Why are my onions so small?

Usually the wrong day-length type for your latitude, or planting too late. Onions bulb in response to day length, and each leaf grown before bulbing becomes a ring — so plant early in cool weather and match long-day (North) or short-day (South) types to where you garden.

Why do my onions rot in storage?

They weren't cured. After lifting, spread onions in a single layer somewhere warm, dry, and airy for 2–3 weeks until the necks are papery and tight. Uncured onions, and thin-skinned sweet types, won't keep.

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How to Grow Onions: Day Length Is Everything

GardenDraft Team · May 6, 2026 · 6 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.

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.

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.

Onions want to sit mostly above the soil as they bulb — bury only the roots, not the bulb.

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. Find your planting window on the planting calendar.

Frequently asked questions

Why are my onions so small?: Usually the wrong day-length type for your latitude, or planting too late. Onions bulb in response to day length, and each leaf grown before bulbing becomes a ring — so plant early in cool weather and match long-day (North) or short-day (South) types to where you garden.
Why do my onions rot in storage?: They weren't cured. After lifting, spread onions in a single layer somewhere warm, dry, and airy for 2–3 weeks until the necks are papery and tight. Uncured onions, and thin-skinned sweet types, won't keep.