It pays to familiarise yourself with the ins and outs of growing this veg plot staple. Get your timing right – starting now – and you can have a supply of home-grown bulbs to hand the whole year round
If you take a moment to consider how many recipes incorporate onions, you’ll begin to appreciate the allencompassing nature of these flavoursome bulbs. Onions are a kitchen mainstay, and with some careful planning there’s a good chance you’ll be able to enjoy your own almost year-round. Whether started off from seed or sets (immature bulbs grown especially for planting) you’ll be producing a valuable staple that’s as flexible in its use as it is potent in its flavour. Plan for a glut and try your hand at stringing onions for winter use. The earliest bulbs (such as the Japanese varieties) will be ready in June, so you needn’t go without for long. Even when ideal soil conditions aren’t met, many gardeners find success with onions started from sets, which are more forgiving than seeds and better able to cope with poorer, lumpier soils.
In recent decades seed companies have made many developments in onion hardiness, leading to a new sowing and planting season from late summer to early autumn. Seeds and sets started into growth during this window will happily overwinter as young plants and provide an early summer harvest, just when stored onions are becoming scarce. Developments have also led to an increased range of shapes – flattened, round and long, stretched forms (perfect for slicing) can all be grown at home. Red- and whiteskinned onions are also increasingly popular: both are suitable for cooking and salad use.
Onions love a sunny, open position with soil that’s of light to medium consistency. Traditionally sandy beds have been favoured due to their free-draining nature but any fertile ground with good drainage is perfectly suitable. The soil should have well-rotted manure or compost added to it the previous autumn with a final boost from a concentrated organic fertiliser (chicken manure pellets are a good choice) raked in shortly before sowing or planting.
Seed-raised onions will need a carefully prepared seedbed if they are to germinate effectively and thrive. Having incorporated your organic fertiliser, rake the soil when it’s dry, then walk over the bed to firm everything down. Give the bed a final raking-over to produce a fine, crumbly soil texture that’s perfect to sow into.
Experienced gardeners with decent soil conditions will be best placed to grow from seed. If you’re doing that in September, look for overwintering varieties such as ‘Reliance’ or Japanese types like ‘Kaizuka Extra Early’ that can be sown up until the end of the month. They’ll give you the earliest harvest next summer.
As a general rule, onion seed can be sown 1cm deep in rows about 20cm apart. Sprinkle the seeds thinly, ideally leaving about 1cm between each to reduce the need for thinning out later on. Alternatively, sow in a dedicated, sheltered seedbed or under cover in modules, for transplanting to the plot proper once the weather warms up in spring. As with most crops, onion seeds started off in modules can be sown a few to each one and thinned to leave the strongest. It’s much simpler, however, to get four or five underway in each module and then just plant out these clumps – the bulbs will simply push themselves apart as they expand.
The next main window for sowing is in spring. Seed can be reliably sown from mid-March in its final growing position (at the same spacings as above). For a super-early crop, sow under cover in January or February (see Giant Onions box) but maintain a temperature of 10–12ºC under glass and ‘harden off’ the onions-inwaiting well before planting them out in spring. Do this by moving them outside for increasingly longer periods each day – it will help acclimatise them to the conditions outside to prepare them for the shock of going from cosy conditions to wintry weather.
Sets are easy to plant, 10–15cm apart in rows that are spaced 20–25cm apart, and so that just the tips are peaking through the soil. Carefully firm them in. Most sets are planted from mid-March to mid-April but Japanese varieties and other autumn-planted types are available for planting in September and October.
Onion seed is plentiful and therefore inexpensive; there is also a bigger range of varieties available to grow this way. Sets are pricier and offer less choice but as they are primed to start into growth they will give you more reliable results with fewer casualties.
Sets should have sent out roots at the base within days, with green shoots following within a fortnight. Newly-planted sets may be lifted out of the ground by birds or frost heave, so firm back any that are dislodged while the roots are developing.
Seeds will germinate within about three weeks and can quickly be thinned to a spacing of 2cm between seedlings. Onions sown in the autumn can be left at this spacing until spring, when they will need to be thinned in stages to an ultimate spacing of 5cm for a good crop of medium-sized bulbs, or 10–15cm apart for fewer but larger bulbs, which will be better for storing or exhibiting. Seedlings started off in a seedbed can be transplanted to their final positions in March. Carefully dig up the young plants with as much of the original soil attached as possible and move them to their new home, at the spacings above.
Spring-sown seedlings can be thinned to 2cm apart on germination and then to their final spacing once they’ve reached 10cm in height. Don’t waste the plants you remove – use them in salads as the first onion flavours of the new season. Individual seedlings grown in modules can be planted out at their final spacings in the spring after being hardened off. If you grew four to five seedlings per module, then space out the clumps a little wider – 25cm in all directions is a good bet – to allow each onion enough room to develop fully.
Like all crops, onions should be kept weed free. The growth of their long, straplike leaves can easily be affected by more boisterous plants using up water and nutrients in the soil, so maintain a clear growing bed by regularly hand-weeding within the rows and hoeing between them.
