Do onions need to be refrigerated?

The short version: no, not a whole one. A dry, papery-skinned onion kept somewhere cool, dark and airy will comfortably outlast a month, and the fridge does not extend that, it shortens it, by putting the onion in exactly the kind of damp air it is built to resist. The moment you cut one open, though, that whole answer flips. A cut onion belongs in the fridge, wrapped, and on a much shorter clock. Two different objects, two different rules, and most of the confusion about onions comes from treating them as one question instead of two.

Why moisture is the enemy, not cold

An onion's papery outer layers are not packaging. They are the onion's own defence. Those dry skins are doing a real job: sealing in just enough moisture to keep the inside firm while keeping outside moisture from reaching the layers underneath, which is what mould and rot actually need to get started. An onion in a cool, dry, ventilated spot can hold that balance for weeks because nothing around it is pushing extra moisture back in.

Cold on its own is not the problem; a fridge is not actually too cold for an onion. The problem is what fridge cold usually comes with: a sealed drawer built to trap humidity, because that humidity is exactly what keeps a lettuce or a bunch of spinach from wilting. An onion reads that same humidity as an attack on the one defence it has. Trapped moisture against the skin is what triggers early sprouting from the root end, softens the layers underneath before anything looks wrong from the outside, and gives surface mould somewhere damp to start.

Not all onions are the same onion

The advice above is really about storage onions, the everyday yellow, white and red onions with thick, dry, papery skins, bred specifically to hold for months in a cupboard. Two other common types do not play by the same rule.

Sweet onions — Vidalia, Walla Walla, and similar mild varieties — carry noticeably more water and sugar and a much thinner skin. That is exactly what makes them mild and good raw, and exactly what makes them spoil faster in the same cupboard that a storage onion handles easily. A sweet onion is closer to a fresh vegetable than a pantry staple: expect weeks, not months, even stored well, and treat any softness or give when you squeeze one as a sign to use it soon rather than a false alarm.
Green onions and spring onions are a different vegetable in every way that matters here. There is no dry outer layer doing protective work — the whole thing is a fresh, leafy stem, closer to a herb than to a storage onion. These do belong in the fridge, loosely wrapped or standing upright in a little water like cut flowers, and are good for one to two weeks rather than one to two months.

Garlic follows the same logic as storage onions, for the same reason: a dry, papery bulb that wants a cupboard, not a crisper drawer, which is why the two so often get grouped together in storage advice.

Once you cut it, everything changes

All of the above describes an intact onion, protected by its own skin. Cut into one and that protection is gone in a single stroke: the moisture-sealing layer is breached, the exposed flesh is now open to whatever bacteria are around, and it starts drying out and oxidising on contact with air the same way a cut apple browns. A cut onion is no longer a pantry item. It is a fresh, cut food, and it needs to move to the fridge — wrapped tightly or in a sealed container, not left loose on a shelf — and be treated as good for a few days, not weeks.

There is a second, very practical reason to keep it sealed rather than just refrigerated: cut onion is unusually good at trading smells with everything around it. Left uncovered, it will pick up odours from whatever else is in the fridge, and just as readily hand its own smell to butter, milk or anything else porous nearby. A tight seal is doing two jobs at once — slowing spoilage and keeping the rest of the fridge from tasting faintly of onion.

The onion-and-potato thing is not just folklore

The old advice to keep onions and potatoes apart in storage holds up, and it runs on the same moisture logic as everything above, just between two vegetables instead of between a vegetable and a fridge. A potato gives off more moisture than an onion wants sitting next to it, which nudges the onion toward softening and mould sooner than it otherwise would, and the reverse happens too, with onions hurrying a potato's dormancy along in a way that shows up as earlier sprouting. Neither vegetable is doing anything wrong on its own; it is specifically the pairing that shortens both of their clocks, which is reason enough to give them separate corners of the same cupboard rather than one shared bowl.

What actually helps

Whole storage onions: a cool, dark, ventilated spot, not the fridge. A wire basket, an open bowl, or a mesh bag lets air move around them, which is doing more of the real work than any specific temperature.
Skip the sealed plastic bag. It traps the exact moisture the papery skin is trying to keep out, which turns a weeks-long onion into a days-long one.
Treat sweet onions as a shorter-window vegetable than a regular yellow or red onion, even when stored the same way, and use them sooner rather than assuming the cupboard buys the same month it does for a storage onion.
Green onions go in the fridge, loosely wrapped or standing in a little water, not in the cupboard with the rest of the onions.
Once cut, wrap it and refrigerate it, and count on a few days rather than weeks — it is a fresh food now, on a fresh food's clock.
Keep onions and potatoes in separate spots, even though they are usually bought, and often stored, together out of habit.

The part that is easy to lose is exactly the moment an onion's rule changes: the day half of it got used in dinner and the other half went back on the counter out of habit, quietly running on borrowed time. Munch tracks where a batch is actually stored — counter, fridge, freezer or pantry — against the item itself, and marking something opened switches it onto its own shorter countdown from that date rather than the one that applied to the whole onion, visible to everyone in the household, so a cut onion does not end up back on the pantry shelf by accident.

None of this needs to be memorised food by food. One idea covers almost all of it: a whole onion is protecting itself with dry air, and every choice — which cupboard, which bag, whether it has been cut — either respects that or works against it. Sweet onions and green onions are just the two cases where the protection was never as strong to begin with.