Why has my onion flowered?

Why do onions flower?

Plants produce flowers to develop seed and reproduce themselves, so flower production is a natural and necessary part of their lifecycle. However, we gardeners often don't want vegetable plants to produce flowers – for crops like onions, garlic, leeks and beetroot, our job is to ensure the plant concentrates its energy on producing the part of the plant we want to eat, in this case a nice, swollen bulb. This is usually achieved by growing onions in moisture-retentive soil, keeping the area weed free and watering consistently and evenly throughout the growing season. But if weather conditions aren't favourable – for example if there's too much or too little sun – the stressed plant may 'bolt' or 'run to seed', which is to say it completes its lifecycle early. With onions, the bulb produces a thickened stem topped with a flower pod, which opens to produce a globe-shaped head of usually white flowers, which are very attractive to pollinators.

The life cycle of onions

Bulb onions are biennial plants, which means they grow in the first year, flower in the second, and then die – we harvest them in their first year, before they bloom.

To produce a good bulb, onions require 90-100 days from seed, which is around four months. The easiest way to grow onions is from sets, or small bulbs, which are ready to harvest after around 80 days, or just under three months. However, onions grown from sets can be more prone to bolting than those grown from seed.

Don’t confuse bulb onions with Welsh or perennial onions, which naturally produce a thickened stem, and often flower.

How the weather can cause an onion to flower early

Growing conditions that are too hot, dry, or cold, can all put onions under stress and so they rush to complete their lifecycle by flowering. Minimise the impact of extreme weather by covering the crop with horticultural fleece to create shade, or to insulate on cold nights, and by giving an occasional thorough watering during periods of drought.

How to deal with bolted onions

To minimise the impact on your crop, cut off the flower stem and bud as soon as possible. While the resulting onion still needs eating within a few weeks, removing the flower should stop bulbs from dividing so they’re still be a useable size. Use young, tender stems of flowered onions in salads, soups, or stews, ideally finely chopped.

In the UK, flowering onions may become more of an issue as the impacts of climate change intensify, so choose a variety that has natural resistance to bolting such as ‘Sturon’ or ‘Stuttgarter’. Look for onion sets that have been heat treated – these have the greatest resistance to bolting and are suitable to plant from early spring.

Benefits of having flowering onions

If you’ve been away and come back to a crop of onion flowers, don’t pull them up. Flowers of the allium (onion) family are wonderful for pollinating insects and are rather ornamental. And you can collect the seed once the flower heads have matured and try using them to grow a new crop next year.