When to plant onion in zone 4a — planting calendar

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Showing USDA Zone 4A

Growing SeasonZone 4a

Last frost ~May 20First frost ~Sep 20Today

Start indoorsTransplant / sowHarvestFrost risk

MARAPRMAYJUNJULAUGSEP

Start indoors Mar 11 · Sow May 6 · First harvest Aug 4 · Wrap by Sep 1

Last spring frostMay 20

Start seeds indoorsMar 11

Direct sowMay 6

First harvestAug 4

Last harvestSep 1

First fall frostSep 20

Why these dates work in zone 4a

Zone 4a's last spring frost averages May 20 and first fall frost Sep 20 — about 123 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 4a?

Onion matures in about 90–110 days from sowing. In zone 4a that means sowing around May 6 and first picking around Aug 4.

Will onion survive frost in zone 4a?

Yes — onion tolerates light frost. In zone 4a you can sow a little before the last frost (around May 20) and keep harvesting past the first fall frost (around Sep 20).

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