When to plant onion 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 · Sow May 1 · First harvest Jul 30 · Wrap by Aug 27

Last spring frostMay 15

Start seeds indoorsMar 6

Direct sowMay 1

First harvestJul 30

Last harvestAug 27

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. 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 4b?

Onion matures in about 90–110 days from sowing. In zone 4b that means sowing around May 1 and first picking around Jul 30.

Will onion survive frost in zone 4b?

Yes — onion tolerates light frost. In zone 4b you can sow a little before the last frost (around May 15) and keep harvesting past the first fall frost (around Sep 25).

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