When to Plant Onions in Arizona
The backbone of the kitchen garden. Choose short-day, intermediate, or long-day varieties based on your latitude.
The Short Answer
Arizona Frost Dates
Your planting dates depend on which part of Arizona you're in. Here are the frost date ranges by region:
| Region | Zones | Last Frost (Spring) | First Frost (Fall) |
|---|---|---|---|
| Northern Arizona (Flagstaff) | 4b, 5a, 5b, 6a | May 15 - Jun 5 | Sep 15 - Oct 10 |
| Central Arizona (Phoenix) | 9a, 9b, 10a | Jan 15 - Feb 10 | Dec 1 - Dec 20 |
| Southern Arizona (Tucson) | 8b, 9a, 9b | Feb 1 - Feb 25 | Nov 20 - Dec 15 |
Onions Planting Schedule for Arizona
Northern Arizona (Flagstaff) (Zones 4b, 5a, 5b, 6a)
Average last frost: May 15 - Jun 5 · Average first frost: Sep 15 - Oct 10
Central Arizona (Phoenix) (Zones 9a, 9b, 10a)
Average last frost: Jan 15 - Feb 10 · Average first frost: Dec 1 - Dec 20
Southern Arizona (Tucson) (Zones 8b, 9a, 9b)
Average last frost: Feb 1 - Feb 25 · Average first frost: Nov 20 - Dec 15
Growing Onions in Arizona
Onions in Arizona's Climate
Short-day onions only. Plant transplants from October through December for spring harvest. Short-day varieties like Granex (Vidalia type) and Texas 1015 bulb when days reach just 10-12 hours — perfectly timed for your spring. Do not plant long-day or intermediate varieties — they will never bulb in your day-length pattern.
Soil Considerations for Arizona
Alkaline, mineral-rich desert soils. Low organic matter. Heavy amendment with compost essential. Caliche layers may need to be broken. Loose, well-drained soil is especially important for onions since the edible portion grows underground. If your soil is heavy clay, consider raised beds.
Arizona Climate & Growing Season
Extreme heat in summer lowlands. Two planting seasons in Phoenix — spring and fall (summer is too hot). High elevation has traditional cold-climate seasons. Onions can handle frost well, which is an advantage in Arizona's climate. You can push planting dates earlier in spring and extend into fall.
Growing season length varies across Arizona: Northern Arizona (Flagstaff) (4b, 5a, 5b, 6a) has a last frost around May 15 - Jun 5, while Southern Arizona (Tucson) (8b, 9a, 9b) sees frost end around Feb 1 - Feb 25. This difference matters for onions — but onions handle frost well, so the timing difference is less critical.
Growing Tips
Day length triggers bulbing. Northern gardeners need long-day varieties. Southern gardeners need short-day varieties.
Companion Planting
Plant onions alongside these companions for better growth:
Keep onions away from:
The Bottom Line
Last reviewed: March 29, 2026