Why do my vegetables run to seed, and can I stop it?

Bolting has three different triggers — heat, day length and a check to growth — and which one applies to a crop decides whether you can prevent it or only avoid it.

Difficulty: beginner
Time: 14 min
Read: 6 min

Short answer

A plant bolts when it switches from growing leaves to producing seed. Heat triggers it in lettuce, spinach, rocket and coriander; day length triggers it in onions and some brassicas regardless of temperature; and any check — drying out, transplant shock, root-bound modules, cold snap — triggers it in almost everything. Steady moisture, shade and correct sowing timing prevent most of it.

Individual crops have their own guides, and lettuce and coriander in particular are covered separately because they are the two people search for most. This is the guide that explains the mechanism across all of them, because once you know which trigger a crop responds to, you know whether shading it will help or whether you simply sowed it in the wrong month.

Step by step

Understand what bolting is.Every annual and biennial vegetable is trying to set seed. Bolting is that switch happening earlier than you want, from producing leaves or roots to producing a flower stem. Once it has switched, it does not switch back — the plant redirects everything into flowering, leaves turn bitter and tough, and roots go woody. Prevention is the only lever.
Identify the heat-triggered crops.Lettuce, spinach, rocket, coriander, pak choi and most oriental leaves bolt when temperatures rise, and faster still if they dry out at the same time. These are the ones where shade, steady moisture and choosing summer-tolerant varieties genuinely work.
Identify the day-length crops.Onions, and to a degree beetroot, spinach and some brassicas, respond to day length as well as heat. An onion set planted too early, or a crop sown at the wrong point in the year, will bolt as the days lengthen no matter how well you water it. For these the answer is sowing date and variety, not shade.
Identify the check-triggered failures.Almost any vegetable bolts after a significant check to growth: drying out even once, being left too long in a module until root-bound, a cold snap after planting out, or rough transplanting. This is the most common cause in practice and the most preventable.
Keep moisture genuinely even.Swinging between bone dry and soaked is a repeated check, and it is the single biggest avoidable trigger. Mulch after watering, water thoroughly rather than lightly, and pay particular attention to modules and containers, which swing fastest.
Sow at the right time rather than the earliest possible time.Sowing early feels productive and produces the most bolting. Salad crops sown in the cooler ends of the season crop far better than ones sown into rising summer heat, and a small sowing every two or three weeks beats one large one.
Do not let modules get root-bound.A seedling that has sat in a small module until its roots circle has already had a serious check before it reaches the ground. Plant out on time, or pot on. This alone accounts for a large share of early bolting in brassicas and lettuce.
Provide afternoon shade in summer.For the heat-triggered crops, light shade during the hottest part of the day extends the season substantially. Shade netting, a taller crop to the south-west, or simply a bed that gets afternoon shade naturally all work.
Choose varieties bred for it.Bolt-resistant and summer varieties exist for lettuce, spinach, beetroot, coriander and onions, and the difference is real rather than marketing. Loose-leaf and cos lettuces bolt far later than butterheads. For coriander, leaf varieties bolt much later than seed varieties.
Use what has bolted rather than binning it.Bolted leaves are bitter but flowers on brassicas, rocket and coriander are edible and good, and they are excellent for pollinators. Leaving one plant to flower and set seed also gives you free seed for next year on many crops.

Tips

Sowing little and often is the whole answer to summer salad. Two short rows every fortnight gives a continuous supply and nothing sits long enough to bolt.
Once a plant has bolted, it will not come back, so there is no point removing the flower stem and hoping — except on onions, where you remove it and eat that bulb first.
Coriander is the crop people most often blame themselves for. Leaf varieties, sowing direct rather than transplanting, and cool conditions are the three things that matter.
A bolted brassica or rocket in flower is one of the best pollinator plants in a summer vegetable garden. Leave one.
Modules dry out several times faster than open ground. If you raise plants in modules, that is where the check usually happens.

Common mistakes

Sowing salad crops as early as possible — Early sowings run straight into rising temperatures and long days at exactly the wrong stage. Later, smaller, more frequent sowings crop for longer.
Letting seedlings sit in modules until convenient — Root-bound seedlings have already had the check that triggers bolting, before they reach the bed. It is the most common preventable cause.
Watering little and often in hot weather — It creates exactly the wet-dry swing that acts as a repeated check. One thorough soak plus mulch is far better.
Blaming heat for onion bolting — Onions respond substantially to day length, so a bolted onion is usually a timing or a set-quality problem rather than a watering one.

If it doesn't work

Lettuce sends up a central stem within weeks

Cause: Heat, or drying out, or both — Fix: Sow in the cooler months, keep moisture even, shade in the afternoon and switch to loose-leaf and cos varieties.

Onions produce a flower stalk

Cause: Day length and temperature after a check, often from planting sets too early — Fix: Plant at the recommended time, use heat-treated sets, and remove the flower stalk — the bulb will not store, so use it first.

Spinach flowers almost immediately

Cause: Long days plus heat, which spinach is unusually sensitive to — Fix: Grow it in spring and autumn, and use chard or New Zealand spinach through summer instead.

Brassicas bolt shortly after planting out

Cause: Root-bound modules or a cold snap after transplanting — Fix: Plant out before roots circle the module and protect from cold after planting with fleece.

Beetroot goes woody with a flower stem

Cause: A check from cold after sowing, or from dry soil — Fix: Sow into warm soil rather than early, keep it evenly watered, and choose a bolt-resistant variety.

Questions people ask

Can I stop a plant that has already bolted?

No. The switch to flowering is irreversible. Harvest what is usable straight away, since leaves get more bitter by the day, and sow a replacement. The exception is onions, where removing the stalk does not save the bulb for storage but the bulb is still perfectly good to eat now.

Are bolted leaves safe to eat?

Yes, they are safe — just bitter and tough, and increasingly so. The flowers of rocket, coriander, brassicas and most salad crops are edible and often much better than the bolted leaves.

Does bolting mean I did something wrong?

Not necessarily. Day-length bolting in onions and spinach happens on schedule regardless of care. Heat-triggered bolting in a heatwave is often unavoidable outdoors. The genuinely preventable kind is the check-triggered sort — dry spells, root-bound modules and rough transplanting.