Zone 9b covers 17 days of last-frost dates. We measured Las Vegas, NV at Feb 6 and Sacramento, CA at Feb 23. 10 of the 15 places we checked in this zone don't dependably freeze at all. Give us your town or ZIP and we'll use ten years of daily lows recorded at your own coordinates instead of the middle of that range.
Why these dates work in zone 9b
Zone 9b's last spring frost averages Feb 9 and first fall frost Dec 8 — about 302 frost-free days. Onion tolerates light frost, so you can sow ahead of the last-frost date and keep harvesting into the cooler fall. Starting seeds indoors 10 weeks ahead gives strong transplants ready the moment the soil warms.
Common questions
How long does onion take to grow in zone 9b?
Onion matures in about 90–110 days from sowing. In zone 9b that means sowing around Jan 26 and first picking around Apr 26.
Will onion survive frost in zone 9b?
Yes — onion tolerates light frost. In zone 9b you can sow a little before the last frost (around Feb 9) and keep harvesting past the first fall frost (around Dec 8).
For soil, spacing, pests and companion planting, see the full onion growing guide →