Why Did My Onion Flower and Go to Seed?

You notice a thick, round stem pushing up from the centre of an onion plant, which quickly develops a globe-shaped flower head. This is bolting — the onion switching from vegetative growth to reproduction. A bolted onion will not form a proper bulb, because all the plant's energy is directed toward producing seed. It is one of the most frustrating outcomes for onion growers, particularly when it affects a significant proportion of the crop. Understanding why it happens gives you the tools to dramatically reduce it in future seasons.

The vernalisation trigger

Onion bolting is triggered by vernalisation — exposure to cold temperatures during a critical early developmental stage. Onion plants that experience cold (below around 10°C, especially below 7°C) for a prolonged period while they are small are tricked into "thinking" they have survived a winter and will then bolt when conditions warm up, following their biennial reproductive programme. With sets, this cold exposure happens either before planting (in poor storage) or in early spring after planting when cold spells occur. With seed-raised onions, it can happen when transplants are put out too early or during a cold spring.

Large sets bolt more readily

Large onion sets — those over about 25 mm in diameter — are significantly more likely to bolt than small sets. This is because large sets have already undergone more physiological development and require less cold exposure to reach the vernalisation threshold. Professional growers specifically sell "heat-treated" sets which have been warm-stored to reduce the vernalisation response, making them much more bolt-resistant. Always check whether sets are heat-treated when buying and choose small-to-medium sets (14–21 mm) if heat-treated ones are not available. Autumn-planted overwintering onion varieties are different: they are bred for cold hardiness but need to be of the right size at the onset of winter to avoid premature bolting in spring.

What to do with bolted onions

As soon as you notice a flower stalk emerging, snap or cut it off as low as possible. This does not reverse the bolting — the bulb has already been compromised — but removing the flower stalk stops the plant wasting further energy on seed production. Harvest bolted onions promptly: the bulb will have a thick, woody central cavity where the stalk was, but the surrounding flesh is perfectly edible. Use bolted onions immediately as they will not store well — the hollow central stalk provides an entry point for rot.

Long-term bolting prevention

Use heat-treated sets. Plant at the correct time — not so early that young plants experience prolonged cold, and not so late that the season is too short for good bulb development. Choose bolt-resistant varieties if your area experiences frequent cold spring weather. Avoid using home-saved sets from bolted plants, as these may be from naturally more bolt-prone individuals. If growing from seed, harden transplants gradually and plant out only when temperatures are reliably above 10°C.