A quick answer from the garden notebook
When to Plant Onions
By David Choi · Updated · How we source and review advice
Short answer
In cold-winter areas, plant onions early in spring. Sow seeds or plant sets outdoors as soon as the soil can be worked. Set out transplants in the 3 weeks before your last spring frost, since onions take a light frost. Start your own transplants indoors 10 to 12 weeks before that date.
Onion planting dates
The timing rule our onion guide uses:
- Start indoors: 10 to 12 weeks before your last spring frost
- Sow outdoors: 0 to 2 weeks after the start of your growing season
- Plant out: 0 to 3 weeks before your last spring frost
Your dates
Enter a U.S. ZIP code or a town to see these dates for your garden.
A U.S. ZIP uses official NOAA frost records. Other countries use weather-station records, or a climate reconstruction where stations are missing. Coordinates are used once and not stored. We save only a place label and climate summary in a private browser cookie. You can clear it anytime.
Example dates for the 2027 season
| Place | Dates |
|---|---|
| Seattle, WALast frost Mar 13 |
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| Minneapolis, MNLast frost Apr 23 |
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| Chicago, ILLast frost Apr 10 |
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| Denver, COLast frost Apr 30 |
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| New York, NYLast frost Mar 30 |
|
| Atlanta, GALast frost Mar 24 |
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| Dallas, TXLast frost Mar 2 |
|
Frost dates are the usual (median) dates from NOAA climate normals for 1991 to 2020. In about half of years the last frost comes later than the usual date, so check the forecast before planting. Where frost is rare, as in much of Florida and the Arizona desert, frost dates do not set planting times.
Day length decides the bulb
An onion forms its bulb once the days reach a certain length. University of Florida Extension says onions are sold as short, mid or long day, based on how many hours of daylight they need. Minnesota Extension says long-day storage onions need 14 or more hours of daylight to make a bulb. Pick a variety made for your area.
This is also why late planting fails. Utah State Extension says onions planted too late in spring mostly grow leaves and never form a bulb.
Seeds, sets or transplants
- Seeds: sow outdoors as soon as the soil can be worked in spring, says Minnesota Extension.
- Sets: plant these small bulbs as soon as the soil can be worked, too. Utah State Extension says they are the easiest to handle.
- Transplants: Utah State Extension says they need about 8 weeks of growing before they go in the garden. Minnesota Extension says to plant them when it reaches 50°F.
Do not rush, either. Utah State Extension says sets and transplants often bolt, or go to seed, if planted too early. Large ones, over 5/8 inch across, bolt more often.
Places with mild winters
In the far south, onions are a fall crop. University of Florida Extension says to plant onions in Florida from September through December, and to choose a variety labeled short day. Minnesota Extension notes that short-day onions make only small bulbs that far north.
Sources
- Growing onions in home gardens (University of Minnesota Extension)
- Onions in the Garden (Utah State University Extension)
- Onions and Shallots (University of Florida IFAS Extension)
The full growing guide
Onion
A cool-season plant grown into a bulb that is dried, cured and stored after the tops fall.
- Harvest
- 100 to 120 days
- Effort
- Season
- Cool
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