Plant Onions in Zone 10a
Seed-starting, transplant & harvest dates for USDA Zone 10a
Zone 10a averages 323 frost-free days, running from a last spring frost around January 31 to a first fall frost around December 20. Onions needs roughly 100 days from planting out to first harvest, so it uses about 31 percent of that window. Onions is a hardy crop, which is what decides where in the window it belongs.
Because onions needs a head start, sow seed indoors around Nov 22, about 10 weeks ahead of the Zone 10a last frost. Transplants go out around Jan 17. Starting earlier than that in Zone 10a usually backfires: plants outgrow their cells and stall when they finally hit cold soil. At 100 days to harvest, a 323-day Zone 10a season has room for about 2 sowings, so stagger them rather than planting everything at once.
Planting onions for a fall harvest in Zone 10a
Yes. Plant onions between November 8 and November 29 in Zone 10a, roughly 6 to 3 weeks before the average first frost, so it roots before the ground freezes.
Short-day onion sets planted in fall overwinter in zone 7 and warmer and bulb up months ahead of a spring planting.
Fall dates are counted backward from the Zone 10a average first frost, not forward from a sowing date. See the fall planting calendar by zone for the full method.
Spring vs fall onions in Zone 10a
| Planting | Sow or start | In the ground | Harvest |
|---|---|---|---|
| Spring | Nov 22 | Jan 17 | Apr 27 |
| Fall | by Nov 29 | Nov 29 | Next spring |
Onions in nearby zones
Further reading on onions
- When to plant garlic by zone
The fall planting window for garlic and other alliums in every zone.
- When to start seeds indoors by zone
Indoor start dates for onions, counted back from your last frost.
- Hardening off seedlings: a schedule
A day by day plan for moving onions seedlings outdoors.
Other crops in Zone 10a
Frequently asked questions
When should I start onions in Zone 10a?
Start onions seeds indoors around Nov 22 (Zone 10a).
Can I direct sow onions in Zone 10a?
Onions is best started indoors and transplanted in Zone 10a rather than direct sown.
Can I plant onions in the fall in Zone 10a?
Yes. Plant onions between November 8 and November 29 in Zone 10a, roughly 6 to 3 weeks before the average first frost, so it roots before the ground freezes.
Will onions survive frost in Zone 10a?
Onions is a hardy crop and keeps going roughly 21 days past the first light frost in Zone 10a. A row cover or a straw mulch buys more.
How many plantings of onions fit in a Zone 10a season?
About 2. Zone 10a averages 323 frost-free days and onions needs roughly 100 days plus a couple of weeks of turnaround, so sowing every 57 days keeps a supply coming.