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Plant diseases
Bolting
Also called running to seed (UK).
Illustration
A crop running to flower and seed too early, set off by heat, lengthening days or a cold spell, so lettuce turns bitter, carrots go woody and onions won't store. Sowing at the right time, choosing slow-to-bolt kinds and keeping plants growing steadily prevent it.
How to spot it
- A flower stalk shooting up from the middle of the plant before the crop is ready.
- Lettuce: the centre stretches up into a tall, leafy stalk and the leaves turn bitter.
- Carrots: a tall flowering stem rises from the middle of the leaves; the root stops swelling and turns woody.
- Onions: a flower stalk grows up through the neck, and the bulb won't store.
- Basil: spikes of small flowers at the shoot tips, and leaves that lose their flavour.
- The trigger often came weeks earlier, such as a cold spell soon after sowing or planting.
Easily confused with
Normal flowering in the second year
Carrots, onions and beetroot left in the ground over winter flower in their second year: that's their normal life. Bolting is flowering in the first year.
Tipburn
Brown, dead edges on the inner leaves of lettuce in hot weather, with no flower stalk.
Where it's a problem
- Worldwide: Common wherever cool-season crops meet heat, long days or a late cold snap. Not regulated.
- United Kingdom: The RHS suggests heat-treated onion sets where onions keep bolting, and notes that red onions bolt more readily than white or brown ones.
What to do
- Sow at the right time for your area, so lettuce and carrots grow in mild weather rather than the hottest, longest days.
- Choose kinds sold as slow to bolt.
- Don't plant onions out too early into cold weather. Small sets bolt less than large ones, and heat-treated sets greatly reduce the risk.
- Keep plants growing steadily: water in dry spells so the soil never dries out.
- Pinch out basil flower buds as soon as they appear, and don't crowd the plants.
- Snap off onion flower stalks as soon as they appear, and use those onions first: they won't store.
- Pick bolted lettuce and carrots straight away, before they turn bitter or woody, or leave one or two to flower and set seed.
Spraying won't help: bolting isn't an infection. Official guides prevent it with sowing time, the choice of kind and steady watering.
Vegetables it affects
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Sources
- RHS: Bolting in vegetables
- RHS: How to grow basil
- RHS: How to grow onions
- University of Illinois Extension: Onion
- University of Missouri IPM: Why onions bolt
- N.C. Cooperative Extension: Spring vegetables will bolt soon
Facts checked Wednesday 21 October 2026. General guidance only: follow your local council, agency and product label.