Zone 5b covers 25 days of last-frost dates. We measured Davenport, IA at Apr 16 and Idaho Falls, ID at May 11. 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 5b
Zone 5b's last spring frost averages Apr 28 and first fall frost Oct 22 — about 177 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 5b?
Onion matures in about 90–110 days from sowing. In zone 5b that means sowing around Apr 14 and first picking around Jul 13.
Will onion survive frost in zone 5b?
Yes — onion tolerates light frost. In zone 5b you can sow a little before the last frost (around Apr 28) and keep harvesting past the first fall frost (around Oct 22).
For soil, spacing, pests and companion planting, see the full onion growing guide →