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🧅 Growing Onion in Zone 10a

Zone 10a has a year-round growing season. Onion can be planted almost any time with proper planning. Last frost: Feb 20. First frost: Dec 1.

Planting Calendar

Indoor

Outdoor

Harvest

Dec 12Dec 1

Season: Feb 20 – Dec 1 (284 days)

Zone10a

Last FrostFeb 20

First FrostDec 1

Season Length284 days

Days to Harvest110

SunFull Sun

Frost ToleranceModerate

Spacing9/sqft (4" apart)

When to Plant Onion in Zone 10a

Start Seeds Indoors: Dec 12

Start Onion seeds indoors around Dec 12 (10 weeks before last frost). Optimal germination temperature: 70°F. Germinates in 7–14 days.

Transplant Outdoors: Jan 23

Transplant Onion seedlings outdoors after Jan 23. Wait until soil temperature reaches 45°F. Space plants at 9/sqft (4" apart). Harden off plants for a week before moving outside.

Harvest: Apr 13

First harvest approximately Apr 13 (~80 days from transplant). Continue harvesting until first frost (Dec 1).

Companion Planting

✅ Strawberries✅ Cabbage✅ Beets✅ Lettuce✅ Carrots✅ Tomatoes

❌ Beans❌ Peas

Onion in Other Zones

1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b 8a 8b 9a 9b 10a 10b 11a 11b 12a 12b 13a 13b

Frequently Asked Questions

When should I plant Onion in Zone 10a?

Start Onion seeds indoors around Dec 12. Transplant outdoors after Jan 23. Expect first harvest around Apr 13.

What is the last frost date for Zone 10a?

The average last spring frost date for Zone 10a is Feb 20. The first fall frost averages Dec 1, giving a 284-day growing season.

Can I grow Onion in Zone 10a?

Yes. Onion needs 110 days to harvest. Zone 10a has a 284-day growing season, which is sufficient.

What is the best month to plant Onion in Zone 10a?

The best time to start Onion in Zone 10a is Dec. Begin seeds indoors around Dec 12, then transplant outdoors after the last frost date of Feb 20.

What companion plants grow well with Onion?

Good companion plants for Onion include Strawberries, Cabbage, Beets, Lettuce, Carrots, Tomatoes. Avoid planting Onion near Beans, Peas.

How long does Onion take to harvest?

Onion takes approximately 110 days from planting to harvest. In Zone 10a, expect your first harvest around Apr 13.

Plan Your Onion Garden

Track Onion from seed to harvest with personalized frost dates, watering recommendations, and companion planting visualization.

Start Free Onion Growing Guide