Crops started off in spring will generally only need watering in very dry weather. However, once the bulbs start to swell you should ensure the soil remains moist – this, combined with warm sunny weather, will see them rapidly put on weight. Onions that have seen through the winter will require a bit more care and will certainly welcome a little extra nutrition in spring in the form of an organic liquid feed. Come spring and summer, any developing bulbs will benefit from a mulch of organic matter, which will help to lock in moisture and add further goodness to the soil.
Sometimes onions, particularly those grown from sets, can bolt (elongate and start flowering). Stop them doing this by breaking off any flower heads that appear, otherwise the plants will put energy towards producing seeds instead of swelling their bulbs. Stop all watering as soon as the bulbs have swollen to their ultimate size (see below), when they should be left to mature in the sun.
Autumn-sown varieties will be ready to harvest from early summer; those started off in spring a couple of months later. Fully-swollen bulbs can be lifted and used fresh as needed but those intended for storage will need a little longer in the ground. You will know when they’re almost ready as the leaves will begin to turn yellow and bend over. Leave them in the ground for another two weeks beyond that point and then gently ease them out of the soil: they are now ready for drying. Space them out on trays or sacking, outside if the weather is fine and dry, or in a shed or greenhouse in damp weather. Try to ensure air can freely circulate around each bulb, to speed up the drying process and reduce the chance of any rot or mould taking hold of your crop.
Drying takes from seven days for smaller bulbs to up to three weeks for the largest. It’s important that they are fully dry before they go into storage to prevent any infections gaining a foothold, as these tend to thrive and spread more rapidly in moist conditions. Dried bulbs will keep for a surprisingly long time, almost closing the gap to next summer’s harvest.
For early maincrop onions, sow seed under cover in pots or modules towards the end of the month. They will be ready for transplanting outside in spring.
Carry on sowing into modules and containers under cover. In mild weather, over-wintered onions may now start back into growth.
Sow seeds directly outdoors and begin planting sets. Thin overwintered onions to their final spacings once they are of sufficient size.
Onions will be swelling rapidly. Autumn-sown and Japanese varieties will already be ready to lift and enjoy.
Now's the time to lift, dry and store mature maincrop onions. You should also begin to sow overwintering and Japanese varieties for early crops next year.
Sow seeds of Japanese and other over-wintering varieties. Also plant suitable onion sets.
Growing Giant Onions
Go to any village show and take a look at the competitive classes of vegetables – I bet there will be some giant-sized onions on display. The Guinness World Record for the heaviest one ever is a whopping 7.5kg, and it was grown here in the UK (the West Midlands to be precise). Good varieties to try if you want to grow a giant onion are ‘Kelsae’ (available from DT Brown) and the appropriately named ‘Mammoth’ (W Robinson & Son).
Start seeds off as early as possible; late December if you can. Sow them under cover in pots of John Innes seed compost and maintain a temperature of around 12°C. Ventilate the greenhouse as often as possible to keep mould at bay. Once the seedlings have two leaves they can be pricked out into individual pots of John Innes compost and left to grow on. They can be planted out in April after hardening off – ideally in a cold frame. Space plants further apart, with up to 30cm between each one to guarantee they sufficient room to develop fully. Assuming the soil has been well prepared and is adequately fertile, there’s no need to apply any extra feed, just make sure bulbs are kept weedfree at all times and water them as they swell. Many champion growers keep their onions in raised beds under the comfort of a polythene tunnel. A combination of warmth and sunlight will lead to a heavier specimen.
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Onions: Varieties To Try
The large, tightly-packed green heads of this sugar loaf type stand upright and look somewhat similar to cos lettuces. The outer leaves are thick and crunchy, and shelter the heart so well they practically blanch it.
This newly-bred variety is highly disease resistant. Even better, it grows rapidly to practically guarantee high yields and one of the earliest onion harvests.
Large, uniform bulbs that are often grown for exhibition. The flesh is sweet and will form with minimal care – a great allrounder that is unlikely to disappoint.
This striking onion has clean white flesh and a shiny outer skin that's a handsome deep red. Coupled with its good looks is a strong flavour and exceptional storage ability.
Once your onions have fully dried they’re ready for storage. Check over each and every bulb for any signs of damage, softness or spots and remove any that are affected for immediate use or freezing. Also reject any that have a thick neck or are too small – only perfect, firm bulbs will be able to survive storage over winter. Carefully place them into nets or sacking; old (but clean) tights are a perfectly adequate cost-cutting option.
Another simple (and arguably more attractive) storage idea is to make an onion string. Create a loop of string and hang it over a strong nail or hook attached to a garage or shed wall. Form a noose at the bottom of the loop and feed through the leaves of your first onion right up to the bulb. Pull the noose closed and allow the onion to weigh down and pull the string loop taut. Now feed in onions one by one, weaving in each dried onion’s leaves between the two string lines in a figure of eight. Move round the strings to create an even spiral of onions, placing each bulb so it sits next to the previous one to create a neat and tidy display. Don’t overfill or you run the risk of it snapping under the weight. As long as your store is sufficiently cool and dry the onions should keep for up to six months.