When to plant onion in zone 5b — planting calendar

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Showing USDA Zone 5B

Growing SeasonZone 5b

Last frost ~May 15First frost ~Sep 30Today

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 30

Why these dates work in zone 5b

Zone 5b's last spring frost averages May 15 and first fall frost Sep 30 — about 138 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 5b?

Onion matures in about 90–110 days from sowing. In zone 5b that means sowing around May 1 and first picking around Jul 30.

Will onion survive frost in zone 5b?

Yes — onion tolerates light frost. In zone 5b you can sow a little before the last frost (around May 15) and keep harvesting past the first fall frost (around Sep 30).

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