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When to plant · 38 cities · zones 5b–6a

When to plant onions in Illinois

Start Spanish Onion seeds indoors around Jan 23 and transplant around Mar 20 — the Illinois statewide median, counted from a last spring frost of Apr 17 across 1,402 ZIP codes and 192 NOAA stations.

Jan 23Start indoors12 weeks before last frost
Mar 20Plant outMar 10 – Mar 30 across the state
Apr 17Last spring frostmedian · 1 in 10 years after May 3
Jul 8First harvest110 days from transplant

The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.

Build a plan from your ZIP →Illinois frost dates by city →

Illinois onions, by step

Statewide median last frost Apr 17, first fall frost Oct 21, 2027 season. Each cultivar's own indoor lead time and days to maturity, same engine as the Plan Builder.

CultivarStart indoorsPlant outFirst harvestDays
Spanish OnionYellow Sweet Spanish (long-day)Jan 23Mar 20Jul 8110 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 Illinois's ZIP codes resolve to.

Onions in Illinois, city by city

Spanish Onion, scheduled from each city's nearest NOAA station — the same dates as its frost page.

CityLast frostStart indoorsPlant outFirst harvest
Chicagozone 6aApr 10Jan 16Mar 13Jul 1
Aurorazone 5bApr 19Jan 25Mar 22Jul 10
Napervillezone 5bApr 28Feb 3Mar 31Jul 19
Jolietzone 5bApr 19Jan 25Mar 22Jul 10
Rockfordzone 5bApr 24Jan 30Mar 27Jul 15
Elginzone 5bApr 26Feb 1Mar 29Jul 17
Springfieldzone 6aApr 15Jan 21Mar 18Jul 6
Peoriazone 6aApr 16Jan 22Mar 19Jul 7
Champaignzone 6aApr 15Jan 21Mar 18Jul 6
Waukeganzone 6aApr 30Feb 5Apr 2Jul 21
Cicerozone 6aApr 11Jan 17Mar 14Jul 2
Bloomingtonzone 6aApr 21Jan 27Mar 24Jul 12
Schaumburgzone 5bApr 30Feb 5Apr 2Jul 21
Evanstonzone 6aApr 26Feb 1Mar 29Jul 17
Arlington Heightszone 6aApr 19Jan 25Mar 22Jul 10
Bolingbrookzone 6aApr 20Jan 26Mar 23Jul 11
Decaturzone 6aApr 14Jan 20Mar 17Jul 5
Palatinezone 5bApr 25Jan 31Mar 28Jul 16
Skokiezone 6aApr 26Feb 1Mar 29Jul 17
Des Plaineszone 6aApr 16Jan 22Mar 19Jul 7
Orland Parkzone 6aApr 20Jan 26Mar 23Jul 11
Oak Lawnzone 6aApr 10Jan 16Mar 13Jul 1
Mount Prospectzone 6aApr 19Jan 25Mar 22Jul 10
Berwynzone 6aApr 11Jan 17Mar 14Jul 2
Tinley Parkzone 6aApr 23Jan 29Mar 26Jul 14
Wheatonzone 5bApr 28Feb 3Mar 31Jul 19
Normalzone 6aApr 25Jan 31Mar 28Jul 16
Oak Parkzone 6aApr 11Jan 17Mar 14Jul 2
Hoffman Estateszone 5bApr 30Feb 5Apr 2Jul 21
Downers Grovezone 6aApr 28Feb 3Mar 31Jul 19
Plainfieldzone 5bApr 20Jan 26Mar 23Jul 11
Glenviewzone 6aApr 19Jan 25Mar 22Jul 10
Elmhurstzone 6aApr 16Jan 22Mar 19Jul 7
Lombardzone 6aApr 28Feb 3Mar 31Jul 19
Buffalo Grovezone 6aApr 19Jan 25Mar 22Jul 10
Molinezone 5bApr 24Jan 30Mar 27Jul 15
Crystal Lakezone 5bMay 1Feb 6Apr 3Jul 22
Romeovillezone 6aApr 20Jan 26Mar 23Jul 11

What these onions become

Sunday GravyTwelve pounds of paste tomatoes, three hours, and a food mill instead of a peeling job. The reduction that makes a homegrown tomato taste like the reason you grew it.Recipe and process →Salsa VerdeBlistered tomatillos, serrano, and a fistful of cilantro. Naturally acidic, ready in forty minutes, and it clings to a taco because tomatillos bring their own pectin.Recipe and process →Chipotles en AdoboRed jalapeños, eight hours of cold smoke, then a braise in a dark ancho-and-vinegar sauce. The single most useful jar in a kitchen.Recipe and process →Habanero-Carrot Hot SauceThree weeks in a 3% brine turns raw habanero heat into something layered and sour, and carrot gives it body and colour with no thickener. The reference build for a homegrown hot sauce.Recipe and process →

What Illinois onions need

SeedsThe cultivar decides the sauce. Everything downstream is damage control.

  • 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.

    Amazon · paid link ↗
StartSoil temperature and light intensity, indoors, weeks before your frost date.

  • 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.

    Amazon · paid link ↗

  • 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.

    Amazon · paid link ↗

  • 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.

    Amazon · paid link ↗
GrowRoot volume, nutrition and structure. Where most home crops actually fail.

  • 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.

    Amazon · paid link ↗

  • 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.

    Amazon · paid link ↗
HarvestTiming, handling, and weighing what you actually got.

  • 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.

    Amazon · paid link ↗

Paid links. We earn a commission from Amazon if you buy; you pay the same. Disclosure.

Onions in Illinois: questions

When should I plant onions in Illinois?

Transplant around Mar 20 at the statewide median; across the state that date runs from Mar 10 in the warmest tenth of Illinois's ZIP codes to Mar 30 in the coldest. To cut the freeze risk to one year in ten, count from May 3 instead of the median last frost. Use the city table for your own town.

When do I start onions seeds indoors in Illinois?

Around Jan 23 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.

When is the last frost in Illinois?

The ZIP-weighted median of NOAA 1991–2020 station medians is Apr 17. In the warmest tenth of Illinois's ZIP codes the median last frost falls on or before Apr 7; in the coldest tenth, on or after Apr 27.

When will Illinois onions be ready to harvest?

Spanish Onion planted on the statewide date is ready around Jul 8: 110 days from transplant.

Which onions grow in Illinois?

Spanish Onion is a long-day onion, correct above roughly 36° N. Every Illinois city listed here is north of that line.

More for Illinois

Onions in every state → · Illinois frost dates → · How frost-date math works →