Onions can last considerably longer than many fresh vegetables, but only when they're stored in the right conditions. A dry, sound bulb kept somewhere cool and airy behaves differently from a peeled onion half or a bunch of spring onions. Putting all three in the same storage drawer is not the best approach.
For whole ordinary brown, white or red onions, the usual recommendation is a cool, dry, ventilated place. The UK gardening guidance from the Royal Horticultural Society also recommends drying homegrown onions thoroughly before long storage. Once an onion is cut, refrigerate it rather than treating it like a whole bulb.
Check the bulbs before storing them
Look for firm bulbs with reasonably dry outer skins and no soft wet spots. A little loose papery skin is completely normal, and a dry neck is a good sign when storing mature onions.
Remove any onion showing mould, wet decay or a strong rotten smell. A single bad onion sitting at the bottom of a bag can make the others damp and difficult to keep.
An onion that's started producing green growth may still have sound edible portions, but it's likely to have a different texture and shorter useful life. Inspect it carefully before use.
If you bought a net containing several bulbs, take a moment to remove any that were damaged during transport.
Why a cool and airy cupboard works
Whole mature onions keep best in conditions that allow moisture to escape. A ventilated pantry, suitable mesh bag or open basket is more useful than a sealed plastic bag in a warm cupboard.
Avoid places exposed to steam from a kettle, dishwasher or cooker. Extra moisture can encourage sprouting and deterioration.
The kitchen may not be ideal in a hot summer. If your cupboards become very warm, buy smaller quantities or use a cooler, suitable storage area.
Don't place onions in direct sunlight, which can increase temperature and affect their quality.
Should onions go in the refrigerator?
Whole dry mature bulbs generally don't need to be kept in the fridge. Cold, damp conditions may soften their surface and change how well they keep once moved back to room temperature.
A cut or peeled onion is a different matter. Cover it properly and refrigerate promptly, using a clean food-safe container to protect the cut surface.
Spring onions are also different: their tender green leaves lose moisture readily and generally benefit from refrigeration.
Rather than asking whether all onions belong in the fridge, first check which kind of onion you're storing and whether its protective outer skin is intact.
Store bulbs where air can circulate
A mesh bag, slatted vegetable rack or ventilated basket allows air to reach the bulbs. You don't need expensive storage equipment, but you do need to avoid a pile of damp onions sealed inside plastic.
Arrange the bulbs so you can spot a soft or sprouting one without emptying an enormous cupboard. Use the oldest onions before buying more.
If you have a large harvest, avoid stacking thick layers where the bottom bulbs become crushed. Individual trays or properly ventilated crates make inspection easier.
Clean the shelf or container occasionally and remove dry skins that collect moisture or dirt.
Keep onions and potatoes apart
Onions and potatoes are often pictured together in rustic storage baskets, but they don't always make ideal neighbours. Both give off moisture and can deteriorate under unsuitable conditions.
For practical kitchen storage, keep them in separate ventilated spaces and inspect each independently.
Potatoes also need protection from light, which can lead to greening. The best potato cupboard may not be the ideal place for your onions.
You can keep both in the same general cool pantry if suitable, without heaping them together in a single closed sack.
Preparing homegrown onions for storage
If you've grown bulb onions, the RHS advises harvesting mature bulbs once the foliage begins dying back and allowing them to dry thoroughly before putting them away.
After lifting, spread them somewhere warm, dry and well ventilated, protected from rain. The outer layers and necks should become dry before long-term storage.
Handle the bulbs carefully to avoid bruises. Damaged onions are better used promptly rather than included in the long-term store.
Trim and store only when the crop is properly cured, following the RHS instructions for your variety and growing conditions.
How to tell whether an onion has gone bad
Press the bulb gently. Widespread softness, leaking fluid or extensive mould usually means it should be discarded.
A dry papery skin that flakes away is not the same as decay. Peel it and inspect the flesh underneath if the bulb otherwise seems sound.
If the inside looks badly discoloured or smells rotten, do not simply cut around a large spoiled area and use the rest in a stew.
Check stored bulbs regularly so one deteriorating onion doesn't remain hidden beneath the others.
Storing half an onion
Put the cut half into a clean lidded container or cover the cut side closely with suitable food wrap. Refrigerate promptly.
Use it within a short period, following relevant food-safety guidance and observing its condition. The longer the cut surface remains exposed, the more likely it is to dry or absorb fridge odours.
Wash the outside before cutting and use a clean knife and board. Keep raw meat juices away from the onion and its storage container.
If a recipe calls for only half an onion, freezing the chopped remainder for a cooked dish can be more practical than leaving it forgotten in the fridge.
Can you freeze chopped onions?
Yes. The National Center for Home Food Preservation advises that peeled diced onion can be frozen without blanching for later cooking.
Cut the pieces to a convenient size, put them into suitable sealed freezer packaging and label the amount and date.
After thawing, the texture will be softer than fresh raw onion. That's why frozen chopped onion is better for soups, curries and sauces than salads.
A tightly sealed bag matters because onion odours can otherwise spread through a freezer.
What about red onions and shallots?
Mature red onions and shallots can often be kept in a similar cool, dry, airy location, although their storage life varies by variety and condition.
Shallots tend to be smaller, and damaged or very thin-skinned ones may dry out more quickly than large storage onions.
Check the bulbs rather than relying on the calendar. A firm shallot stored in suitable conditions is more useful than a sprouting one left in a hot kitchen for months.
If you've cut or peeled them, follow the same basic refrigeration and hygiene precautions as other prepared onions.
Making the storage habit easy
When unpacking groceries, remove damaged onions, keep dry whole bulbs ventilated and use cut onion portions promptly from the fridge.
Don't keep buying new onions while forgotten ones are sprouting at the back of a cupboard. Arrange them where you can see and use the older ones.
For a garden harvest, curing and inspection matter as much as the storage location. For a supermarket bag, simply keeping the bulbs dry and out of the heat can make a noticeable difference.
Sources checked
Royal Horticultural Society: growing onions and National Center for Home Food Preservation: freezing onions.