How to grow onions

An onion grows leaves until the days reach a certain length, and then it stops making leaves and starts making a bulb. Each leaf becomes one ring, so the size of the bulb is set by how many leaves the plant managed before the switch flipped. This is the whole crop in one sentence.

Long-day varieties need about fourteen to sixteen hours of daylight to trigger bulbing. They suit roughly the northern half of the United States — anywhere above about 37 degrees latitude, which includes the whole Pacific Northwest.

Short-day varieties bulb at about ten to twelve hours, and suit the south. Grow a short-day onion in Oregon and it will start bulbing in spring while the plant is still small, and give you a crop of golf balls no matter how well you feed it.

Day-neutral or intermediate varieties bulb at twelve to fourteen hours and work across a broad middle band, which makes them the safe choice if you are unsure.

Seed racks very often do not say. It is worth checking the catalogue description before buying, because no amount of good growing recovers from this — and it is the reason a great many people conclude they cannot grow onions.

Sets are small immature bulbs grown the previous year, planted like a bulb and by far the easiest route. They are forgiving, they establish quickly, and they are what most gardeners should use for a first crop. Our calendar treats onions as a sets crop, going in about four weeks before the last frost.

The catch with sets is bolting. A set is a plant that has already been through a winter once, and if it experiences another cold spell after planting it can conclude it is in its second year and run to seed. Choosing small sets — under about three quarters of an inch — markedly reduces this, because very small sets are less likely to have vernalised.

Seed gives a much wider variety choice, better keeping quality and no bolting risk, at the cost of a longer season and more work. Onion seed is notoriously short-lived — noticeably worse in its second year — so buy fresh every year rather than using leftovers.

Bought transplants are the middle path: variety choice close to seed, convenience close to sets.

Whichever you use, plant shallow. A set should sit with its tip just at the surface, not buried. Onions bulb at the soil surface and burying them produces tall necks and small bulbs.

Onions have sparse, shallow roots and thin upright leaves that shade almost nothing. They are among the worst competitors in the vegetable garden, and a weedy onion bed produces small onions — not sometimes, reliably.

So the single most valuable thing you can do for this crop after choosing the right variety is keep the bed clean from the start. Hoe shallowly and often rather than deeply and occasionally; the roots are close to the surface and easily damaged.

Feed early and stop early. Nitrogen while the plant is making leaves is what builds bulb size, because leaves become rings. But nitrogen once bulbing has begun produces soft bulbs with thick necks that will not cure and will not keep. Stop feeding once you see the bulbs starting to swell.

Water steadily through the leaf-building phase and then ease off as the bulbs mature. A wet finish makes onions that rot in store.

A bolting onion sends up a thick, hollow flower stem from the centre. It happens because the plant has been convinced it has been through a winter — a cold spell after planting, most often, or an oversized set that had already vernalised.

Once the stem is up, that bulb will not keep. The flower stalk leaves a hollow core running down through the bulb which lets rot in, and the plant stops putting energy into the rings.

So the response is simple: pull bolted onions and eat them first. They are perfectly good, just not storable. Do not bother snapping the stem off in the hope of saving them — that is folk wisdom and it does not undo the hollow neck.

The photograph below is what a whole field looks like when it has gone this way, or when it has been deliberately grown on for seed, which looks the same. It is a useful thing to be able to recognise.

Prevention: small sets, planting into soil that has warmed rather than at the earliest possible date, and growing from seed if bolting is a recurring problem.

This is where home crops are lost, and it is entirely avoidable.

Onions are ready when the tops fall over of their own accord and start to yellow. Wait for that rather than bending the tops over yourself, which is a very old practice that mostly just damages the plant and brings the harvest forward without improving it.

Lift them and lay them out to dry somewhere with air moving — in the field if the weather is settled, as in the photograph at the top of this page, or under cover if it is not. They need one to three weeks depending on conditions.

The test is the neck. Curing is finished when the neck has shrivelled to a dry, papery, tightly closed point and the outer skins rustle. A neck that is still thick or soft means the onion is not cured, and an uncured onion rots in storage within weeks no matter how good it looked coming out of the ground.

Then trim the roots, cut or plait the tops, and store cool, dark and dry with air around them. A well-cured long-day storage onion keeps until spring. A poorly cured one keeps until roughly Christmas, and you will lose a lot of it.

Onion fly is the main pest: the maggots feed at the base and the plant yellows, wilts and collapses. Rotation and fine mesh from planting are the effective answers. Sets are less vulnerable than seedlings because they establish faster.

Downy mildew appears as pale patches on the leaves with grey growth in damp weather, which in a maritime spring is most of it. Space the rows so air moves, avoid overhead watering, and rotate — it persists in the soil.

White rot is the serious one: white fluffy growth and small black specks at the base, and it stays in the soil for many years. There is no cure. Do not move soil or plants between beds, and rotate alliums as far and as long as possible.

Thrips cause silvery streaking in hot dry spells and are largely cosmetic on a bulb crop.

And a common non-problem: onions with thick necks and soft bulbs that will not cure. That is almost always nitrogen applied too late in the season rather than any disease.