Why Did My Onions Bolt?

Spotted a fat flower stem shooting up from the middle of an onion? That is bolting — the plant has decided to make seed instead of a bulb. The usual UK trigger is a cold spell after planting, with stress (drought or oversized sets) the runner-up. The key thing to know: a bolted onion stops swelling and will not store, so use it soon.

Most likely causes (and the fix)

Onions are biennials. A run of cold weather convinces them they have been through a winter, so they flower in their first year instead of bulbing up. Here are the culprits, roughly in order.

1. A cold spell after planting. This is the big one. Spring-planted sets that meet a sharp cold snap — frost, a fortnight of chilly nights — read it as winter and rush to flower. There is no fix once a plant bolts; snap off the flower stem as soon as you see it to slow the bulb going woody, then plan to eat that one first.

2. Oversized sets. Sets larger than about 2cm across (roughly pea to small-marble size is ideal) are far more prone to bolting. The bigger the set, the more it behaves like a mature plant ready to flower. Pick out and use the largest sets in the kitchen, and plant the small-to-medium ones.

3. Drought or stress after planting. Onions with shallow roots hate drying out, especially in a warm, breezy spring. A check to growth — dry soil, a sudden hot spell, root disturbance — can tip a borderline plant into bolting. Keep the soil evenly moist through dry weeks, particularly while bulbs are forming.

4. Sowing or planting too early. Get sets or seed in while the ground is still cold and you simply expose them to more cold-snap risk. Earlier is not better with onions — wait for the soil to warm.

How to prevent it next year

Most bolting is avoidable with a few habits.

• Use heat-treated sets. Many UK suppliers sell "heat-treated" onion sets, which have had the embryo flower killed off — these bolt far less. Worth the small extra cost. See the onion growing guide for varieties.
• Plant at the right time. Spring sets go in from mid-March to mid-April, once the soil is workable and warming. In a cold, late spring, hold off a couple of weeks rather than fighting frosty ground. Use the frost-date checker and the planting calendar to time it.
• Choose smaller sets. When buying, favour packs of small, even sets over a few jumbo ones. If your pack is mixed, plant the small ones and cook the big ones.
• Water steadily. Don't let onions dry out in spring dry spells. A good soak in dry weeks beats a daily splash. The onion guide covers feeding and spacing too.
• Firm soil, weed-free bed. Settle sets into firm soil and keep competition down — stressed, crowded onions bolt more readily.

Using bolted onions promptly

A bolted bulb is perfectly edible — it just won't keep. The flower stem runs as a tough, woody core down through the bulb, and the onion will rot in storage rather than cure.

So lift bolted onions and use them within a week or so. Cut out the hard central stem, then chop and use the rest in soups, stews, stocks or anything cooked. Don't try to plait or store them with your keepers.

If a bolting onion has begun to swell at the base into a small flower bud, you can also leave a couple to flower for the bees — onion flowers are a magnet for pollinators.

One more thing: bolting and rot are different problems. If your onions are collapsing with white fluffy mould at the base rather than throwing flower stems, that is likely onion white rot, not bolting — and it needs a different response.

For the full sowing-to-harvest method, head back to the how to grow onions guide.