When to Start Onion Seeds Indoors (Onions, Leeks, Shallots)
Onions are a cool-season crop. The University of Idaho Extension guide says cool-season crops like broccoli and onions can be transplanted outside 2 to 5 weeks before the average date of last frost. The group "onions, leeks and shallots" needs 4 to 8 weeks growing indoors. Find your last frost date with the frost date tool.
What the source says
| Onions, leeks, shallots | |
|---|---|
| Ideal soil temperature for germination | 65 to 70 °F |
| Days to germination | 10 to 14 |
| Weeks to grow indoors | 4 to 8 |
| Move outdoors | 2 to 5 weeks before the average last frost |
The math
- Move-out window = last frost − 5 weeks to last frost − 2 weeks.
- Earliest start = earliest move-out date − 8 weeks.
- Latest start = latest move-out date − 4 weeks.
The range of indoor weeks is wide (4 to 8), so the start window is wide too. If you want one date, pick the day you want to move seedlings out, then subtract the weeks you give them.
Three stations, same rules
All dates use NOAA's 50% last freeze at 32 °F and a 2027 calendar.
| Weather station | Average last freeze (50%) | Move onions outdoors | Start seeds indoors |
|---|---|---|---|
| Dallas-Fort Worth AP, Texas | Mar 8 | Feb 1 to Feb 22 | Dec 7 (2026) to Jan 25 |
| Port Columbus Intl AP, Ohio | Apr 18 | Mar 14 to Apr 4 | Jan 17 to Mar 7 |
| Denver Intl AP, CO | May 5 | Mar 31 to Apr 21 | Feb 3 to Mar 24 |
Check one row: Columbus, April 18 − 5 weeks = March 14, − 2 weeks = April 4. March 14 − 8 weeks = January 17. April 4 − 4 weeks = March 7.
Hardening off
Indoor seedlings are soft. The guide says to harden them off by moving pots outdoors for progressively longer periods each day, which usually takes at least a week.
Sources
- University of Idaho Extension, Start Seeds Indoors with Success (CIS 1209).
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, stations named in the table.