When to plant Onion
A hardy biennial grown as an annual for its bulb. Bulbing is triggered by day length, so the variety must match your latitude: long-day in the north, short-day in the south, day-neutral anywhere. Stop watering once tops flop to cure for storage.
Companions: Carrot, beets, lettuce, cabbage — avoid beans and peas
How to grow Onion
Onion is usually started indoors and transplanted out, giving this half-hardy crop the head start it needs. Give it full sun and space plants about 4 inches apart. It takes roughly 100 days from planting to its first harvest, so count that against your frost dates to be sure it can finish. Classic companions include carrot, beets, lettuce, cabbage — avoid beans and peas.
Grower's tip: Day-length sensitive — match the variety (long/short day) to your latitude for big bulbs.
When to plant Onion by USDA zone
Timing shifts with your climate — the same onion goes in weeks earlier in a mild zone than a cold one. Find your zone below for its planting window, or open that zone's full calendar.
| Zone | When to plant |
|---|---|
| Zone 1a | Start seeds indoors Apr 6–Apr 20 Transplant outdoors May 18–Jun 1 |
| Zone 1b | Start seeds indoors Apr 1–Apr 15 Transplant outdoors May 13–May 27 |
| Zone 2a | Start seeds indoors Mar 27–Apr 10 Transplant outdoors May 8–May 22 |
| Zone 2b | Start seeds indoors Mar 21–Apr 4 Transplant outdoors May 2–May 16 |
| Zone 3a | Start seeds indoors Mar 16–Mar 30 Transplant outdoors Apr 27–May 11 |
| Zone 3b | Start seeds indoors Mar 11–Mar 25 Transplant outdoors Apr 22–May 6 |
| Zone 4a | Start seeds indoors Mar 6–Mar 20 Transplant outdoors Apr 17–May 1 |
| Zone 4b | Start seeds indoors Mar 1–Mar 15 Transplant outdoors Apr 12–Apr 26 |
| Zone 5a | Start seeds indoors Feb 24–Mar 10 Transplant outdoors Apr 7–Apr 21 |
| Zone 5b | Start seeds indoors Feb 19–Mar 5 Transplant outdoors Apr 2–Apr 16 |
| Zone 6a | Start seeds indoors Feb 10–Feb 24 Transplant outdoors Mar 24–Apr 7 |
| Zone 6b | Start seeds indoors Feb 4–Feb 18 Transplant outdoors Mar 18–Apr 1 |
| Zone 7a | Start seeds indoors Jan 27–Feb 10 Transplant outdoors Mar 10–Mar 24 |
| Zone 7b | Start seeds indoors Jan 21–Feb 4 Transplant outdoors Mar 4–Mar 18 |
| Zone 8a | Start seeds indoors Jan 11–Jan 25 Transplant outdoors Feb 22–Mar 8 |
| Zone 8b | Start seeds indoors Jan 4–Jan 18 Transplant outdoors Feb 15–Mar 1 |
| Zone 9a | Start seeds indoors Dec 20–Jan 3 Transplant outdoors Jan 31–Feb 14 |
| Zone 9b | Start seeds indoors Dec 7–Dec 21 Transplant outdoors Jan 18–Feb 1 |
| Zone 10a | Start seeds indoors Nov 22–Dec 6 Transplant outdoors Jan 3–Jan 17 |
| Zone 10b | Start seeds indoors Nov 6–Nov 20 Transplant outdoors Dec 18–Jan 1 |
| Zone 11a | Start seeds indoors Oct 23–Nov 6 Transplant outdoors Dec 4–Dec 18 |
| Zone 11b | Start seeds indoors Oct 23–Nov 6 Transplant outdoors Dec 4–Dec 18 |
| Zone 12a | Start seeds indoors Oct 23–Nov 6 Transplant outdoors Dec 4–Dec 18 |
| Zone 12b | Start seeds indoors Oct 23–Nov 6 Transplant outdoors Dec 4–Dec 18 |
| Zone 13a | Start seeds indoors Oct 23–Nov 6 Transplant outdoors Dec 4–Dec 18 |
| Zone 13b | Start seeds indoors Oct 23–Nov 6 Transplant outdoors Dec 4–Dec 18 |