When to plant · 14 cities · zones 5a–5b
When to plant onions in Iowa
Start Spanish Onion seeds indoors around Feb 3 and transplant around Mar 31 — the Iowa statewide median, counted from a last spring frost of Apr 28 across 964 ZIP codes and 160 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
Iowa onions, by step
Statewide median last frost Apr 28, 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 3 | Mar 31 | Jul 19 | 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 Iowa's ZIP codes resolve to.
Onions in Iowa, 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 |
|---|---|---|---|---|
| Des Moineszone 5b | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| Cedar Rapidszone 5b | Apr 28 | Feb 3 | Mar 31 | Jul 19 |
| Davenportzone 5b | Apr 23 | Jan 29 | Mar 26 | Jul 14 |
| Sioux Cityzone 5a | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Ankenyzone 5b | Apr 23 | Jan 29 | Mar 26 | Jul 14 |
| Iowa Cityzone 5b | Apr 21 | Jan 27 | Mar 24 | Jul 12 |
| West Des Moineszone 5b | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| Ameszone 5b | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| Waterloozone 5a | Apr 27 | Feb 2 | Mar 30 | Jul 18 |
| Council Bluffszone 5b | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| Dubuquezone 5a | Apr 19 | Jan 25 | Mar 22 | Jul 10 |
| Urbandalezone 5b | Apr 18 | Jan 24 | Mar 21 | Jul 9 |
| Marionzone 5a | Apr 28 | Feb 3 | Mar 31 | Jul 19 |
| Cedar Fallszone 5a | Apr 27 | Feb 2 | Mar 30 | Jul 18 |
What these onions become
What Iowa 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 Iowa: questions
When should I plant onions in Iowa?
Transplant around Mar 31 at the statewide median; across the state that date runs from Mar 23 in the warmest tenth of Iowa's ZIP codes to Apr 5 in the coldest. To cut the freeze risk to one year in ten, count from May 11 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in Iowa?
Around Feb 3 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in Iowa?
The ZIP-weighted median of NOAA 1991–2020 station medians is Apr 28. In the warmest tenth of Iowa's ZIP codes the median last frost falls on or before Apr 20; in the coldest tenth, on or after May 3.
When will Iowa onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Jul 19: 110 days from transplant.
Which onions grow in Iowa?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every Iowa city listed here is north of that line.
More for Iowa
- 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 → · Iowa frost dates → · How frost-date math works →