How to Grow Onions: Day Length Is Everything

GardenDraft Team · May 6, 2026 · Updated August 1, 2026 · 8 min read

Part of: Garden Planning Guides · How to Grow Vegetables — Crop Guides A–Z

Onions hide a quirk that trips up almost everyone the first time: the size of your bulb is decided by day length and by how much leaf the plant grows before it starts bulbing. Understand that one fact and onions go from mysterious to straightforward.

Get the day-length type right for your latitude

Onions bulb in response to day length, and there are three types. Long-day onions need 14–16 hours of daylight and are for the northern US. Short-day onions bulb at 10–12 hours and suit the South. Day-neutral types bulb across a wide range and are the safe choice if you're unsure. Plant a long-day onion in the Deep South and it never gets enough daylight to bulb; plant a short-day onion up north and it bulbs while it's still tiny. Matching the type to your latitude is the single most important onion decision.

A rough dividing line runs across the country at about the 35th parallel: north of it, grow long-day; south of it, short-day; near it, day-neutral types shine. Within your day-length class you still choose for the kitchen — pungent yellow storage onions that keep all winter, milder reds for salads, and sweet types that trade shelf life for gentleness. The onion catalog page spans the range, with maturities running roughly 90 to 130 days for full bulbs from seed or transplants.

Sets, transplants, or seed

You can start onions three ways. Sets (tiny dormant bulbs) are the easiest and most forgiving for beginners. Transplants (pencil-thin seedlings) give the biggest bulbs. Seed is cheapest and offers the most varieties but needs an early indoor start, around 10–12 weeks before transplanting. Whichever you choose, get them in early — onions want to grow as much leaf as possible in cool spring weather, because each leaf becomes a ring, and bulbing later stops new leaves forming.

The catalog timeline runs like this: seed indoors 8–12 weeks before planting out, a 1–2 week hardening off, and everything — sets, transplants, or your homegrown seedlings — goes into the garden 2–4 weeks before your last frost (how frost dates work). Onions are cold-tolerant to the low 20s°F, so early planting is safe and, given the leaf-count logic, mandatory for big bulbs. In the South, short-day onions flip the calendar: they're planted in fall and grow through winter for a late-spring harvest. One warning for seed buyers: onion seed is famously short-lived, so buy fresh packets each year rather than trusting the drawer.

There's a built-in bonus crop hiding in the spacing math, too. Plant sets or seedlings at a tight 2 inches, then eat every second plant as a green onion in late spring — the survivors inherit the full 4 inches just as bulbing begins. You get two harvests from one bed with zero extra ground, and the thinning trip doubles as a weeding trip. Just finish the thinning before bulbs start to swell; crowding after that point costs you bulb size that never comes back.

Sun, even moisture, and a clean bed

Plant in full sun, about 4 inches apart, shallow — the bulb wants to sit mostly on top of the soil as it swells. Onions have skimpy roots and compete poorly with weeds, so keep the bed clean and watered evenly; a dry spell during bulbing means smaller onions. Ease off nitrogen once bulbs start to form. A light mulch keeps weeds down and moisture steady.

That 4-inch figure doubles as the intensive square-foot value — nine onions per square, with 3 inches workable for smaller-bulbed types you'll eat fresh. Prepare the bed with compost and, if you test your soil, aim near neutral. The feeding rhythm mirrors the leaf-then-bulb logic: nitrogen every few weeks while the plant is building leaves in spring, then stop feeding when the soil at the shoulder of each plant starts to crack and the bulb swells (fertilizing guide). Shallow roots also make onions the crop that repays drip irrigation most visibly.

Troubleshooting: bolting, maggots, and thrips

Three problems cover most onion grief. Bolting — a flower stalk in the first year — is triggered when a plant that's already pencil-thick sits through a cold snap; it's most common from oversized sets, so pick the smallest sets in the bin, and eat any bolter promptly since it won't store. Onion maggots tunnel seedlings in cool wet springs, especially where alliums grew last year; a row cover at planting and honest crop rotation are the fixes. Thrips — tiny slivers that rasp silvery streaks in hot dry weather — sap bulb size in bad years; hard water sprays and even moisture keep them tolerable. Onions repay the neighborhood, too: extension-documented pairings put alliums beside carrots and parsnips to mask the carrot rust fly's host-finding scent (companion planting that works).

Reading the plant through the season

Onions telegraph their progress if you know the signals. Through spring, count leaves: a plant carrying eight or more healthy leaves at bulbing time has the makings of a big onion, since each leaf wraps into a ring. When day length trips the bulbing switch, the soil around each plant cracks and the shoulders emerge — normal and good, so resist the urge to re-bury them. From that point the leaves stop coming and the plant pumps everything downward; this is when even moisture matters most and feeding should already have stopped. A flower bud instead of new leaves means that plant has quit on you — snap the stalk to buy time, but move it to the front of the eating queue. And when tops begin flopping on their own in high summer, don't bend the rest over by hand as the old advice went; let the plant finish on its own schedule for the best-storing bulbs.

Curing is what makes onions keep

When about half the tops flop over and yellow, the bulbs have finished sizing up. Stop watering, and after a few days lift them on a dry day. Curing is the step that separates onions you store from onions that rot: spread them in a single layer somewhere warm, dry, and airy for two to three weeks until the necks are papery and tight. Then trim the tops, and well-cured storage varieties will keep for months. Sweet onions, by contrast, are thin-skinned and meant to be eaten soon.

Store cured bulbs cool, dry, and dark with airflow — mesh bags or old tights hung in a cellar are traditional for a reason — and check monthly, cooking any bulb that softens at the neck first. Don't refrigerate whole onions long-term; the humidity wakes them up. And if you want green onions rather than bulbs at all, that's a different, faster game covered in growing green onions, while their perennial cousins shallots and leeks round out the allium bench. Find your planting window on the planting calendar.