Zone 6b covers 17 days of last-frost dates. We measured Springfield, MO at Apr 6 and Detroit, MI at Apr 23. 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 6b
Zone 6b's last spring frost averages Apr 19 and first fall frost Nov 6 — about 201 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 6b?
Onion matures in about 90–110 days from sowing. In zone 6b that means sowing around Apr 5 and first picking around Jul 4.
Will onion survive frost in zone 6b?
Yes — onion tolerates light frost. In zone 6b you can sow a little before the last frost (around Apr 19) and keep harvesting past the first fall frost (around Nov 6).
For soil, spacing, pests and companion planting, see the full onion growing guide →