When to plant · 24 cities · zones 6a–8b
When to plant onions in Utah
Start Spanish Onion seeds indoors around Feb 10 and transplant around Apr 7 — the Utah statewide median, counted from a last spring frost of May 5 across 295 ZIP codes and 136 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
Utah onions, by step
Statewide median last frost May 5, first fall frost Oct 10, 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 10 | Apr 7 | Jul 26 | 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 Utah's ZIP codes resolve to; 3 of 298 ZIPs have no datable freeze and are left out.
Onions in Utah, 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 |
|---|---|---|---|---|
| Salt Lake Cityzone 7b | Mar 31 | Jan 6 | Mar 3 | Jun 21 |
| West Valley Cityzone 7b | Apr 8 | Jan 14 | Mar 11 | Jun 29 |
| West Jordanzone 7b | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| Provozone 7a | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| St. Georgezone 8b | Mar 14 | Dec 20 | Feb 14 | Jun 4 |
| Oremzone 7a | Apr 19 | Jan 25 | Mar 22 | Jul 10 |
| Lehizone 7a | Apr 30 | Feb 5 | Apr 2 | Jul 21 |
| Sandyzone 7b | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Ogdenzone 6b | Apr 21 | Jan 27 | Mar 24 | Jul 12 |
| South Jordanzone 7a | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| Laytonzone 7a | Apr 19 | Jan 25 | Mar 22 | Jul 10 |
| Millcreekzone 7b | Apr 25 | Jan 31 | Mar 28 | Jul 16 |
| Herrimanzone 7a | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| Eagle Mountainzone 6b | Apr 30 | Feb 5 | Apr 2 | Jul 21 |
| Taylorsvillezone 7b | Apr 8 | Jan 14 | Mar 11 | Jun 29 |
| Saratoga Springszone 7a | Apr 30 | Feb 5 | Apr 2 | Jul 21 |
| Loganzone 6a | Apr 30 | Feb 5 | Apr 2 | Jul 21 |
| Murrayzone 7b | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Draperzone 7a | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| Spanish Forkzone 7a | Apr 25 | Jan 31 | Mar 28 | Jul 16 |
| Rivertonzone 7a | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| Bountifulzone 6b | Apr 25 | Jan 31 | Mar 28 | Jul 16 |
| American Forkzone 7a | Apr 17 | Jan 23 | Mar 20 | Jul 8 |
| Tooelezone 7a | May 2 | Feb 7 | Apr 4 | Jul 23 |
What these onions become
What Utah 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 Utah: questions
When should I plant onions in Utah?
Transplant around Apr 7 at the statewide median; across the state that date runs from Mar 12 in the warmest tenth of Utah's ZIP codes to May 4 in the coldest. To cut the freeze risk to one year in ten, count from May 28 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in Utah?
Around Feb 10 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in Utah?
The ZIP-weighted median of NOAA 1991–2020 station medians is May 5. In the warmest tenth of Utah's ZIP codes the median last frost falls on or before Apr 9; in the coldest tenth, on or after Jun 1.
When will Utah onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Jul 26: 110 days from transplant.
Which onions grow in Utah?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every Utah city listed here is north of that line.
More for Utah
- 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 → · Utah frost dates → · How frost-date math works →