When to plant zinnia in zone 4b — planting calendar

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Showing USDA Zone 4B

Growing SeasonZone 4b

Last frost ~May 15First frost ~Sep 25Today

Start indoorsTransplant / sowHarvestFrost risk

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Start indoors Apr 17 · Sow May 15 · First harvest Jul 4 · Wrap by Sep 18

Last spring frostMay 15

Start seeds indoorsApr 17

Direct sowMay 15

First harvestJul 4

Last harvestSep 18

First fall frostSep 25

Why these dates work in zone 4b

Zone 4b's last spring frost averages May 15 and first fall frost Sep 25 — about 133 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 4b?

Zinnia matures in about 50–70 days from sowing. In zone 4b that means sowing around May 15 and first picking around Jul 4.

Will zinnia survive frost in zone 4b?

No — zinnia is frost-sensitive. In zone 4b, wait until after the last frost (around May 15) to plant, and harvest before the first fall frost (around Sep 25).

For soil, spacing, pests and companion planting, see the full zinnia growing guide →