When to plant onion in zone 3b — planting calendar

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Showing USDA Zone 3B

Growing SeasonZone 3b

Last frost ~May 25First frost ~Sep 15Today

Start indoorsTransplant / sowHarvestFrost risk

MARAPRMAYJUNJULAUGSEP

Start indoors Mar 16 · Sow May 11 · First harvest Aug 9 · Wrap by Sep 6

Last spring frostMay 25

Start seeds indoorsMar 16

Direct sowMay 11

First harvestAug 9

Last harvestSep 6

First fall frostSep 15

Why these dates work in zone 3b

Zone 3b's last spring frost averages May 25 and first fall frost Sep 15 — about 113 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 3b?

Onion matures in about 90–110 days from sowing. In zone 3b that means sowing around May 11 and first picking around Aug 9.

Will onion survive frost in zone 3b?

Yes — onion tolerates light frost. In zone 3b you can sow a little before the last frost (around May 25) and keep harvesting past the first fall frost (around Sep 15).

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