Why are my plant leaves curling?

Curling is a physical response to something the plant cannot control, and the pattern of the curl narrows the cause faster than any other symptom.

Step by step

1. Look underneath the leaves first.Before anything else. Aphids, whitefly and red spider mite all cause curling and all sit on the underside. This takes ten seconds and settles the commonest cause.
2. Note whether it is new or old growth.Distortion confined to the newest leaves points at sap-sucking pests or a nutrient problem. Curling across older leaves points at water or root trouble.
3. Check the direction of the curl.Downward and inward, often with drooping, suggests underwatering or root damage. Upward and inward, cupping like a taco, is typically a response to heat or intense light.
4. Push a finger into the compost.Two knuckles deep. Bone dry means underwatering; wet and heavy several days after watering means the roots are drowning, which produces the same wilting curl.
5. Feel the weight of a pot.Lift it. A dry rootball is startlingly light and it is the quickest test there is, particularly for houseplants where the surface tells you nothing.
6. Check the roots if watering is not the answer.Slide the plant out. Healthy roots are firm and pale. Brown, soft and smelling sour means root rot, which curls and wilts the leaves exactly like drought.
7. Consider heat and direct sun.Leaves curl upward to reduce the surface exposed to the sun. On a hot windowsill or against a south wall in a heatwave, that is a sensible response rather than a fault.
8. Look for weedkiller damage.Twisted, elongated, fern-like new growth with strong curling is characteristic of hormone weedkiller drift, including from contaminated manure or compost. Tomatoes are especially sensitive.
9. Check for wind and draughts.Persistent wind outdoors, or a draught from a door or a radiator indoors, dries leaves faster than roots can supply and produces curling with crisp edges.
10. Rule out normal behaviour.Some plants curl at night, in strong midday sun, or as older leaves age and are shed. If new growth is healthy, there may be nothing wrong.
11. Deal with what you found.Pests: a strong jet of water, then repeat treatment. Dry: water thoroughly and check drainage. Wet: stop watering, improve drainage, repot if roots are rotting. Heat: shade or move it.
12. Change one thing and wait.Give it a week or two. Changing watering, position and feed at once means you learn nothing, and existing curled leaves generally do not uncurl even after the cause is fixed.
13. Judge success on new growth.Healthy new leaves are the sign it worked. Curled leaves that were already damaged stay that way and can be removed once the plant is growing again.

Tips

• Check under the leaves first. Aphids are the single most common cause and they are easy to miss.
• Lift the pot. Weight tells you about the rootball far better than poking the surface.
• Curled leaves do not uncurl. Judge whether the fix worked by looking at the new growth.

Common mistakes

• Watering more because the leaves are curling and drooping — Overwatered roots cannot take up water, so the plant wilts exactly like a dry one. Watering again drowns it faster. Always check the compost before reaching for the watering can.
• Changing several things at once — New position, new feed, different watering — when it recovers or fails you have no idea which mattered, and the next plant gets the same guesswork.

If it doesn't work

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