When to plant · 4 cities · zones 4b–5b
When to plant onions in Montana
Start Spanish Onion seeds indoors around Feb 25 and transplant around Apr 22 — the Montana statewide median, counted from a last spring frost of May 20 across 332 ZIP codes and 181 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
Montana onions, by step
Statewide median last frost May 20, first fall frost Sep 22, 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 25 | Apr 22 | Aug 10 | 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 Montana's ZIP codes resolve to; 37 of 369 ZIPs have no datable freeze and are left out.
Onions in Montana, 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 |
|---|---|---|---|---|
| Billingszone 5a | May 5 | Feb 10 | Apr 7 | Jul 26 |
| Missoulazone 5b | May 10 | Feb 15 | Apr 12 | Jul 31 |
| Great Fallszone 4b | May 6 | Feb 11 | Apr 8 | Jul 27 |
| Bozemanzone 5a | May 23 | Feb 28 | Apr 25 | Aug 13 |
What these onions become
What Montana 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 Montana: questions
When should I plant onions in Montana?
Transplant around Apr 22 at the statewide median; across the state that date runs from Apr 11 in the warmest tenth of Montana's ZIP codes to May 9 in the coldest. To cut the freeze risk to one year in ten, count from Jun 4 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in Montana?
Around Feb 25 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in Montana?
The ZIP-weighted median of NOAA 1991–2020 station medians is May 20. In the warmest tenth of Montana's ZIP codes the median last frost falls on or before May 9; in the coldest tenth, on or after Jun 6.
When will Montana onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Aug 10: 110 days from transplant.
Which onions grow in Montana?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every Montana city listed here is north of that line.
More for Montana
- 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 → · Montana frost dates → · How frost-date math works →