When to plant onions
Onions are a cool-season crop that shrugs off frost and light freezes, so it goes in well before the last frost. Type your town to see the dates for your weather station.
Onions in Columbus, OHZONE 6B
- Start seeds indoors
- Dec 27 to Mar 7
- Set plants outside
- Mar 7 to Apr 4
- Or sow seed outside
- Mar 7 to Apr 4
- Plant onion sets
- Feb 28 to Apr 4
- First harvest
- Jun 5 to Jul 25
Counted from the average last frost, Apr 18, and first frost, Oct 27, at Columbus Port Columbus Intl Airport, OH (NOAA 1991-2020).
The seed packet, in short
| Planting depth | 1/4 to 1/2 in |
|---|---|
| Spacing in the row | 2 to 6 in |
| Between rows | 8 to 24 in |
| Soil temperature | at least 35 °F, best 65 to 85 °F |
| Days to sprout | 7 to 21 days |
| Days to harvest | 90 to 140 days from planting |
| Row per person | 3 to 20 feet |
Onion planting dates by zone
Each row uses the typical frost dates of a zone: the middle value among towns of 10,000 people or more in it. In zone 6b, for example, the typical last frost is Apr 20, so the first step for onions, start seeds indoors, falls Dec 29 to Mar 9. Towns in one zone can differ by two weeks, so your own town's dates above are the better guide.
| Zone | Last frost | Start seeds indoors | Set plants outside | Or sow seed outside | Plant onion sets | First harvest |
|---|---|---|---|---|---|---|
| 4 | May 5 | Jan 13 to Mar 24 | Mar 24 to Apr 21 | Mar 24 to Apr 21 | Mar 17 to Apr 21 | Jun 22 to Aug 11 |
| 5a | Apr 30 | Jan 8 to Mar 19 | Mar 19 to Apr 16 | Mar 19 to Apr 16 | Mar 12 to Apr 16 | Jun 17 to Aug 6 |
| 5b | Apr 28 | Jan 6 to Mar 17 | Mar 17 to Apr 14 | Mar 17 to Apr 14 | Mar 10 to Apr 14 | Jun 15 to Aug 4 |
| 6a | Apr 24 | Jan 2 to Mar 13 | Mar 13 to Apr 10 | Mar 13 to Apr 10 | Mar 6 to Apr 10 | Jun 11 to Jul 31 |
| 6b | Apr 20 | Dec 29 to Mar 9 | Mar 9 to Apr 6 | Mar 9 to Apr 6 | Mar 2 to Apr 6 | Jun 7 to Jul 27 |
| 7a | Apr 14 | Dec 23 to Mar 3 | Mar 3 to 31 | Mar 3 to 31 | Feb 24 to Mar 31 | Jun 1 to Jul 21 |
| 7b | Apr 3 | Dec 12 to Feb 20 | Feb 20 to Mar 20 | Feb 20 to Mar 20 | Feb 13 to Mar 20 | May 21 to Jul 10 |
| 8a | Mar 27 | Dec 5 to Feb 13 | Feb 13 to Mar 13 | Feb 13 to Mar 13 | Feb 6 to Mar 13 | May 14 to Jul 3 |
| 8b | Mar 16 | Nov 24 to Feb 2 | Feb 2 to Mar 2 | Feb 2 to Mar 2 | Jan 26 to Mar 2 | May 3 to Jun 22 |
| 9a | Mar 3 | Nov 11 to Jan 20 | Jan 20 to Feb 17 | Jan 20 to Feb 17 | Jan 13 to Feb 17 | Apr 20 to Jun 9 |
| 9b | Jan 31 | Oct 11 to Dec 20 | Dec 20 to Jan 17 | Dec 20 to Jan 17 | Dec 13 to Jan 17 | Mar 20 to May 9 |
Good to know about onions
- Pick a day-length type that fits your latitude: long-day onions for the North, short-day for the South; the wrong type makes small or no bulbs.
- Plant early, because bulbing is triggered by day length; late plantings make small bulbs.
- Small sets (smaller than a dime) give the best dry bulbs; large sets are best used for green onions.
- Seed depth is 1/4 to 1/2 in.; sets and transplants go 1 to 2 in. deep.
How to plant onions
- Give onions full sun and well-drained, fertile soil with plenty of organic matter.
- For bulbs, leave about 3 to 6 in. between plants; you can plant closer and pull every other one early as a green onion.
- Thin seedlings before they crowd each other and start to bulb; the thinnings can be eaten as green onions.
- Onion roots are shallow, so keep weeds out from the start by hand-pulling or hoeing very lightly; a thin straw mulch between rows also helps.
- In dry weather, water slowly and deeply about once a week.
Types that change the timing
- Day-length groups: short-day onions bulb at about 11 to 12 hours of daylight, intermediate-day at about 12 to 13, and long-day at about 14 to 16; intermediate types fit middle latitudes.
- Sweet, mild onions keep only a few weeks after harvest, while pungent storage onions can last for months.
- Bunching onions never form a real bulb and are grown only for green onions.
Common mistakes
- Planting oversized sets or thick transplants, which tend to bolt to flower and make poor bulbs.
- Hoeing too deep around the row, which cuts the shallow roots and damages bulbs.
- Leaving mature onions in wet soil or watering heavily near harvest, which delays curing and leads to rot in storage.
When onions are ready
- Bulbs are mature when most of the tops have fallen over on their own
Timing and spacing: compiled from these university extension guides, each figure checked against at least two of them:
- Colorado State University Extension: Vegetable Planting Guide (CMG GardenNotes #720)
- Virginia Cooperative Extension (Virginia Tech): Virginia's Home Garden Vegetable Planting Guide: Recommended Planting Dates and Amounts to Plant (426-331)
- University of Georgia Extension: Home Garden Planting Chart (B577)
- Kansas State University Research and Extension: Vegetable Garden Planting Guide (MF315)
- University of California Agriculture and Natural Resources, UC Master Gardener Program: Seed Germination Temperature and Timing
- Oregon State University Extension Service: When to start seeds indoors in central Oregon
- University of Maine Cooperative Extension: Maine Vegetable Gardening: Keep Your Garden Growing, Plant from Spring to Fall
- University of Illinois Extension: When should I start my seeds? (Good Growing)
- Cornell Cooperative Extension, Rockland County: Vegetable Planting Guide
- University of Illinois Extension: Watch Your Garden Grow: Onion
- Utah State University Extension: Onions in the Garden
- yardandgarden.extension.iastate.edu
- agrilifeextension.tamu.edu
Frost dates: NOAA NCEI U.S. Climate Normals 1991-2020. General timing based on published university extension guidance and your local frost dates. Varieties and weather differ; your local extension office has the final word.