How to Grow Onions: Day Length Decides Your Bulb Size

Onions are the crop most often grown wrong for a reason nobody mentions on the seed packet rack. They do not bulb when they reach a certain age or size. They bulb when the days reach a certain length — and if you buy the wrong type for your latitude, the plant will do everything right and still hand you something the size of a golf ball.

The day-length rule

This is the whole game, so it is worth being precise about it.

An onion plant grows leaves until the day reaches a trigger length, then switches over and pours everything into the bulb. Each leaf it managed to grow before that switch becomes one ring of the bulb. More leaves at the moment of switching means a bigger onion — and that is why planting date and variety type matter more than anything you do later.

Plant a short-day onion in Minnesota and it will begin bulbing in late spring, after only a handful of leaves, and stop. Plant a long-day onion in Texas and the day never gets long enough to trigger bulbing at all — you get a large green plant and no onion.

Seed, sets, or transplants

All three work. They trade convenience against size and storage life.

Sets are small dormant bulbs from last season. They are the easiest and quickest option and tolerate rough handling, but because they are already in their second year of growth they bolt readily if stressed by cold or drought, and they generally produce smaller bulbs. Choose sets under about ¾ inch across — larger ones bolt more.

Seed takes the longest and needs an indoor start, but gives you the full variety range, the biggest bulbs, and the best keepers. Onion seed is notoriously short-lived; buy it fresh every year rather than sowing what is left in the packet.

Transplants — bare-root bundles sold in spring — are the middle ground, and are how most serious growers who do not start their own seed do it.

Planting and spacing

Onions are terrible competitors. They have fine, shallow roots, upright leaves that shade nothing, and no ability to push through compacted ground. Almost every cultural problem traces back to one of those three facts.

Give them full sun and soil forked loose with 2–3 inches of compost. Set plants 1 inch deep — deep enough to stand up, no deeper, because a bulb buried too far cannot expand sideways. Space at 4 inches for cooking onions and 6 inches if you want the big ones; spacing directly limits final diameter.

Plant as early as the ground can be worked in spring. Established onions shrug off frost down to about 20°F, and the earlier they go in the more leaves they build before day length flips the switch.

Feeding, and when to stop

Nitrogen builds leaves, and leaves become rings. Side-dress every two to three weeks through spring with a nitrogen-rich fertilizer — blood meal, feather meal, or a balanced organic feed.

Then stop, and stop deliberately. Once the bulbs begin to swell and push the soil up, further nitrogen goes into thick, green, sappy necks. A thick neck never dries down properly, and an onion that does not dry at the neck is an onion that rots in storage within a month.

Water an inch a week and keep it even. Onions that dry out and then get soaked during bulbing split their skins, and split onions do not keep.

Harvest and curing

When about three-quarters of the tops have collapsed at the neck of their own accord, the crop is done. Lift the whole bed with a fork on a dry day.

You will read advice to bend the tops over yourself to hurry things along. Do not — it interrupts the plant’s own dry-down and measurably shortens storage life. Let them fall.

Cure with the tops left on, somewhere dry, shaded, and well-ventilated, for two to three weeks. They are ready when the necks are papery all the way through and the outer skins rustle. Then trim the roots close, cut the tops to about an inch, and store cold and dry — 32–40°F, in mesh bags, crates, or braids.