Zone 4b covers 52 days of last-frost dates. We measured Pierre, SD at Apr 28 and Jackson, WY at Jun 19. 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 4b
Zone 4b's last spring frost averages May 4 and first fall frost Oct 20 — about 169 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 4b?
Onion matures in about 90–110 days from sowing. In zone 4b that means sowing around Apr 20 and first picking around Jul 19.
Will onion survive frost in zone 4b?
Yes — onion tolerates light frost. In zone 4b you can sow a little before the last frost (around May 4) and keep harvesting past the first fall frost (around Oct 20).
For soil, spacing, pests and companion planting, see the full onion growing guide →