When to Plant Onions in Arizona
Published: April 21, 2026
Arizona gardeners in Zone 9b enjoy an extended 50-week growing season for onion. Cool-season crops like onion do best in fall, winter, and early spring in Arizona — the hot summers bolt these plants before they can be harvested. This guide gives exact dates for Arizona based on your local frost calendar.
Onion Planting Dates for Arizona
| Start seeds indoors | November 6–13 |
| Last frost (average) | January 15 |
| Transplant outdoors | December 18–28 |
| Direct sow outdoors | December 18–28 |
| Expect first harvest | March 18 – April 17 |
| First fall frost (average) | December 31 |
Dates above assume Zone 9b, the most common zone in Arizona. Gardeners in cooler San Tan Valley (Zone 7b) plant 1–2 weeks later; warmer Tucson (Zone 9a) can plant 1–2 weeks earlier.
⚠ CRITICAL: Choose variety based on your latitude. Short-day varieties for south of 35°N (Texas, Florida, California). Long-day varieties for north of 35°N (Ohio, Minnesota, New York). Intermediate varieties work in the middle band.
Growing Onions in Arizona's Climate
Arizona's warm Zone 9b climate makes onion a fall, winter, and early spring crop rather than a summer one. The cool-season window runs roughly October through April in Arizona — plant outside this window and the onion will bolt (send up a flower stalk and turn bitter) before it produces a harvestable crop.
Succession planting stretches the onion harvest window. Sow small amounts every two weeks from October through February — you will have staggered harvests rather than a single wave to process all at once. Bolt-resistant varieties extend the usable window an extra three to four weeks into warming weather.
Common onion pests to watch for in Arizona include Onion Thrips and Onion Maggot. The first line of defense is companion planting: Carrot and Tomato planted nearby discourage several of these pests by confusing host-plant identification or repelling adults before they can lay eggs. Weekly garden walks in the early morning catch problems when they're still manageable.
Onion Garden Calendar for Arizona
| Month | Task | Notes |
|---|---|---|
| January | Harden off seedlings | Set seedlings outdoors 1–2 hours daily, increasing gradually |
| February | — | Off-season |
| March | First harvest window opens | Onion begins producing 90–120 days after transplant |
| April | — | Off-season |
| May ← | Watch for bolting | Harvest promptly; spring heat arrives quickly |
| June | — | Off-season |
| July | — | Off-season |
| August | — | Off-season |
| September | — | Off-season |
| October | — | Off-season |
| November | Start seeds indoors | Use grow lights; warm soil to 70–75°F for best germination |
| December | Transplant outdoors | Soil temp must be 35°F+; nighttime lows above 50°F |
Onion Growing Tips for Arizona Gardeners
- •Match variety to latitude: short-day south of 35°N, long-day north of 35°N, intermediate in between.
- •Growing from sets (small bulbs) is fastest; growing from transplants gives the most variety options.
- •Arizona's cool-season onion plants best in October, November, and December for winter and early spring harvest. Spring plantings bolt before producing a harvestable crop.
- •Reduce watering as tops begin to fall over — dry conditions encourage tight, firm bulbs that store longer.
Companion Planting for Onions in Arizona
In Arizona's warm climate, planting onion with Carrot and Tomato helps deter pests and improve soil. Avoid planting near Bean and Pea, which inhibit onion growth through root chemistry or shared pest pressure. See the full onion companion planting guide for the complete list.
Onions Planting Dates by City in Arizona
Top cities in Arizona — select for city-level frost dates and planting calendars.
Frequently Asked Questions
When should I plant onion in Arizona?
In Arizona (Zone 9b), start onion seeds indoors around November 6–13 and transplant outdoors around December 18–28 — 4 weeks before the state's average last frost of January 15.
What is the last frost date in Arizona?
Arizona's average last frost date is January 15 in the state's most common Zone 9b. Frost dates vary by city — higher elevations and northern counties run 1 to 3 weeks later than the state average. See the city list below for city-level frost dates.
Can I grow onion year-round in Arizona?
onion is a cool-season crop in Arizona, with spring and fall windows. Summer heat bolts the plants before they can produce a harvestable crop. Succession planting every 2–3 weeks stretches the harvest window.
What onion varieties grow best in Arizona?
For Arizona's Zone 9b, Short-day varieties like Texas 1015, Granex, and Yellow Bermuda bulb under the 10–12 hour days south of 35°N.
When do I start onion seeds indoors in Arizona?
Start onion seeds indoors in Arizona around November 6–13, which is 10 weeks before the state's average last frost of January 15. Use grow lights and keep soil temperatures at 70–75°F for fastest germination.