At 42.23 degrees north, Michigan’s Detroit-area reference point gets 15 hours and 5 minutes of daylight on the longest day of the year — enough for long-day and intermediate-day onions to bulb, but not for short-day varieties, which will only make leaves before rotting. That single number decides which onion sets belong in your garden and which ones don’t.
How long your longest day is
The reference station for this territory sits at 42.23 degrees north latitude (NOAA NCEI station USW00094847, in the Detroit area), and on the longest day of the year it logs 15 hours and 5 minutes of daylight. That number matters more than any frost date or soil test you’ll run this spring, because onions don’t bulb because the weather turns warm. They bulb because the day gets long enough to trigger the switch from leaf-growth to bulb-growth. Clemson Cooperative Extension puts that trigger at around 12 hours of daylight for short-day types, and long-day varieties hold off until the day stretches to 14 or even 16 hours. Michigan’s 15 hours and 5 minutes lands squarely inside the range where long-day and intermediate-day onions do their work. Short-day onions never get the memo, they start bulbing too early, while the plant is still small, and the result is disappointing.
Where the 42.23N figure comes from
That 15-hour-5-minute figure is calculated from the latitude of the reference city using the standard sunset hour-angle formula, with a solar declination of 23.44 degrees at the June solstice. It’s a geometric measurement, atmospheric refraction adds roughly seven more minutes of real daylight that isn’t counted here, so the true number is a touch higher. The gap doesn’t change which onion group you should plant, but it’s worth knowing the figure is a floor, not a ceiling.
What a large state means for your own yard
Michigan runs from around 41.7 degrees north near the Ohio line to well past 47 degrees north in the Upper Peninsula, a spread of more than five degrees of latitude. That’s a meaningful difference in summer daylight. A gardener in Ironwood or Marquette gets noticeably more daylight on the solstice than one in Monroe or Detroit, which pushes the northern edge of the state toward varieties bred for even longer days. If you garden near either edge of Michigan, it’s worth checking your own latitude rather than assuming Detroit’s number applies to your yard exactly.
Which varieties bulb in Michigan
Twenty-one varieties documented on this site have a published latitude range that covers 42.23 degrees north, meaning their bulbing trigger lines up with the daylight Michigan actually delivers. That’s the good news, and it’s a longer list than most gardeners expect.
- Ailsa Craig (long-day, 38-60N)
- Barolo (long-day, 38-55N)
- Blush (long-day, 38-55N)
- Bridger (long-day, 35-50N)
- Cabernet (intermediate-day, 35-45N)
- Calibra (intermediate-day, 35-45N)
- Elsye (intermediate-day, 32-45N)
- Expression (intermediate-day, 32-45N)
- Frontier (long-day, 40-50N)
- Monastrell (intermediate-day, 35-46N)
- New York Early (long-day, 38-50N)
- Patterson (long-day, 38-55N)
- Red Carpet (long-day, 40-45N)
- Red Long of Florence (intermediate-day, 38-50N)
- Red Long of Tropea (intermediate-day, 35-48N)
- Rossa di Milano (long-day, 38-50N)
- Safrane (long-day, 42-50N)
- Scout (intermediate-day, 32-46N)
- Sierra Blanca (intermediate-day, 32-45N)
- Talon (long-day, 42-50N)
- Walla Walla Sweet (long-day, 35-55N)
Two more sit right at the edge of their published range: Powell (long-day, 43-47N) and Redwing (long-day, 43-65N) are catalogued for latitudes just above Michigan’s reference point. They may still bulb here, but the daylight cutoff is a continuum rather than a hard line, so treat these as a gamble rather than a sure thing, especially in the southern half of the state.
Varieties that will not bulb here
This is the list that actually saves a season. Five varieties documented on this site are catalogued for latitudes below 42.23 degrees north, which means Michigan’s daylight never reaches their trigger point:
- Candy (intermediate-day, 33-40N)
- Gabriella (short-day, 20-35N)
- Madalyn (short-day, 20-35N)
- Red Rock (short-day, 30-36N)
- White Castle (short-day, 30-38N)
Plant any of these in a Michigan garden and here’s what actually happens: the plant grows lush, green tops all summer, looking perfectly healthy. Then, as the days start to shorten in late summer, it either never bulbs at all or forms a tiny, undersized bulb that promptly starts to rot in the ground. A gardener who’s grabbed a bag of supermarket sets without checking the daylight requirement has probably seen exactly this, good-looking leaves, a season of watering, and nothing worth pulling in the fall. That’s a variety mistake, not a feeding or watering mistake, and no amount of fertilizer fixes it.
When to plant onions in Michigan
Planting season follows the daylight group, and that’s why it looks so different from state to state. In mild-winter regions of the South, growers plant short-day onions in the fall and let them overwinter as small plants. Michigan can’t do that, its winters are too harsh, and its onions are long-day and intermediate-day types anyway, which need a spring start.
Working around Michigan’s frost dates
MSU Extension puts the last spring frost around April 30 in the Detroit area, with early to mid May typical across most of the Lower Peninsula. Onion sets and transplants can go in a few weeks before that average date, since young onion plants tolerate a light frost far better than a warm-season crop like tomatoes or peppers. Many Michigan gardeners aim for late March through April, giving the plants a long run of cool weather to build leaf mass before the June solstice triggers bulbing.
On the other end of the season, MSU Extension lists the first fall frost arriving between October 6 and October 21 across zone 6a, which covers much of the state’s growing regions. That gives a long-day onion planted in April roughly five months to grow leaves, bulb, and cure before the ground turns cold, plenty of time, provided the start date isn’t pushed too late.
Both of those frost dates are averages pulled from decades of weather records, not guarantees for any single spring. A late cold snap after April 30 or an early frost before October 6 happens often enough that treating these dates as flexible windows, rather than fixed deadlines, will save a crop more years than it costs one.
Sweet onions or onions that keep
Michigan’s daylight numbers point toward a garden that leans more toward storage onions than sweet ones, and the reason has nothing to do with soil or luck. It’s chemistry. The bite in an onion comes from volatile sulfur compounds, and those same compounds are what protect the bulb once it’s cured and sitting in a pantry. University of Missouri Extension notes it plainly: sweet onions do not store as well as the more pungent types, because they simply contain fewer of those protective compounds.
What that means for a Michigan harvest
Day length and pungency are linked. Short-day varieties, bred for southern gardens with shorter summer days, tend to run sweeter and milder, and they don’t keep. Long-day and intermediate-day varieties, the ones that actually bulb under Michigan’s 15-hour-5-minute solstice, tend to run sharper and store longer. That’s not a trade-off you can breed around with better fertilizer; it’s built into the same mechanism that makes the onion bulb at all in this latitude.
| Garden type | Typical day-length group | Flavor | Storage life |
|---|---|---|---|
| Southern garden | Short-day | Sweet, mild | Weeks, not months |
| Michigan garden | Long-day / intermediate-day | Sharper, more pungent | Several months in a cool, dry space |
What that means practically: a Michigan gardener growing Ailsa Craig, Patterson, or Talon should expect an onion that’s better suited to cooking than to eating raw off the cutting board, but one that will still be sound in a root cellar or a cool basement in March. Don’t expect a sweet Vidalia-style bite from anything that actually bulbs reliably at this latitude, and don’t plant a sweet variety expecting it to carry you through winter, because the same chemistry that gives it a mild flavor is what leaves it soft and spoiled well before spring.