there are drainage holes and use a peat-free, well-draining compost rather than dense garden soil.
• If root rot is suspected, lift the plant, cut away dark or mushy roots, allow to dry briefly, and replant in improved conditions.

Mosaic virus

Mosaic virus, transmitted by aphids, can occasionally affect lemon balm. Infected plants develop irregular yellow or pale green mosaic patterns on the leaves alongside distortion and curling. Unlike aphid-caused curling, the discolouration is not confined to where aphids are present and the pattern tends to be more random and patchy across the whole leaf. There is no cure for mosaic virus in herb plants.

What to do

• If mosaic virus is confirmed by the characteristic yellow mosaic leaf pattern, remove and dispose of the affected plant in the bin (not the compost heap) to prevent spread.
• Control aphid populations on remaining plants, as aphids are the primary transmission route for the virus.
• Wash hands and tools after handling infected plants before touching other herbs.
• Lemon balm grows readily from seed or divisions, so replacing an infected plant is straightforward.

Frequently asked questions

Why are my lemon balm leaves curling?

Lemon balm leaves most often curl because of aphid infestation or drought stress. Aphids cluster on the undersides of young leaves and shoot tips, causing the leaves to pucker and curl as they feed on the sap. Drought causes the whole plant to wilt and the leaves to curl inward to reduce water loss.

How do I get rid of aphids on lemon balm?

The safest approach on an edible herb is to wash aphids off with a strong jet of water directed at the undersides of leaves, then follow up by squeezing any remaining colonies between your fingers. If the infestation is heavy, an insecticidal soap spray is safe to use on edible herbs and effective against aphids.

Why are my lemon balm leaves curling and turning yellow?

Yellow curling leaves can indicate aphid infestation, overwatering and root rot, or mosaic virus. Check the undersides of leaves for aphid colonies first as this is the most common cause. If no pests are present and the roots are healthy, mosaic virus (which causes yellow mosaic patterns alongside curling) may be responsible.

Is it safe to eat lemon balm leaves that have been curled by aphids?

Yes, once you have washed the leaves thoroughly to remove aphids and their residue, the leaves are safe to eat. Aphid damage is cosmetic: the leaves may be slightly misshapen or discoloured but the plant itself is not made toxic by the infestation.