Onion: plant running up to flower before it is ready to eat

What it is likely to be on onion

1. Bolting

   Candidate 1very common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Bolting

   Checked 2026-08-13

   A plant switching from leaf or root production to flowering and seed. It is the plant doing exactly what it evolved to do under stress, and it cannot be reversed — which makes bolting a problem of timing and cultivar choice rather than of treatment.

   Points towards it

   • · heat is the usual trigger, and lettuce responds to it within days
   • · a check from drying out brings it on as reliably as heat does
   • · a growing-season problem

   Argues against it

   • · a recent repot points to transplant check rather than to the plant deciding to flower

   Check this next

   Look at the centre of the plant. A lettuce running a stem up from the middle, or an onion sending up a thick round flower stalk, has bolted — and no amount of watering reverses it once the stem has started.

   What to do now

   Harvest what is usable now rather than waiting: a bolted lettuce turns bitter within days, and a bolted onion will not store. Sow successionally and keep the crop evenly watered to reduce it next time — bolting is a response to a check, not a disease.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Bolting

   Checked 2026-08-13

2. Transplant shock

   Candidate 2common on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Transplanting

   Checked 2026-08-13

   Wilting, leaf drop or a stall in growth in the days after repotting, planting out or moving. Often not a problem at all but a normal adjustment, and the commonest mistake is to react to it by changing something else — which turns a recoverable check into a real decline.

   Points towards it

   • · a check at transplanting is a distinct trigger from heat, and it acts weeks earlier in the season
   • · it follows planting out from modules or a seedbed

   Argues against it

   • · hot weather is the commoner trigger and points to ordinary bolting rather than to a transplant check
   • · the plant has not been moved, so a transplant check cannot be the trigger — this rules it out entirely.

   Check this next

   Work out when it was moved. A crop that runs to seed within a few weeks of being planted out, before any hot spell, was checked at transplanting — root disturbance and a dry fortnight afterwards do it as reliably as heat.

   What to do now

   Nothing rescues the affected plants; harvest what is usable. Next time sow direct where the crop is to grow, or plant out earlier and smaller and water well for the first fortnight — the aim is to avoid the check rather than to treat the result.

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Transplanting

   Checked 2026-08-13

3. Cold damage

   Candidate 3common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Onions — bolting
   • Agriculture and Horticulture Development Board — AHDB: Alliums — bolting and set size

   Checked 2026-08-13

   Blackened, translucent or limp tissue appearing suddenly after a cold night or a spell beside a draught. Tender plants show it well above freezing — basil blackens around 10 °C — so 'there has been no frost' does not rule it out.

   Points towards it

   • · a cold spell after planting is read by a set that is already large enough as a winter, and it switches from bulbing to flowering
   • · it is the largest sets in the bag that bolt, so the affected plants are scattered through the row rather than being all of it
   • · a thick hollow flower stem pushes up through the centre of the neck
   • · the trigger is spring temperature on a bed of sets in open ground

   Argues against it

   • · warm settled weather after planting is what suppresses this, and a hot spring is the least likely to produce it
   • · if literally every plant in the bed has run, look at the batch of sets and the planting date rather than at a cold snap

   Check this next

   Look at which plants ran and remember how big the sets were. Vernalisation bolting takes the largest sets — anything much over 2cm across when it went in — and it takes them scattered through the row while their smaller neighbours bulb normally. The stem is thick and hollow and comes up through the middle of the neck, and if you cut a bolted bulb across, the flower stalk runs down through the centre of it. That solid core is why a bolted onion will not store, which is the practical consequence.

   What to do now

   Snap the flower stems off as soon as you see them, then lift and use those bulbs first — a bolted onion is perfectly good to eat now and will rot in store because of the hollow core running through it, so it should never go in the net. For next year, buy heat-treated sets, which have been through a controlled warm period that removes the flowering signal, plant sets under 2cm across, and plant in March rather than February so they meet less cold in the ground.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Onions — cultivation and storage
   • Agriculture and Horticulture Development Board — AHDB: Alliums — heat-treated sets

   Checked 2026-08-13

Narrow it down

This page lists everything at once, which is the right shape for reading and the wrong shape for deciding. The interactive tool asks one question at a time, starting with whichever would separate these causes fastest, and stops as soon as further answers would not help.

Opens with the plant and the symptom already set. Nothing you answer is stored or sent anywhere — the answers live in the address bar and nowhere else.

What this does not cover

These are the causes GrowingHQ holds rules for on onion — not every cause there is. Where several problems look alike, the honest answer is often that they cannot be told apart by eye, and a page that named one would be guessing. If none of the above fits, the general plant running up to flower before it is ready to eat page lists causes across all plants, and the Onion profile lists the problems it is prone to.