When to plant rosemary 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 Mar 6 · Transplant May 15 · First harvest Aug 3 · Wrap by Sep 18

Last spring frostMay 15

Start seeds indoorsMar 6

Transplant outMay 15

First harvestAug 3

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. Rosemary 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 10 weeks ahead gives strong transplants ready the moment the soil warms.

Common questions

How long does rosemary take to grow in zone 4b?

Rosemary matures in about 80–100 days from transplanting. In zone 4b that means setting plants out around May 15 and first picking around Aug 3.

Will rosemary survive frost in zone 4b?

No — rosemary 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 rosemary growing guide →