Why Are My Tarragon Leaves Curling?

Spider mite

Two-spotted spider mite (Tetranychus urticae) is the most damaging pest of tarragon in UK gardens and under glass. Mites feed by piercing the individual leaf cells, leaving a characteristic pale, bronze, stippled mottling on the upper leaf surface. In heavier infestations, fine silky webbing appears on the underside of leaves and between stems, and the leaves curl, yellow, and drop. Spider mites are at their worst on tarragon in hot, dry conditions from July to September and are especially problematic on plants in sheltered positions, against south-facing walls, or grown under glass.

What to do

• Inspect the underside of tarragon leaves weekly from June, looking for the characteristic stippled mottling and any fine webbing. Early intervention before populations build is much more effective than tackling a large infestation.
• Increase humidity around the plant: spider mites cannot reproduce effectively in genuinely humid conditions. Mist the leaves with water in the morning, place pots on trays of damp gravel, and move plants away from dry, hot, sheltered positions.
• The predatory mite Phytoseiulus persimilis gives excellent biological control when introduced early in the season before mite populations are established. Particularly useful for tarragon in a greenhouse or polytunnel.
• Insecticidal soap or neem oil applied thoroughly to the underside of leaves every 5 to 7 days provides contact control. Multiple applications are needed to break the mite life cycle.

Drought

Tarragon is moderately drought-tolerant once established but not immune to drought stress. In prolonged dry spells, the narrow leaves wilt and curl inward. Severe drought also weakens the plant's resistance to spider mite, which thrives in the same hot, dry conditions that stress the plant. Drought is much more damaging on tarragon growing in light, free-draining soils or in pots than on plants in heavier, moisture-retentive soil.

What to do

• Water tarragon during extended dry spells, allowing the soil to dry out somewhat between waterings but not to the point of the leaves wilting. Tarragon in pots needs more frequent watering than garden-grown plants.
• Apply a layer of compost or bark mulch around the base of garden-grown tarragon to retain soil moisture and reduce the frequency of watering needed.

Aphids

Aphids colonise tarragon growing tips in spring and early summer, causing the young leaves to curl inward around the feeding colonies. The most common species on tarragon are greenfly. Aphid damage on tarragon is usually self-limiting as natural predators (ladybirds, hoverfly larvae, parasitoid wasps) are attracted to the plant and control populations within a few weeks.

What to do

• Inspect tarragon growing tips from May for aphid colonies. Small colonies can be squashed between the fingers or knocked off with a jet of water. Insecticidal soap spray is effective against larger colonies; avoid using systemic insecticides on a culinary herb.

Root rot

Tarragon growing in heavy, poorly drained soil or in waterlogged pots develops root rot. The symptoms appear above ground as yellowing and wilting of the lower leaves, which then curl and drop. The plant loses vigour progressively from the base upward. Root rot is most common in the UK during wet winters and on plants in containers without adequate drainage.

What to do

• Grow tarragon in well-drained soil: if your soil is heavy clay, incorporate coarse grit at planting and grow the plant on a slight mound. In containers, use a free-draining compost mix (compost plus grit, approximately 2:1) and ensure drainage holes are clear.
• If root rot is already present, carefully lift the plant and inspect the roots: healthy roots are white or cream, rotted roots are dark and mushy. Remove rotted material, treat the remainder with a copper-based fungicide if available, and replant in fresh, well-drained compost or a new, better-drained garden position.

Cold damage

French tarragon is mostly hardy in the UK but the top growth is killed by hard frosts: the leaves turn grey-brown and collapse. This is not usually fatal; the roots survive below ground and re-shoot in spring. The crown is more vulnerable to winter cold in waterlogged soil than in well-drained soil.

What to do

• Cut tarragon back to 5 to 10 centimetres above the ground after the first frosts. In cold areas, cover the crown with a dry mulch of straw or dead leaves from November to March. In pots, move containers to a frost-free shed or garage for the winter if temperatures regularly fall below -10°C in your area.

Frequently asked questions

Why are my tarragon leaves curling?

Tarragon leaves curl most often from spider mite infestation, drought, or aphid feeding. Spider mites (Tetranychus urticae) produce characteristic pale, bronze stippling on the leaves with fine webbing on the undersides, and the leaves curl and yellow as the infestation progresses. Mites are worst in hot, dry conditions in summer and are particularly common on tarragon grown in sheltered positions or under glass. Drought causes the narrow leaves to wilt and curl inward. Aphids colonise the growing tips and cause the young leaves to curl around the feeding colonies. Tarragon grown in heavy, poorly drained soil may also develop root rot, which manifests as yellowing and wilting from the base upward.

What is the difference between French and Russian tarragon?

French tarragon (Artemisia dracunculus var. sativa) and Russian tarragon (Artemisia dracunculus) are very different plants in terms of flavour, despite their similar appearance. French tarragon has the characteristic warm, anise-flavoured, aromatic leaves used in classic French cooking (béarnaise sauce, fines herbes, tarragon vinegar). It is sterile and cannot be grown from seed; it must be propagated by division or from cuttings. Russian tarragon looks almost identical to French tarragon but has completely different, virtually tasteless leaves that lack the anise flavour. It grows readily from seed and is much more vigorous and cold-tolerant than French tarragon, which is why seed packets labelled simply 'tarragon' almost always contain Russian tarragon. To ensure you have genuine French tarragon, buy from a reputable herb nursery, grow from cuttings or division, and taste a leaf before purchase.

How do I overwinter French tarragon in the UK?

French tarragon is generally hardy in most of the UK (USDA zone 5 to 8) and dies back to the ground each autumn, overwintering as dormant roots and re-shooting from the crown in spring. In very cold areas or in prolonged hard winters, the crown can be killed, particularly in wet, heavy soil. To overwinter French tarragon successfully: cut the stems back to about 5 to 10 centimetres above the ground in autumn; in cold areas, cover the crown with a layer of straw, dry leaves, or horticultural fleece from November to March; ensure drainage is good, as waterlogging in winter kills the roots more reliably than frost does; divide and replant every 2 to 3 years in spring to maintain vigour, as older plants become woody and less productive.

How do I propagate French tarragon?

French tarragon is sterile and produces no viable seed, so it must be propagated vegetatively. The two main methods are division and stem cuttings. Division: lift the entire plant in spring as new growth begins, tease the root mass apart into sections each with several shoots, and replant the sections in well-drained soil 30 to 40 centimetres apart. Division also rejuvenates older, woody plants that have become less productive. Stem cuttings: take 8 to 10 centimetre cuttings from the growing tips in late spring or early summer, remove the lower leaves, dip in rooting hormone, and insert into free-draining compost. Place in a propagator or cover with a clear bag to maintain humidity. Cuttings typically root within 3 to 4 weeks.