When to plant · 29 cities · zones 6a–6b
When to plant onions in Michigan
Start Spanish Onion seeds indoors around Feb 8 and transplant around Apr 5 — the Michigan statewide median, counted from a last spring frost of May 3 across 988 ZIP codes and 196 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
Michigan onions, by step
Statewide median last frost May 3, first fall frost Oct 14, 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 8 | Apr 5 | Jul 24 | 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 Michigan's ZIP codes resolve to; 4 of 992 ZIPs have no datable freeze and are left out.
Onions in Michigan, 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 |
|---|---|---|---|---|
| Detroitzone 6b | Apr 27 | Feb 2 | Mar 30 | Jul 18 |
| Grand Rapidszone 6a | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Warrenzone 6b | Apr 21 | Jan 27 | Mar 24 | Jul 12 |
| Sterling Heightszone 6b | Apr 30 | Feb 5 | Apr 2 | Jul 21 |
| Ann Arborzone 6a | May 5 | Feb 10 | Apr 7 | Jul 26 |
| Lansingzone 6a | May 1 | Feb 6 | Apr 3 | Jul 22 |
| Dearbornzone 6b | Apr 27 | Feb 2 | Mar 30 | Jul 18 |
| Livoniazone 6b | May 1 | Feb 6 | Apr 3 | Jul 22 |
| Troyzone 6b | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| Farmington Hillszone 6a | May 1 | Feb 6 | Apr 3 | Jul 22 |
| Westlandzone 6b | May 1 | Feb 6 | Apr 3 | Jul 22 |
| Flintzone 6a | May 3 | Feb 8 | Apr 5 | Jul 24 |
| Wyomingzone 6a | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Rochester Hillszone 6a | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| Southfieldzone 6b | May 1 | Feb 6 | Apr 3 | Jul 22 |
| Kalamazoozone 6a | Apr 28 | Feb 3 | Mar 31 | Jul 19 |
| Novizone 6a | May 1 | Feb 6 | Apr 3 | Jul 22 |
| Pontiaczone 6a | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| Taylorzone 6b | Apr 20 | Jan 26 | Mar 23 | Jul 11 |
| Dearborn Heightszone 6b | May 1 | Feb 6 | Apr 3 | Jul 22 |
| Royal Oakzone 6b | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| St. Clair Shoreszone 6b | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Kentwoodzone 6a | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Battle Creekzone 6a | Apr 30 | Feb 5 | Apr 2 | Jul 21 |
| Portagezone 6a | Apr 28 | Feb 3 | Mar 31 | Jul 19 |
| East Lansingzone 6a | Apr 28 | Feb 3 | Mar 31 | Jul 19 |
| Rosevillezone 6b | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Saginawzone 6a | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Midlandzone 6a | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
What these onions become
What Michigan 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 Michigan: questions
When should I plant onions in Michigan?
Transplant around Apr 5 at the statewide median; across the state that date runs from Mar 29 in the warmest tenth of Michigan's ZIP codes to Apr 25 in the coldest. To cut the freeze risk to one year in ten, count from May 20 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in Michigan?
Around Feb 8 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in Michigan?
The ZIP-weighted median of NOAA 1991–2020 station medians is May 3. In the warmest tenth of Michigan's ZIP codes the median last frost falls on or before Apr 26; in the coldest tenth, on or after May 23.
When will Michigan onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Jul 24: 110 days from transplant.
Which onions grow in Michigan?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every Michigan city listed here is north of that line.
More for Michigan
- 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 → · Michigan frost dates → · How frost-date math works →