When to plant onions in Arizona
Arizona’s average last spring frost is March 15, and the first fall frost typically arrives around November 15 — about 245 frost-free days (USDA zones 4b, 5b, 6b, 7b, 8b, 9b, 10b). Onion needs 90–120 days to mature, which fits comfortably within Arizona's growing season.
Arizona covers a wide range of climates, so your exact frost dates may differ from the state average by two weeks or more. For dates tuned to your town, use the calendar generator.
When to plant onions in Arizona: the short answer
For a spring planting, plant out Feb 15 – Mar 4, with seeds started indoors Dec 28 – Jan 11. Seeds germinate in soil from 40 to 90°F.
Windows are ~1–2 week bands around each date. Frost dates are a statewide average from NOAA/NCEI normals; your town may differ by two weeks or more. Calendar year 2026.
How to grow onions in Arizona, step by step
- Start seeds indoors (Dec 28 – Jan 11). Sow seeds ½ inch deep in seed-starting mix under bright light. They take 7-14 days to sprout, fastest in soil around 70°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. They need at least six to eight hours of direct sun a day and loose soil at least 12 inches deep, with a pH between 6.0 and 7.0. Dig in compost before planting.
- Plant out (Feb 15 – Mar 4). Space plants about 4 inches apart in rows 12 inches apart. It handles frost well, so it can go out before the last frost date. As a cool-season crop it does best before summer heat arrives in Arizona.
- Water and feed. Give it steady, even moisture, about an inch of water a week from rain or the hose.
- Watch for pests and problems. Pests: onion maggot, onion thrips. Diseases and disorders: downy mildew, white rot, neck rot, purple blotch.
- Harvest (from about May 23 – Jun 22). Onion takes 90-120 days to reach harvest. Match the day-length type to your latitude: long-day onions for the north, short-day for the south, day-neutral anywhere. Bulbing is triggered by day length, not calendar date.
Figures come from our crop data; dates use Arizona’s average frost dates. More in the full onion growing guide.
Onion planting dates by USDA zone in Arizona
Arizona spans USDA zones 4b, 5b, 6b, 7b, 8b, 9b, 10b. Colder zones, usually higher or further inland, have a later last spring frost and an earlier first fall frost than the state average, so the onion dates shift with them.
| Zone | Last frost | First frost | Season | Start indoors | Plant onion |
|---|---|---|---|---|---|
| Zone 4 | May 10 | Sep 25 | 135 days | Feb 22 – Mar 8 | Apr 12 – Apr 29 |
| Zone 5 | Apr 30 | Oct 5 | 160 days | Feb 12 – Feb 26 | Apr 2 – Apr 19 |
| Zone 6 | Apr 15 | Oct 15 | 185 days | Jan 28 – Feb 11 | Mar 18 – Apr 4 |
| Zone 7 | Apr 5 | Oct 25 | 200 days | Jan 18 – Feb 1 | Mar 8 – Mar 25 |
| Zone 8 | Mar 20 | Nov 5 | 230 days | Jan 2 – Jan 16 | Feb 20 – Mar 9 |
| Zone 9 | Feb 20 | Nov 30 | 280 days | Dec 5 – Dec 19 | Jan 23 – Feb 9 |
| Zone 10 | Jan 30 | Dec 15 | 330 days | Nov 14 – Nov 28 | Jan 2 – Jan 19 |
When to plant onions in nearby states
| State | Last frost | Plant out |
|---|---|---|
| Arizona (this page) | March 15 | Feb 15 – Mar 4 |
| New Mexico | April 20 | Mar 23 – Apr 9 |
Questions gardeners in Arizona ask
When is the best time to plant onions in Arizona?
Plant onion transplants outdoors in Arizona between February 15 and March 4. That timing is set by Arizona's average last spring frost of March 15; Arizona spans several climates, so cooler and higher parts of the state run later.
When should I start onion seeds indoors in Arizona?
Start seeds indoors between December 28 and January 11, about 10 weeks before the last frost, so the seedlings are ready to plant out from February 15.
When will onions planted in Arizona be ready to harvest?
Planted on time, onions are usually ready from about May 23 to June 22, 90-120 days after transplanting.
When is the last frost in Arizona?
On average around March 15, with the first fall frost around November 15 - about 245 frost-free days. Your town may differ by two weeks or more.