Last spring frostMay 6
Start seeds indoorsFeb 25
Direct sowApr 22
First harvestJul 21
Last harvestAug 18
First fall frostOct 22
Why these dates work in zone 3b
Zone 3b's last spring frost averages May 6 and first fall frost Oct 22 — 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 3b?
Onion matures in about 90–110 days from sowing. In zone 3b that means sowing around Apr 22 and first picking around Jul 21.
Will onion survive frost in zone 3b?
Yes — onion tolerates light frost. In zone 3b you can sow a little before the last frost (around May 6) and keep harvesting past the first fall frost (around Oct 22).
For soil, spacing, pests and companion planting, see the full onion growing guide →