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When to plant onions in zone 4a

In zone 4a, where the last spring frost typically comes around May 13, start onions indoors January 21 to March 4 and plant out April 15 to 29.

The zone 4a schedule for onions

Start seeds indoors
January 21 to March 4
Plant out
April 15 to 29
Sow outdoors
April 1 to 15

From the median frost dates of the NOAA stations assigned to the 739 ZIP codes in zone 4a, and the timing recommended by Cooperative Extension guides for onions.

Dates before the last frost assume the soil has thawed and is dry enough to work: a squeezed handful crumbles instead of sticking. After a late or wet spring, wait for that even if the date has passed.

Frost risk in zone 4a

Zones are based on winter cold, not on frost dates, so the last spring frost varies a lot within a zone. Across zone 4a, the typical last frost ranges from about May 6 in the earliest tenth of ZIP codes to May 24 in the latest tenth, and the first fall frost from September 20 to October 3. For a typical place in the zone:

  • April 30Risky: a later frost happens in 9 years out of 10
  • May 13Typical: frost after this date in half of years
  • May 27Safe: a later frost only 1 year in 10

Your dates for onions

Set your ZIP code at the top of the page and we will work out when to plant onions where you live, from the frost records of the nearest NOAA weather station. Your ZIP code is remembered on this device only.

Enter your ZIP code

Onions in zone 4a cities

CityTypical last frostStart indoorsPlant out
Fargo, NDMay 10Jan 18 to Mar 1Apr 12 to 26
Bismarck, NDMay 14Jan 22 to Mar 5Apr 16 to 30
Grand Forks, NDMay 10Jan 18 to Mar 1Apr 12 to 26

How to grow onionsAll zonesOnions in zone 3bOnions in zone 4bAbout zone 4a

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