Why Onions Need a Proper Harvest and Store

There is little more satisfying on an allotment than lifting a row of fat, golden onions that you grew from tiny sets back in March. But the difference between onions that see you through to March and a slimy, sprouting mess in the shed comes down to two things: how you dry them and where you keep them. Get these right and a modest bed can supply your kitchen for eight months or more. Get them wrong and you will be binning perfectly good bulbs by November.

Onions are biennials. Left to their own devices, a bulb's whole purpose is to sprout again the following spring. Our job after lifting is to persuade it that nothing is going to happen for a very long time, so it stays dormant in the meantime. Warmth, moisture and bruising all break that dormancy, so every step from fork to shelf should be about keeping the bulbs cool, dry and unmarked.

Knowing When to Lift

Timing matters more than most gardeners realise. Onions are ready when the tops begin to yellow and flop over, usually from late July into August depending on variety and where you garden. The classic mistake is bending the tops over deliberately to speed things up. This damages the neck and lets rot in, and the bulb has not finished swelling. Let them fall on their own.

Lift with a fork rather than pulling by hand, working a good few inches away from the bulb so you do not spear it. On heavy clay, gently lever rather than dig. If rain is forecast, lift anyway and dry them under cover — a wet spell at this stage is far more damaging than a day or two out of the ground.

• Wait until roughly half the tops have gone over naturally.
• Choose a dry, breezy morning if you can.
• Loosen the soil first, then lift with the fork.
• Leave the roots and the dried outer skin intact.

Do not wash them. A rub with your hands once dry is all that is needed.

Drying: The Stage Nobody Should Skip

Drying, or curing, is the single most important step and the one most often rushed. The aim is to dry the outer skins and, crucially, the neck, until it is papery and tight. A green, thick neck is an open door for neck rot, which will quietly spread through your stored crop.

In a good summer, lay the onions in a single layer on a slatted tray or on the ground on dry newspaper in full sun, turning them every day or two. A week to ten days is usually enough outdoors, longer in a dull year. Bring them in at night if rain threatens, or better still, move the whole lot into a greenhouse, polytunnel or well-ventilated shed to finish. Air movement matters more than heat; a warm, still room will not cure them.

You will know they are done when the skins rustle, the necks are completely dry and closed, and the roots look like little brittle wires. If you can twist the top off cleanly with no moisture at the break, you are there.

• Aim for at least two to three weeks of drying in an average year.
• Turn them regularly so no damp patches develop underneath.
• Keep them off bare soil to avoid splashing and slug damage.
• Never stack them wet in a heap — that is how rot starts.

Plaiting, Stringing or Trays

Once cured, onions can be plaited into ropes, tied in bunches by the dried tops, or laid in single layers in shallow trays or nets. Plaiting looks lovely hanging in a cool porch and saves space, but only works if the tops are still supple enough to handle. If they have gone brittle, string them instead, or simply lay them in trays.

Whichever method you choose, the principle is the same: air must be able to move around every bulb. Onions pressed against each other in a bucket or a sealed box will sweat and rot. If you use nets or orange bags, hang them so air circulates all round. Do not pile them more than two bulbs deep in a tray.

Where to Store Them

The ideal spot is cool, dark, dry and airy — around 2 to 5°C is perfect, though anything under about 10°C will do well. A frost-free shed, garage, unheated porch or a spare room works. The kitchen is the worst place in the house: warm, humid and full of light, all of which encourage sprouting.

Darkness matters because light makes bulbs green and start to grow. Good ventilation matters because trapped damp causes botrytis and neck rot. And keep them away from other ripening fruit and vegetables, particularly apples and potatoes, which give off ethylene and can shorten storage life. Check the crop every few weeks and remove any bulb that feels soft, shows grey mould or has started to sprout — one bad onion can quickly spoil its neighbours.

Which Varieties Keep Best, and What to Do With the Rest

Not all onions store equally. Long-keeping brown-skinned varieties such as 'Sturon', 'Stuttgarter Giant' and 'Red Baron' (for reds) will comfortably last into spring if cured properly. Sweeter, milder types — including most white onions and many of the giant exhibition sorts — are better eaten fresh from late summer to autumn. Shallots, which are naturally hardier in store, will often outlast onions by months and are well worth a row of their own.

Any bulb that is thick-necked, damaged or soft at harvest should be set aside and used first. Chop and freeze the surplus, or make a batch of onion chutney or caramelised onion relish. Thick-necked onions are also ideal for drying as flakes if you have a dehydrator.

Looked after properly, a decent bed of onions lifted in August will still be giving you sound, sweet bulbs the following March — long after the supermarket ones have given up.