When to plant · 5 cities · zone 4a
When to plant onions in North Dakota
Start Spanish Onion seeds indoors around Feb 17 and transplant around Apr 14 — the North Dakota statewide median, counted from a last spring frost of May 12 across 388 ZIP codes and 127 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
North Dakota onions, by step
Statewide median last frost May 12, first fall frost Sep 28, 2027 season. Each cultivar's own indoor lead time and days to maturity, same engine as the Plan Builder.
| Cultivar | Start indoors | Plant out | First harvest | Days |
|---|---|---|---|---|
| Spanish OnionYellow Sweet Spanish (long-day) | Feb 17 | Apr 14 | Aug 2 | 110 from transplant |
Tight means first harvest lands within two weeks of the median first fall frost. Statewide dates are the ZIP-weighted median of every NOAA 1991–2020 station North Dakota's ZIP codes resolve to.
Onions in North Dakota, city by city
Spanish Onion, scheduled from each city's nearest NOAA station — the same dates as its frost page.
| City | Last frost | Start indoors | Plant out | First harvest |
|---|---|---|---|---|
| Fargozone 4a | May 5 | Feb 10 | Apr 7 | Jul 26 |
| Bismarckzone 4a | May 10 | Feb 15 | Apr 12 | Jul 31 |
| Grand Forkszone 4a | May 10 | Feb 15 | Apr 12 | Jul 31 |
| Minotzone 4a | May 10 | Feb 15 | Apr 12 | Jul 31 |
| West Fargozone 4a | May 10 | Feb 15 | Apr 12 | Jul 31 |
What these onions become
What North Dakota onions need
Yellow Sweet Spanish Onion Seed
A long-day storage onion with enough sugar to sweat down without scorching. The aromatic base under every tomato sauce on this site.
Organic Seed Starter Potting MixMust have
Seed-starting mix is not potting soil with a nicer label — it is finer, lighter, and low in nutrients on purpose, because a seedling's first roots want air and moisture, not fertiliser.
72-Cell Trays with 1020 Bottoms, 5-PackMust have
The cheap black nursery trays crack after one season and flex enough to break roots when you lift them. These do not. Over five seasons the price per use is lower than the throwaways.
T8 Full-Spectrum Grow Lights, 1 ft 4-packMust have
A south window in March delivers a fraction of the light a pepper seedling needs, and the plant tells you by stretching. Two inches of LED above the canopy for 16 hours a day fixes it permanently.
Rapitest Soil Test Kit
In-ground growers are guessing until they test. pH outside 6.0–6.8 locks nutrients out no matter how much fertiliser you add, and no amount of feeding fixes that.
Pure Organic Worm Castings, 15 lb
Not a fertiliser so much as a soil-biology inoculant with a mild nutrient charge. A cup in each planting hole is a cheap, unglamorous improvement you can measure by August.
Garden Harvest Basket with Tool Caddy
A harvest basket that doubles as a colander means the hose rinse happens in the garden, and the dirt stays outside. On a 20-pound tomato day that is a genuine time saving.
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Onions in North Dakota: questions
When should I plant onions in North Dakota?
Transplant around Apr 14 at the statewide median; across the state that date runs from Apr 8 in the warmest tenth of North Dakota's ZIP codes to Apr 21 in the coldest. To cut the freeze risk to one year in ten, count from May 26 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in North Dakota?
Around Feb 17 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in North Dakota?
The ZIP-weighted median of NOAA 1991–2020 station medians is May 12. In the warmest tenth of North Dakota's ZIP codes the median last frost falls on or before May 6; in the coldest tenth, on or after May 19.
When will North Dakota onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Aug 2: 110 days from transplant.
Which onions grow in North Dakota?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every North Dakota city listed here is north of that line.
More for North Dakota
- When to plant tomatoes
- When to plant peppers
- When to plant tomatillos
- When to plant basil
- When to plant cilantro
- When to plant carrots
- When to plant mustard greens
Onions in every state → · North Dakota frost dates → · How frost-date math works →