Starting Method Guide
Growing Onions From Sets: The Fastest Way From Garden to Harvest
Onion sets — sometimes loosely called “onion bulbs” at garden centers — are small, dormant onions grown the previous season. Plant them pointed end up, about 1 inch (2.5 cm) deep, in full sun as soon as the soil can be worked, and you can be harvesting in about 2 months: the quickest route to a crop of the three starting methods.
Onion Sets at a Glance
| Quick Fact | What to Know |
|---|---|
| What they are | Small, dormant onions grown the prior season, not seeds or mature bulbs |
| Best size to buy | Smaller than a dime, for dry storage bulbs |
| Planting depth | About 1 inch (2.5 cm), pointed end up |
| Spacing for dry bulbs | 2 to 6 inches (10 to 15 cm), sources vary |
| Spacing for green onions | Close enough to touch, about 1.5 in (4 cm) deep |
| Time to harvest | About 2 months, the quickest of the three methods |
| Bolting risk | Higher in sets larger than a dime |
Why Buy Sets Instead of Seed or Transplants?
Illinois Extension calls sets probably the simplest method for a home gardener, and that’s the honest case for them: no indoor sowing, no hardening off, just plant and wait about two months. The trade-off is variety choice (garden centers stock far fewer set varieties than seed catalogs) and a higher chance of bolting if you pick the wrong size. See the full seed-starting method if variety choice matters more to you than speed. To put the convenience in perspective: a one-pound bag typically holds 75-100 sets, enough for a household that cooks with onions several times a week for a season, without the eight to ten weeks of indoor seed-tray management that method requires.
How to Plant Onion Sets, Step by Step
- Sort your sets before planting. Discard any that feel soft. Separate out anything larger than a dime — plant those as green onions instead, since Illinois Extension notes they’re more likely to bolt.
- Plant pointed end up, about 1 inch (2.5 cm) deep, in full sun and loose, well-drained soil, as soon as the ground can be worked.
- Space for your goal. 4 to 6 inches (10 to 15 cm) apart for full-size dry bulbs, or as close as 2 inches if you plan to pull every other plant as a green onion.
- Water well after planting, then keep soil evenly moist, about 1 inch (25 mm) a week.
- Feed early, stop late. Nitrogen while leaves grow; stop once bulbs begin to swell to avoid thick necks that won’t cure well.
- Don’t hill soil around them. Unlike potatoes, mounding soil over onion sets can cause neck rot at harvest.
Why Did My Sets Bolt Instead of Bulbing?
Bolting is the single most common complaint about sets, and it’s driven by size and cold, not bad luck. Illinois Extension is specific: sets larger than a dime in diameter are best used for green onions because they’re more likely to bolt, while sets smaller than a dime make the best dry bulbs and usually don’t. A cold snap shortly after planting can also trigger a set that was borderline in size to bolt. If flower stalks have already appeared, pull those plants and use them right away as green onions — Illinois Extension notes bolted onions don’t store well. For the full explanation of why cold (not day length) causes bolting, see our main growing guide.
Sets vs. Seed vs. Transplants
| Factor | Sets | Seed or Transplants |
|---|---|---|
| Time to harvest | About 2 months, fastest | 2 to 5 months |
| Effort | Lowest — plant and go | Higher: sowing, hardening off, or buying transplants |
| Variety choice | Limited to what’s stocked | Much wider, especially from seed |
| Bolting risk | Higher for large sets | Lower |
| Cost | Low to medium | Lowest (seed) to highest (transplants) |
Our verdict: sets are the easiest first crop for a beginner or a short, unpredictable spring. If you want the biggest, most reliable bulbs and don’t mind paying more, see growing onions from transplants on our main guide instead.
Frequently Asked Questions
What’s the difference between onion sets and onion bulbs?
“Onion bulbs” is often used loosely to mean sets at garden centers, but they’re the same thing: small, dormant onions from last season’s crop, planted to grow into this year’s mature bulb. A mature “bulb onion” is the harvested, full-size onion.
How deep do you plant onion sets?
About 1 inch (2.5 cm) deep, pointed end up. Planting deeper can inhibit bulbing.
How long do onion sets take to grow?
About 2 months to harvest, the fastest of the three common starting methods.
What size onion set should I buy?
Smaller than a dime for the best dry storage bulbs with the lowest bolting risk. Save any larger sets for green onions.
Final Thoughts
Sets are the shortcut to a real onion harvest: buy small, firm ones, plant them shallow and on time, and you’ll have bulbs in about two months with almost none of the effort seed or transplants demand. The one rule that matters most is size — stick to sets smaller than a dime and bolting mostly takes care of itself. For everything else, from day-length varieties to harvest and curing, see the main growing guide.
How This Guide Was Checked
Related Onion-Growing Guides
- Growing Onions From Seed: The Complete Step-by-Step Method
- Onion Sets vs Transplants vs Seeds: Which Should You Choose?
- How Long Does It Take to Grow Onions? A Timeline by Method
- How to Harvest, Cure, and Store Onions for Months
See the full How to Grow Onions guide for the complete seed-to-harvest walkthrough.
