Plot · field guide
How to Grow Onions from Sets and Seed
Onions are cheap to start and store for months in a cool, dry spot. The one thing that trips up most growers is not spacing or watering, it is picking the wrong type for where they live. Onions bulb up based on day length, so a variety that thrives in Georgia makes tiny bulbs in Minnesota. Get the type right first, then the rest is easy.
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Match the type to your latitude
This is the rule that decides your whole crop. Onions start forming bulbs when the days reach a certain length, and that length depends on the variety. Plant the wrong one and the plant either bulbs too early and stays small, or never bulbs at all.
Short-day onions bulb when days hit about 10 to 12 hours, so they suit the South, roughly below the line from Atlanta across. Long-day onions need about 14 to 16 hours, so they fit the North. Day-neutral, sometimes called intermediate, bulb at 12 to 14 hours and grow well almost anywhere, which makes them the safe pick if you are unsure.
- Short-day: for the South, bulbs at 10 to 12 hours of daylight.
- Long-day: for the North, bulbs at 14 to 16 hours.
- Day-neutral: works almost anywhere, bulbs at 12 to 14 hours.
Sets, seed, or transplants
There are three ways to start onions and they trade speed for control. Sets are tiny dried bulbs from last year; they are the easiest and fastest but come in the fewest varieties and are prone to bolting. Seed gives you the most varieties and the best storage bulbs, but it is the slowest and needs an early start indoors.
Transplants, young plants you buy in bunches, are the sweet spot for most gardeners. From transplants a bulb onion is ready in about 95 days, versus about 110 days when you direct sow from seed. That two-week head start matters because onions race the calendar to bulb before the days get too long.
- Sets: easiest and fastest, but fewer types and more bolting.
- Seed: most varieties and best keepers, but slowest to mature.
- Transplants: about 95 days to a bulb, versus about 110 from direct seed.
Space them close and keep the weeds out
For full-size bulbs, set onions 4 inches apart in the row, with 12 inches between rows. Plant them shallow, only about half an inch deep, so the bulb can swell at the surface. Full sun is a must, 6 hours or more, or the bulbs stay small.
Onions have thin, grassy leaves that lose badly to weeds, so keep the bed clean. Pull weeds by hand near the bulbs rather than hoeing deep, because onion roots are shallow and easy to cut. For green onions instead of bulbs, plant a bunching onion type about 1 inch apart and pull them young.
- Space bulb onions 4 inches apart, rows 12 inches apart.
- Plant shallow, about half an inch deep, in full sun.
- For green onions, grow a bunching type 1 inch apart and pull young.
Stop watering when the tops fall
Onions need steady water while the leaves grow, about 1 inch a week, because every leaf becomes a ring in the bulb. More leaves means a bigger onion. But that changes late in the season.
When the tops naturally flop over and start to yellow, the bulb is done growing. Stop watering then and let the necks dry. Watering a mature bulb only invites rot at the neck, which is exactly the spot that decides whether the onion stores or spoils.
- Give about 1 inch of water a week while leaves are growing.
- When the tops flop over and yellow, the bulb is finished.
- Stop watering once tops fall so the necks dry and store better.
Cure before you store
A bulb onion takes about 110 days from seed, or 95 from a transplant, to reach harvest. Pull them after most of the tops have fallen, on a dry day, and shake off the loose dirt.
Do not stack them away yet. Cure the onions first: lay them in a single layer in a warm, airy, shaded spot for 2 to 3 weeks until the necks are papery and tight. Cured onions keep for months. Skip curing and they rot at the neck within weeks. See the full curing steps below.
- Harvest at about 110 days from seed, 95 from a transplant.
- Cure 2 to 3 weeks in a warm, airy, shaded spot until necks are dry.
- Only cured onions store for months; uncured ones rot fast.
Keep going
Questions, answered straight
It is about latitude. Short-day onions bulb at 10 to 12 hours of daylight and suit the South. Long-day onions need 14 to 16 hours and suit the North. Day-neutral types bulb at 12 to 14 hours and grow well almost anywhere, so they are the safe pick if you are unsure.
Sets are easiest and fastest but bolt more. Seed gives the most varieties and best keepers but is slowest. Transplants are the middle ground: a bulb is ready in about 95 days versus about 110 from direct seed. Transplants are the easy choice for most gardeners.
When the tops naturally flop over and start to yellow, the bulb is done growing. Stop watering then and let the necks dry. Watering a mature bulb invites neck rot, which is the spot that decides whether it stores or spoils.
Grow a bunching onion type instead of a bulb type. Plant them about 1 inch apart and pull them young, around 60 days, before they try to bulb. Bunching types are bred to stay slim and mild.