Onion in Zone 10a & 10b
Allium cepa
When to plant onion in zone 10b
Zones 10a and 10b share the same average frost dates in our data, so this page covers both.
Method: Start indoors: Start indoors from seed, then transplant outside. Onion needs a head start most zones can't give it by direct-sowing alone.
| Window | Fall window |
|---|---|
| Start indoors | Aug–Nov |
| Direct sow / transplant outside | Oct–Jan |
| Estimated harvest | Jan 9 '27–May 11 '27 (~100 days after transplanting) |
Spacing: 16 per square foot (square-foot-gardening spacing). Mature height: 1-1.5 ft.
Zone 10b dates sourced from growinginthegarden.com.
Onions are one of the slower crops in the garden, around 100 days from transplant to harvest, and they start life indoors well before most of the garden is even being planned, which catches a lot of first-time growers off guard. There's no shortcut here: get the timing right up front and the rest of the crop mostly takes care of itself.
How onion fits zone 10's season
Zone 10b's frost-free window runs about 324 days, from January 30 to December 20. Onion needs about 100 days from transplant to first harvest.
Good companions
- Carrot: A real, peer-reviewed field study (Uvah & Coaker 1984) found intercropping onions with carrots reduced carrot rust fly damage, on the theory that mixed scents mask the host plant from pests hunting by smell.
- Tomato: Onion scent can deter tomato pests.
Where to buy onion seeds
See why we recommend these suppliers →
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See it growing
Search YouTube for "growing onion" videos → (a live search, not a hand-picked video; results vary and aren't vetted by us)
← Back to the full zone 10b calendar · Full companion planting guide · Full plant profile · Plan your beds in the Garden Designer →
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Growing Onion in USDA zone 10b: when to start seeds indoors, when to direct-sow, and when to expect a harvest.
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