Why are my tomato leaves curling?

Tomatoes curl their leaves for several unrelated reasons, and the most common one requires no action at all. Working out which you have takes a minute and prevents a lot of unnecessary intervention.

Step by step

1. Look at which leaves and which direction.Older lower leaves rolling upward and inward, still green and slightly leathery, is physiological leaf roll. New growth at the top curling down and twisting is a different problem entirely.
2. Rule out normal leaf roll.It is a response to a mismatch between water uptake and demand, common in greenhouses, after heavy pruning, and in wide temperature swings. The plant crops normally and needs nothing done.
3. Check for heat and water stress.Curling that appears in the afternoon and eases overnight in a hot greenhouse is heat. Improve ventilation, shade the glass and water consistently rather than in cycles.
4. Check your watering rhythm.Alternating drought and flood produces leaf roll, split fruit and blossom end rot together. Consistency matters more than volume.
5. Look hard for herbicide damage.Growing tips twisted, fern-like, thin and cupped downwards, with stems that look distorted, is aminopyralid or similar contamination. It is the single most under-recognised cause.
6. Trace the herbicide source.Contaminated manure or compost is the usual route — the chemical survives the animal's gut and the composting process. Grass clippings from a treated lawn used as mulch is the other.
7. Test suspect compost with beans.Sow broad beans or peas in a pot of the suspect material and a pot of clean compost. Distorted growth in the suspect pot confirms contamination.
8. Check for virus.Mottled yellow and green, distorted leaves, stunting and poor fruit set. There is no cure. Remove and bin the plant, wash your hands, and do not save seed from it.
9. Check underneath for aphids.Heavy aphid infestation distorts and curls new growth. Look on the undersides of the curled leaves before assuming anything more serious.
10. Act according to what you found.Nothing for physiological roll. Ventilation and consistent watering for heat. Removal for virus. For herbicide contamination, remove the source and expect a lost crop.

Tips

• Old leaves rolling up and staying green is harmless. New leaves twisting downwards is not.
• The bean test on suspect compost costs nothing and settles the herbicide question.
• Consistent watering prevents leaf roll, split fruit and blossom end rot at the same time.

Common mistakes

• Feeding or spraying a plant with physiological leaf roll — There is nothing wrong with it. Extra feed on a plant already struggling with water balance can make the rolling worse.
• Using farmyard manure of unknown origin on tomatoes — Aminopyralid herbicides used on grazing pasture persist through the animal and through composting, and tomatoes are among the most sensitive plants there are.

If it doesn't work

Questions people ask