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Growing Onions

Easy to grow and easy to store, onions are a staple of our diet all over the world. There are loads of different types available and some quite different approaches to growing them, so the gardener is spoiled for choice. Bulbing onions can be grown from seed or from immature bulbs known as ‘sets’. Growing from sets is easier but more expensive, and there isn’t such a good range available. Yellow onions store best; red and white onions less so. Some onions aren’t intended for storage and should be used fresh (see below).

Whether you choose sets or seeds, onions prefer a sunny, well-drained location. They don’t do well with competition, so make sure you keep up with the weeding, but have shallow root systems that are easily disturbed by over-zealous cultivating. They also don’t like freshly-manured earth, so dig this in several weeks beforehand if possible. Overall pH should be neutral or close to it, erring on the alkaline side.

red white and salad onions, image

Red, white and salad onions

Growing from seed

Sow in modules in January or February, two seeds per pot, and then thin to the best. Thinning in this way won’t attract onion fly, which can be a serious pest. Some growers set aside a bed for permanent onion production but this can lead to disease and soil depletion, so they should be included in a crop rotation plan along with almost everything else in the garden. Transplant them to their final positions when they are 10cms (4″) tall.

Positives: wide variety available, bulbs store better than set-grown ones.
Negatives: need a longer growing season. Slightly more prone to disease than set-grown bulbs. Seeds have a short life span (1-2 years).

Growing from sets

Onion sets are small bulbs grown from closely-sown seed during the previous year. If they have been heat treated as well, which destroys the flower embryo, they are much less likely to bolt when re-planted. Sets are more reliable than seeds, and if properly cared for should they produce large bulbs for harvest, although these won’t store quite as well as seed-grown bulbs. The range available as sets is small, reflecting the ones which are commercially grown, so if you want a particular variety you are much more likely to find it as seeds than as sets.

Plant sets in March, roughly 15cms (6″) apart each way, just below the surface. Make sure you put them in the right way up, with the little cluster of roots at the bottom.

Positives: Less work than seeds, shorter growing season.
Negatives: Fewer varieties available, and they don’t store quite as well.

Both seed- and set-grown plants are prone to attack from slugs when they are young. Onion fly targets seed-grown bulbs, particularly if they are sown direct: you are much less likely to have problems with set-grown bulbs. Storage rot can also be a problem, but less so with varieties described as ‘thin necked’.

Once the bulbs have formed (during late summer for spring-sown varieties) the leaves begin to yellow and fall over. Some gardeners hasten this along and bend any remaining plants over just above the bulb, but doing so can trap moisture in the neck making storage rot more likely. Instead, when three quarters of your plants have fallen over use a hand fork to loosen (but not lift) the roots of the remaining plants. Restricting the water the plant can get in this way hurries them along without promoting rot.

Harvesting onions

Onions should be lifted once the leaves have fallen. The bulk of the earth should be gently removed from the roots, but don’t cut off the leaves. Lay the bulbs on a rack in a dry, warm place (a shelf in a polytunnel or greenhouse is ideal) to ‘cure’ for storage. After a few days, any remaining earth can be cleaned away from the roots. Onions are ready for storage only when they pass the ‘roll’ test: grip the neck between finger and thumb, and roll it. If it feels brittle and dry, it’s ready. However, if it feels at all soft or squishy they need to dry further. Sometimes this takes several weeks, but so long as they aren’t subjected to frost, this is fine. It just takes as long as it takes. When completely dry, the leaves can either be cut off and the bulbs stored in paper or net sacks, or they can be left in place for braiding. Either way, store them in a cool, dry, well-ventilated and dark place.

onions drying, image

Yellow onions laid out on a mesh to dry

Other onion types

Spring onions, also known as ‘salad’ onions or ‘scallions’ are a non-bulbing type that can be grown under cover for harvesting at any time. Because there are no bulbs you don’t usually need to thin them, and they can be eaten as soon as you think they’re big enough.

Topsetting onions, also known as ‘walking’ or ‘tree’ varieties, form small bulblets near the top of the leaves. When the leaves fall the bulblets take root, so the next generation grows in a slightly different position – and so on, eventually ‘walking’ away from the original site. Bulblets don’t usually store well so remove them and plant them in modules to overwinter, preferably under cover, until the following spring.

walking onions, image

Topsetting or 'walking' onions, forming at the top of a stem

Bunching or ‘multiplier’ onions form a bulb cluster rather than a single bulb and are related to shallots. Some store well while others should be used fresh, and some varieties are also topsetting.

Avoiding the onion gap

Stored onions tend to sprout or rot in early summer, leaving an awkward gap until the new season bulbs appear. This can be reduced or even eliminated using spring onions, sown at three week intervals throughout the year, either outside (Feb – July) or under cover (any time). However, spring onions are not so versatile as their bulbing cousins. An alternative is to use overwintering varieties of bulbing onions which are designed to be sown in the summer. Often called ‘Japanese’ onions (because they were first developed in Japan), these can be sown outside in mid-August for a crop the following June or July. Casualty rates can be high, particularly in bad winters, but if you can make space in a polytunnel or large greenhouse you can plant overwintering onions as late as September for a spectacular crop of fat bulbs around the end of May with very few losses.

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