Last spring frostMay 7
Start seeds indoorsFeb 26
Direct sowApr 23
First harvestJul 22
Last harvestAug 19
First fall frostOct 13
Why these dates work in zone 4a
Zone 4a's last spring frost averages May 7 and first fall frost Oct 13 — about 159 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 4a?
Onion matures in about 90–110 days from sowing. In zone 4a that means sowing around Apr 23 and first picking around Jul 22.
Will onion survive frost in zone 4a?
Yes — onion tolerates light frost. In zone 4a you can sow a little before the last frost (around May 7) and keep harvesting past the first fall frost (around Oct 13).
For soil, spacing, pests and companion planting, see the full onion growing guide →