Arizona · zones 5b–10a
When to plant onions in Arizona.
Sow or transplant between Nov 28 to Jan 11 in a typical year, keyed to Arizona's median last frost around Feb 5. A second, fall sowing fits Sep 20 to Nov 1. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Arizona season.
1 · Start indoors
Oct 17 to Nov 30
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Nov 28 to Jan 11
Keyed to the ~Feb 5 last frost. Sow 0.5" deep, 4" apart.
Fall round
Sep 20 to Nov 1
Counted back from the ~Nov 15 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Arizona spans more than one climate: Zone 9 is the most common zone, but the state runs 5b to 10a. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
In Arizona: watch for
The pests that find onions here.
With about 283 frost-free days, onion, bulb spends longer in the ground here than in most of the country, which widens the window in which onion thrips can find it. Scout weekly from midseason and act on the first signs.
Silvery streaks and curled leaf tips in hot spells.
Organic fix: Insecticidal soap or neem; hang sticky traps.
Seedlings wilt; white maggots in the bulb base.
Organic fix: Float a row cover over the row; rotate beds each year.
Pale patches with fuzzy gray growth in damp weather.
Organic fix: Space for airflow, water early in the day, spray neem.
White rot
Yellowing and dieback, then wilt; fluffy white mold at the bulb base and roots so rotted the plant lifts straight out.
Organic fix: No cure once it is in the soil. Plant clean stock, never follow onions with onions, and keep infested soil off tools and boots.
Your season math: Arizona gives onions about 283 frost-free days, and it needs roughly 95 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting onions in Arizona: questions
When should I plant onions in Arizona?+
In a typical Arizona year, sow onions or set out transplants between Nov 28 to Jan 11, keyed to the state's median last frost around Feb 5. There's also a fall window: Sep 20 to Nov 1. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Arizona?+
Start seeds inside about 6 weeks before set-out: roughly Oct 17 to Nov 30 for a typical Arizona season.
What USDA zone is Arizona?+
Arizona spans zones 5b–10a, mostly zone 9. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many onions plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.