Onion in Zone 10a & 10b
Allium cepa
Yes, onion grows in zones 10a and 10b. Start seeds indoors in Aug–Nov and plant out in Oct–Jan. For a fall crop, start seeds indoors in Aug–Nov and plant out in Oct–Jan.
When to plant onion in zone 10b
Zones 10a and 10b share the same average frost dates, so this page covers both.
Method: Start indoors: Start indoors from seed, then transplant outside. It 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 | ~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.
Fall window: the last sowing date is the average first frost minus days to maturity, the method in Purdue Extension's fall garden guide, which lists onion for late-summer planting.
Plant alongside it in zone 10b
These go outside within two weeks of onion in zone 10b, so one planting day can cover them.
- GarlicOct–Nov
- ArtichokeOct–Feb
- NasturtiumOct–Jan
- DillOct–Jan
- Black-Eyed SusanOct–Mar
- Chamomile (German)Oct–Mar
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 goes in during the fall and grows through the winter, so it isn't limited by this window: expect first harvest about 100 days after planting.
Onion planting windows in every zone
Zone 10b is highlighted. Useful for comparing against a neighbouring zone, or for a garden somewhere else.
| Zone | Start indoors | Sow or plant out | Harvest |
|---|---|---|---|
| 3a & 3b | Apr 4–Apr 11 | May 9–May 16 | Aug 17–Aug 24 |
| 4a & 4b | Mar 20–Mar 27 | Apr 24–May 1 | Aug 2–Aug 9 |
| 5a & 5b | Mar 15–Mar 22 | Apr 19–Apr 26 | Jul 28–Aug 4 |
| 6a & 6b | Feb 28–Mar 7 | Apr 4–Apr 11 | Jul 13–Jul 20 |
| 7a & 7b | Feb 13–Feb 20 | Mar 20–Mar 27 | Jun 28–Jul 5 |
| 8a & 8b | Jan 28–Feb 4 | Mar 4–Mar 11 | Jun 12–Jun 19 |
| 9a & 9b | Dec 21–Dec 28 | Jan 25–Feb 1 | May 5–May 12 |
| 10a & 10bFall window (your zone) | Aug–Nov | Oct–Jan | ~100 days after transplanting |
Harvest dates are about 100 days after the sow or plant-out date, not after starting seeds indoors.
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
Botanical Interests · True Leaf Market · Burpee · High Mowing Organic Seeds (each supplier's onion seeds)
Varieties we recommend, and where to get them:
- Ailsa Craig Exhibition: Also highly rated in Cornell's database at 4.5 stars, a large, slightly oval long-day type prized for sweetness over long storage life. Buy: High Mowing Organic Seeds Full notes
- 'Walla Walla Sweet' (best for fresh eating, not storage): Johnny's lists this long-day Northwest classic as ultra-mild and juicy, best fall-planted where winters stay above -10°F, but explicitly not a storage onion the way Copra is. Buy: True Leaf Market · High Mowing Organic Seeds Full notes
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)
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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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