Bolting is triggered by day length, heat and cold, and watering only fixes part of it

Bolting is a vegetable running to seed before it has given you a crop. The leaves turn bitter, the root stops swelling and the plant puts its energy into a flower stem. Once that stem is up, there is no way back.

The cause depends on the crop. That matters, because the remedy for one group makes no difference to the other.

Two triggers

Long days and heat. Lettuce, spinach, radish and rocket are annuals. They flower as the days lengthen, and heat and dry soil speed it up. A lettuce sown in April is running out of time by June, whatever you do.

Cold. Onions, leeks, carrots, beetroot and celery are biennials. They expect a winter before flowering. A cold spell in spring, especially while the plants are young, convinces them they have had one. They bolt in summer instead of making a bulb or a root.

Some crops sit between the two. Florence fennel, radicchio and oriental greens bolt when the nights stay warm, around 10 to 13°C. Dry soil pushes cauliflower, rocket and spinach towards flowering.

What it looks like

On lettuce, the centre of the plant stretches upwards and a stalk appears. The leaves get darker and bitter. Spinach sends up a tall stem with small leaves. Onions produce a hard flower bud at the top; beetroot and carrots produce a flower stem from the centre of the rosette.

Cut a lettuce stem and taste a leaf. If it is bitter, the plant is finished. Pull it and put it on the compost.

What actually helps

Choose bolt-resistant varieties. Seed catalogues label them, and they are the simplest defence for early sowings. Boltardy beetroot is the classic example. For lettuce, crisphead and cos types generally hold longer than soft butterheads.

Sow at the right time. Lettuce and spinach do best in spring and autumn. Sowing in the heat of June and July is asking for trouble unless you have shade and water. For overwintered onions, plant sets after the worst of the cold, and use heat-treated sets if bolting has been a problem.

Sow in small batches. A row of lettuce that all bolts in the same week is a wasted row. Sow a short row every two or three weeks, and you will have something to pick while the older plants go over.

Keep the soil moist. Dry soil is a documented trigger for cauliflower, rocket and spinach, and it makes heat-triggered bolting worse. Water deeply rather than little and often. When to water vegetables covers the amounts.

Use light shade in midsummer. A sheet of netting over lettuce and spinach cuts the midday heat. It will not stop the day-length signal, but it buys a week or two.

Pick cut-and-come-again. Loose-leaf lettuce and spinach can be picked leaf by leaf, which keeps the plant growing and delays the moment it decides to flower.

What does not help

Pinching out the flower stem of a lettuce does not make it edible again. The leaves are already bitter.

Watering an onion that has bolted does not reverse it. The bulb may still be usable if you cut the flower stem early, but it will not store well.

Shading a biennial that bolted after cold does nothing. The trigger has already happened.

After it bolts

Clear the plant and sow again. In midsummer, sow lettuce in the shade or wait until the end of August. Spinach can be sown again in late summer for an autumn crop. Coriander is best sown in spring and autumn, not high summer.

If you leave a few plants to flower, the bees will thank you, and you can save seed from open-pollinated varieties. Just do not expect a crop from the same plant.

For the wider picture, see growing vegetables in a changing climate and how to protect vegetables in a heatwave.