If you are growing onions you may have noticed that some of them have produced flower stems – bolted. Some onions are more prone to this than others and it is usually red onions that bolt, especially those grown from sets – small onion bulbs that are planted in spring.

Bolting is caused by a check to growth – it may be dryness, a lack of nutrients or a cold spell while they are in growth, usually the result of a frosty period in April or early May.

To understand why this happens it helps to understand what onions are. Unlike many other alliums, including all those lovely ornamental onions, true onions are biennials. They germinate in summer/autumn and, being native to warmer parts of northern Africa (possibly – they have been cultivated for so long) they complete their life cycle before the extreme heat of summer the following year. The cooler winter temperatures probably stimulate flowering in the wild. Most biennial plants have this habit – growing one year and flowering the second, stimulated by winter chill: sweet WIlliams, foxgloves, parsley, aquilegias and forget-me-nots.

We sow them in spring and they make new leaves until midsummer and then they store sugars in the bulb from then onwards until they go dormant. That is why onions do not get swollen until about now – you should see your bulbs swelling now, ready to sprout and flower again next year. Every leaf means a ‘ring’ in the bulb so the more leaves they have before midsummer, the bigger the bulbs, potentially.

It is not just onions that bolt when conditions do not suit: we know that many plants flower more freely when potbound or slightly starved. It is almost as though the plants panic and think they are in danger so try to produce seeds before they die. Some plants can measure the length of sunlight each day and will flower prematurely if sown at the ‘wrong’ time. Oriental veg and Florence fennel are typical examples in the veg garden and if sown in spring will run straight to bloom rather than make good edible plants – they are usually sown after midsummer.

When you buy onion sets they should be heat-treated to kill the flower buds inside but it does not always work.

When you see the flower stems growing you should pull up these plants because they will not form good bulbs and if they  do make bulbs they will have a large hole in the centre where the flower stem was – the same applies to leeks where the flower stem will be woody and make the plant less pleasant to eat.

why onions bolt