Onions are one of the most important of all vegetables and popular garden crops. I approach the subject with trepidation because there are so many kinds and growing them can be complex so I will cut the waffle for once and be as useful as I can.
I will ignore the non-bulbing, salad onions for now and stick to bulbing onions. Onions can be white, brown or red and, of course, there are colours in between. The bulbs themselves can be globe-shaped, flattened or more upright and torpedo shaped. In the UK and Ireland globe-shaped brown onions are the most popular although red onions are gaining favour. Red onions cost more, partly, I am sure because people will pay more for something that is apparently good for them, and partly because, as you will discover if you grow them, they are slightly more tricky to grow.
You can grow onions from seed or from sets. Sets are small, immature onions that are produced by sowing the seeds the previous summer. There is not time for the plants to make big bulbs so they are harvested small and sold for planting the next year. For this reason some varieties are available as sets and seed. Onions (Allium cepa) are naturally biennial plants, unlike most ornamental alliums which are perennials. This means that in their second year of growth they use all the sugars stored in the bulb to form a flower stem with a ball of white flowers (usually pollinated by flies), set seed and die.
Obviously it is easier to plant an onion set than sow tiny seeds – you can space them out more easily for a start – but you may have noticed a problem here – they naturally want to bloom the second year. If an onion plant ‘bolts’ or tries to bloom it will not make a good bulb and the centre of the bulb will have tough, woody stem. This is a big issue with onion sets but they are usually heat treated to kill the embyonic flower. Even so, bolting can be an issue.
Like most biennials, flowering is triggered by a check to growth, naturally a cold snap or perhaps drought. It tells the plant that the bad times are over and it is time to bloom. This is all very good but it means that if we cause a check to growth, once the plant has started to grow, it triggers flowering. Among the things we do that check growth can be transplanting (fine with onions and carrots if we want to collect our own seeds), a cold snap after growth starts in spring, drought or a really hot spell. All these can cause onions to bolt. It is most common in red onions and in onions from sets. It is not usually a big issue and if I have 5% bolt I am not bothered but I have had 30% of red onions from sets bolt in a hot, dry spring. Luckily these are not that common in Ireland. If onions do bolt they should be pulled up immediately because they will make bulbs.
Onions have a strange growth cycle. Once they start to grow they make new leaves right until the end of June. An onion bulb is composed of the concentric leaf bases so the more leaves a plant has, the bigger the bulb will be. So we need to get the plants with as many leaves as possible before the end of June. For this reason it is traditional to sow giant onions at Christmas but to do this you need a propagator in a cold or cool greenhouse. Otherwise you can sow under glass in March. If the weather is good you can sow outside in March too. You can sow in a tray indoors and transplant the seedlings carefully but I sow a pinch in cell trays and plant then out, as clusters, in April. They are allowed to grow as a clump and I pull out the small ones to use as scallions, leaving three or just one to grow to maturity.
Onions need a fertile, well-drained soil. Although they need sun to ripen them, they do like moisture when growing. A plot that has been manured for a previous crop is fine. I would add a granular fertiliser once the plants have started to make three or four leaves. You must keep the young plants free from weeds at all times.
I would plant sets or seedlings 10cm apart in rows 30-40cm apart. If sowing direct, thin out in stages, eating the pulled seedlings, to leave them 10cm apart to mature.
Keep them watered in dry spells. And then, in July, they will stop making new leaves and the bulbs will start to swell. Don’t panic that the bulbs do not seem to be getting any bigger before then! But it means that the size of your onion is more or less determined by July. By August the plants will be starting to die back and you can hasten that by pushing a fork under them and severing the roots. By September you need to dry them off, either by lifting them and storing them in a greenhouse or by covering them, in the garden, with a cloche.
There are a few issues, of course. White rot is a fungal disease in the soil that make the bulbs rot and if it is present you can’t really grow onions in your garden.
And, lastly, I should mention autumn-sown or planted onions. These are extra hardy and are sown in August or September and they mature at least a month earlier than regular onions. They need well-drained soil and a sheltered spot and can be prone to downy mildew.
Only well ripened onions will store well. Some will have thick necks and these will not store as well as those with thin necks that have a ‘closed top’ to the bulb.