When to Harvest Onions (and How to Cure Them for Storage)

Onions are one of the few crops that tell you, unmistakably, when they're done: the tops keel over. It looks alarming the first time — a bed of upright green leaves suddenly lying flat as if trampled — but it's exactly what you want to see. Harvest at the right moment and cure the bulbs properly, and homegrown onions will keep for months; rush them or skip the curing, and they'll soften and sprout within weeks. Here's how to read the signals and get the storage part right.

The signs your onions are ready

The headline signal is the flop. As a bulb finishes swelling, the neck — the point where the leaves meet the bulb — softens, and the top folds over while it's still partly green. That's the plant shutting up shop: it has stopped moving energy into the leaves and sealed its resources into the bulb. Once about half the tops in the bed have gone over, the crop is as big as it's going to get, and leaving it much longer gains you nothing but a rising risk of rot, regrowth and thick necks that never dry properly.

The supporting signs: foliage yellowing from the tips down, papery skin starting to form on any bulb shoulders showing above the soil, and — for those keeping track — the calendar. Autumn-planted sets and overwintered onions typically go over in early to mid summer, while spring-planted sets and seed-grown onions usually finish from mid summer into early autumn, around 100 to 120 days after planting. Day length is the underlying trigger: bulbing is switched on by summer's long days, which is why an onion won't simply keep growing bigger the longer you leave it.

Warning: don't bend the tops over yourself to 'hurry things along'. It's an old allotment trick, but breaking the necks early interrupts bulb filling and bruises the neck tissue — and damaged necks are the main gateway for the rots that spoil stored onions. Let the plant fold on its own schedule.

How to lift them

Pick a dry day, ideally at the start of a dry spell. Loosen the soil under each bulb with a hand fork and lift the onion by the bulb, not by yanking the fragile neck. Shake or brush off loose soil — don't wash them, and don't bang bulbs together to knock dirt off, because every bruise is a future soft spot. If a bulb has sent up a flower stalk (bolted), set it aside for the kitchen: bolted onions have a woody core and a hollow stem that funnels moisture into the bulb, so they won't store.

If the weather is fine, you can leave the lifted onions lying right on the soil surface for a day or two to start drying in the sun, roots facing the light. If rain threatens, move them straight under cover — a wet re-soaking undoes the drying you've started.

Curing: the step that makes them keep

Curing is simply thorough drying, and it's what turns a fresh onion into a storable one. Spread the bulbs in a single layer — on wire mesh, slatted trays, or an old pallet — somewhere warm, dry and airy, out of rain and direct scorching sun. A greenhouse with the vents open, a covered porch, or a garage with the door ajar all work. Good airflow matters more than heat; if the air is still, a small fan makes a real difference.

Give them two to three weeks. You're waiting for three things: the outer skins to turn papery and rustling, the roots to become dry and wiry, and — most importantly — the necks to shrivel tight and dry right through. A cured neck feels like paper string; an uncured one still feels fleshy and green. Only when the necks are fully dry should you trim the tops to a couple of centimetres (or plait them, leaves on, into strings) and snip the roots.

Storing your crop

• Store only sound bulbs — firm, dry-necked, with no bruises or thick green necks. Anything doubtful goes to the kitchen first.
• Keep them cool, dark, dry and ventilated: a garage, shed or cool spare room at roughly 0–10°C is ideal. Avoid the fridge for whole bulbs — it's too humid — and never a plastic bag.
• Use mesh sacks, net bags, old tights or plaited strings hung up, or single layers in slatted trays — anything that lets air move around every bulb.
• Use thick-necked bulbs and any that bolted first; well-cured storage varieties can last four to six months or more.
• Check monthly and remove anything softening or sprouting before it spreads.

One thing worth knowing: sweet, mild onions with thin skins are naturally poor keepers — a few weeks to a couple of months at best — while pungent, thick-skinned storage varieties last deep into winter. The compounds that make an onion sharp are the same ones that protect it in storage, so grow some of each and eat the sweet ones first.

Tip: where your onions grew this year matters for next year. Onion white rot and downy mildew persist in soil, so rotate — don't grow onions, garlic or leeks in the same bed for at least three years, and never compost obviously diseased bulbs.

And if something looks off in the bed before harvest — leaves collapsing early with fluffy grey mould, or yellowing long before midsummer — photograph it and let PlantMe take a look. The app recognises 500+ plant diseases from a photo, and telling downy mildew from natural die-back is exactly the call that decides whether you lift early or leave the crop to finish.