Plant Onions in Zone 9b
Seed-starting, transplant & harvest dates for USDA Zone 9b
Zone 9b averages 298 frost-free days, running from a last spring frost around February 15 to a first fall frost around December 10. Onions needs roughly 100 days from planting out to first harvest, so it uses about 34 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 Dec 7, about 10 weeks ahead of the Zone 9b last frost. Transplants go out around Feb 1. Starting earlier than that in Zone 9b usually backfires: plants outgrow their cells and stall when they finally hit cold soil. At 100 days to harvest, a 298-day Zone 9b season has room for about 2 sowings, so stagger them rather than planting everything at once.
Planting onions for a fall harvest in Zone 9b
Yes. Plant onions between October 29 and November 19 in Zone 9b, 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 9b 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 9b
| Planting | Sow or start | In the ground | Harvest |
|---|---|---|---|
| Spring | Dec 7 | Feb 1 | May 12 |
| Fall | by Nov 19 | Nov 19 | 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 9b
Frequently asked questions
When should I start onions in Zone 9b?
Start onions seeds indoors around Dec 7 (Zone 9b).
Can I direct sow onions in Zone 9b?
Onions is best started indoors and transplanted in Zone 9b rather than direct sown.
Can I plant onions in the fall in Zone 9b?
Yes. Plant onions between October 29 and November 19 in Zone 9b, roughly 6 to 3 weeks before the average first frost, so it roots before the ground freezes.
Will onions survive frost in Zone 9b?
Onions is a hardy crop and keeps going roughly 21 days past the first light frost in Zone 9b. A row cover or a straw mulch buys more.
How many plantings of onions fit in a Zone 9b season?
About 2. Zone 9b averages 298 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.