When to plant · 22 cities · zones 5b–7a
When to plant onions in Colorado
Start Spanish Onion seeds indoors around Feb 12 and transplant around Apr 9 — the Colorado statewide median, counted from a last spring frost of May 7 across 493 ZIP codes and 190 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
Colorado onions, by step
Statewide median last frost May 7, first fall frost Oct 6, 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 12 | Apr 9 | Jul 28 | 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 Colorado's ZIP codes resolve to; 34 of 527 ZIPs have no datable freeze and are left out.
Onions in Colorado, 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 |
|---|---|---|---|---|
| Denverzone 6a | May 4 | Feb 9 | Apr 6 | Jul 25 |
| Colorado Springszone 6a | May 3 | Feb 8 | Apr 5 | Jul 24 |
| Aurorazone 6a | May 5 | Feb 10 | Apr 7 | Jul 26 |
| Fort Collinszone 5b | May 3 | Feb 8 | Apr 5 | Jul 24 |
| Lakewoodzone 6a | May 3 | Feb 8 | Apr 5 | Jul 24 |
| Thorntonzone 6a | May 2 | Feb 7 | Apr 4 | Jul 23 |
| Arvadazone 6a | May 5 | Feb 10 | Apr 7 | Jul 26 |
| Westminsterzone 6a | May 2 | Feb 7 | Apr 4 | Jul 23 |
| Greeleyzone 5b | May 2 | Feb 7 | Apr 4 | Jul 23 |
| Pueblozone 6a | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| Centennialzone 6a | May 5 | Feb 10 | Apr 7 | Jul 26 |
| Boulderzone 6a | May 6 | Feb 11 | Apr 8 | Jul 27 |
| Longmontzone 5b | May 7 | Feb 12 | Apr 9 | Jul 28 |
| Castle Rockzone 5b | May 9 | Feb 14 | Apr 11 | Jul 30 |
| Lovelandzone 5b | May 3 | Feb 8 | Apr 5 | Jul 24 |
| Broomfieldzone 6a | May 2 | Feb 7 | Apr 4 | Jul 23 |
| Grand Junctionzone 7a | Apr 28 | Feb 3 | Mar 31 | Jul 19 |
| Commerce Cityzone 6a | May 3 | Feb 8 | Apr 5 | Jul 24 |
| Parkerzone 5b | May 9 | Feb 14 | Apr 11 | Jul 30 |
| Littletonzone 6a | May 2 | Feb 7 | Apr 4 | Jul 23 |
| Brightonzone 5b | May 3 | Feb 8 | Apr 5 | Jul 24 |
| Windsorzone 5b | May 3 | Feb 8 | Apr 5 | Jul 24 |
What these onions become
What Colorado 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 Colorado: questions
When should I plant onions in Colorado?
Transplant around Apr 9 at the statewide median; across the state that date runs from Apr 2 in the warmest tenth of Colorado's ZIP codes to May 9 in the coldest. To cut the freeze risk to one year in ten, count from May 21 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in Colorado?
Around Feb 12 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in Colorado?
The ZIP-weighted median of NOAA 1991–2020 station medians is May 7. In the warmest tenth of Colorado's ZIP codes the median last frost falls on or before Apr 30; in the coldest tenth, on or after Jun 6.
When will Colorado onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Jul 28: 110 days from transplant.
Which onions grow in Colorado?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every Colorado city listed here is north of that line.
More for Colorado
- 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 → · Colorado frost dates → · How frost-date math works →