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. Bok Choy tolerates light frost, so you can set plants out ahead of the last-frost date and keep harvesting into the cooler fall. Starting seeds indoors 4 weeks ahead gives strong transplants ready the moment the soil warms. There's also room for a fall succession sowing around Aug 25 for a second harvest before frost.
Common questions
How long does bok choy take to grow in zone 4b?
Bok Choy matures in about 45–60 days from transplanting. In zone 4b that means setting plants out around Apr 13 and first picking around May 28.
Will bok choy survive frost in zone 4b?
Yes — bok choy tolerates light frost. In zone 4b you can set plants out a little before the last frost (around May 4) and keep harvesting past the first fall frost (around Oct 20).
Can I plant a second crop of bok choy in zone 4b?
Yes — in zone 4b there's time for a fall sowing around Aug 25 for a second harvest before frost.
For soil, spacing, pests and companion planting, see the full bok choy growing guide →