Last spring frostMay 6
Start seeds indoorsApr 8
Direct sowMay 6
First harvestJun 25
Last harvestSep 17
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. Zinnia is frost-sensitive, so the around last frost timing builds in a safety margin against a late cold snap — setting plants out too early risks losing them to frost. Starting seeds indoors 4 weeks ahead gives strong transplants ready the moment the soil warms.
Common questions
How long does zinnia take to grow in zone 3b?
Zinnia matures in about 50–70 days from sowing. In zone 3b that means sowing around May 6 and first picking around Jun 25.
Will zinnia survive frost in zone 3b?
No — zinnia is frost-sensitive. In zone 3b, wait until after the last frost (around May 6) to plant, and harvest before the first fall frost (around Oct 22).
For soil, spacing, pests and companion planting, see the full zinnia growing guide →