Zone 5a covers 27 days of last-frost dates. We measured Sioux City, IA at Apr 20 and Bozeman, MT at May 17. 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 5a
Zone 5a's last spring frost averages Apr 30 and first fall frost Oct 20 — about 173 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 5a?
Onion matures in about 90–110 days from sowing. In zone 5a that means sowing around Apr 16 and first picking around Jul 15.
Will onion survive frost in zone 5a?
Yes — onion tolerates light frost. In zone 5a you can sow a little before the last frost (around Apr 30) and keep harvesting past the first fall frost (around Oct 20).
For soil, spacing, pests and companion planting, see the full onion growing guide →