Starting Method Guide
Growing Onions From Seed: The Complete Step-by-Step Method
To grow onions from seed, sow ¼ inch (6 mm) deep indoors 8 to 12 weeks before your last frost, keep the mix at about 70°F (21°C) until germination, then harden off and transplant seedlings once they are 3 to 4 inches tall. Expect up to 5 months from sowing to harvest — the slowest start of the three methods, but the cheapest and the widest choice of varieties.
Onion Seed at a Glance
| Quick Fact | What to Know |
|---|---|
| Germination time | 4 to 10 days at about 70°F (21°C) |
| Seedling emergence | 7 to 14 days at 60 to 85°F (16 to 29°C) |
| Seed viability | Only 1 to 2 years — buy fresh seed each season |
| Sowing depth | ¼ inch (6 mm) |
| Start indoors | 8 to 12 weeks before your last frost |
| Transplant size | Move outside at 3 to 4 inches (7.5 to 10 cm) tall |
| Time to harvest | Up to 5 months from sowing |
| Cost | Lowest of the three methods; widest variety choice |
Why Grow Onions From Seed Instead of Sets or Transplants?
Seed is the slowest route to a bulb, but it is also the cheapest per plant and gives you access to far more varieties than any garden center sells as sets. Illinois Extension notes that transplants are the method that most reliably produces large, dry storage onions, while seed rewards the grower who can start trays indoors in winter with that same large-bulb potential at a fraction of the cost. If you want the full breakdown against sets and transplants, see our seed vs. sets vs. transplants comparison on the main growing guide.
How to Start Onion Seed Indoors
- Use a fresh seed-starting mix in cell trays or small pots, 4 to 6 seeds per cell is a common home approach.
- Sow ¼ inch (6 mm) deep and water gently; a plastic dome helps hold humidity until seeds sprout.
- Keep the mix around 70°F (21°C). Expect germination in 4 to 10 days, with seedlings fully up in 7 to 14 days.
- Remove the dome once seedlings emerge and give them 14 to 16 hours of light a day under a grow light, or a very bright south-facing window.
- Trim tall, floppy seedlings back to about 4 inches. This keeps them upright and pushes the plant to build a stronger root system and thicker stem instead of top growth — a technique documented by several experienced home seed-starters, repeated every couple of weeks until transplanting.
- Feed lightly once seedlings have a few leaves. A diluted fish emulsion (roughly 1 tablespoon per gallon of water) every other week, alternated with plain water, is a commonly used routine that avoids burning young roots.
- Harden off for about a week before planting out, giving seedlings increasing time outdoors each day.
Transplanting Seedlings Outdoors
Move seedlings to the garden once they are 3 to 4 inches tall and hardened off. Plant 1 to 1.5 inches (2.5 to 4 cm) deep and 4 to 5 inches (10 to 13 cm) apart for large bulbs, in full sun and loose, well-drained soil. Match your variety’s day-length type to your latitude first — a long-day seed packet planted in the Deep South may never bulb properly, regardless of how well the seedlings grow. See our day-length guide on the main page before you buy seed.
Real example: Meg Cowden of Seed to Fork, gardening in Minnesota, sows onion seed indoors from mid-February into March and counts roughly 180 days between sowing and harvest. She grows 250 to 300 onions a year, harvests in mid-August, cures for 3 to 6 weeks, and still loses about 10% to rot by late winter — bulb size, she notes, comes down to how well the roots developed and how much transplanting disturbed them.
Seed to Fork
Why Won’t My Onion Seed Germinate?
Poor germination is one of the most common complaints among home growers starting onions from seed, and it usually comes down to one of a few fixable causes. University extension trials on fresh, properly stored onion seed commonly report germination rates of 85-95% under ideal warm, moist conditions, while seed older than two years can fall well below 50%, which is why age is usually the first thing worth ruling out before blaming technique. If you’ve ever seeded a full tray and watched barely a third come up, it’s rarely bad luck; it’s almost always one of the causes below.
Old seed
Onion seed stays viable for only about 1 to 2 years. Seed kept longer than that germinates at a much lower rate, even if it looks fine. Buy fresh seed each season rather than using up a leftover packet.
Soil too cold or too dry
Germination is fastest around 70°F (21°C). Cooler mix slows sprouting dramatically, and a mix that dries out between waterings can stall or kill emerging roots. A humidity dome and a warm spot (on top of a fridge works) solve most of this.
Seed sown too deep
Onion seed only needs ¼ inch (6 mm) of cover. Sown deeper, seedlings use up their energy reserves before reaching the surface.
Damping off
Thin, collapsing seedlings at the soil line point to damping off, a fungal issue favored by poor air flow and overly wet soil. A small fan running near trays and avoiding waterlogged mix both help prevent it.
Seed vs. Sets vs. Transplants: Where Seed Wins and Loses
| Factor | Seed | Sets or Transplants |
|---|---|---|
| Cost per plant | Lowest | Higher, especially transplants |
| Variety choice | Widest | Limited to what’s stocked locally |
| Time to harvest | Up to 5 months | 2 to 4 months |
| Bolting risk | Lower | Higher, especially large sets |
| Effort required | Highest (indoor sowing, hardening off) | Lowest (plant and go) |
Our verdict: seed is the best value over time for gardeners who can start trays indoors in late winter and want the largest choice of keeper varieties. If you want the simplest possible start instead, see growing onions from sets. In practice, most home gardeners who stick with seed for three or four seasons in a row report it gets noticeably easier each year, mainly because they learn their own windowsill or grow-light setup’s quirks rather than any change in the seed itself.
Frequently Asked Questions
How long does onion seed take to germinate?
4 to 10 days at about 70°F (21°C), with seedlings fully emerged in 7 to 14 days. Cooler conditions slow this down considerably.
Can you direct-sow onion seed outdoors?
Yes, once soil has warmed, but indoor starting gives seedlings a head start that usually produces larger bulbs by harvest, since onions need a long season to size up.
Do onion seedlings need a grow light?
Not strictly, but 14 to 16 hours of strong light a day keeps seedlings stocky instead of thin and floppy. A very bright south-facing window can work if a grow light isn’t available.
How old can onion seed be and still grow?
Onion seed keeps for only about 1 to 2 years. After that, germination rates drop sharply, so it’s worth buying fresh seed each season rather than relying on an older packet.
When should I start onion seed indoors?
Count back 8 to 12 weeks from your last expected frost date. Many northern growers start in February; adjust earlier or later depending on your own last-frost date.
Final Thoughts
Growing onions from seed takes patience — up to five months from tray to harvest — but it rewards you with the cheapest cost per plant and access to varieties no garden center sells as sets. Get the day-length type right for your latitude, keep germination warm and the mix evenly moist, and don’t skip hardening off. If you’d rather skip the indoor-sowing stage entirely, onion sets and transplants are both covered on our main growing guide, and if you’re just trying to figure out which method suits you, start there.
How This Guide Was Checked
Related Onion-Growing Guides
- Growing Onions From Sets: The Fastest Way From Garden to Harvest
- Onion Sets vs Transplants vs Seeds: Which Should You Choose?
- How Long Does It Take to Grow Onions? A Timeline by Method
- Why Onion Seeds Aren’t Germinating (and How to Fix It)
See the full How to Grow Onions guide for the complete seed-to-harvest walkthrough.
