Plan on 60 to 90 days from planting to the first ears, counted from the day the soil is warm enough, not from the last frost. In the Twin Cities that count fits the season with room to spare. In northern Minnesota the margin is thin. Once an ear is ripe, a second clock starts, and it is short: about five days at best before sweetness slips. The broader growing picture is in Growing Sweet Corn in Minnesota.
The short answer
Soil warms after the frost ends
Minnesota soil is likely to reach 60 F weeks after the last frost, not on the same day. Utah State University Extension sets the floor for sweet corn at soil above 60 F. The NOAA NCEI Climate Normals for the Twin Cities put the last spring frost between April 17 and April 30. Those are two different events. A frost date tracks air on a clear night, while soil is a cold, wet mass that needs sun and warm nights to climb. This page gives no calendar date for 60 F soil, because none is published here. A thermometer is the right tool.
Clemson Cooperative Extension gives the reading and the depth: “Do not plant before the soil temperature at the 4-inch depth is at least 50 ºF. Wait to plant extra-sweet varieties until soil temperatures reach 65 °F.” The depth matters, since a surface reading runs warmer and sends people out too early. The two thresholds are not interchangeable either. Ordinary sweet corn can go in at 50 F, but extra-sweet (sh2) seed needs 65 F. For the timing question in more detail, see When to Sow Sweet Corn in Minnesota.
The other end of the season is the first fall frost, which the same NOAA NCEI normals place between October 11 and October 23. Sixty to ninety days must fit between warm soil and that window. For most of the state they do, as the next section works out.
Then a second clock starts
The planting clock says whether a variety fits the season. It does not say which week to pick. That is a separate count: ears are typically ready 17 to 24 days after silks first emerge. Within that stretch, judge the ear by three signs together: dry brown silks, husks still moist and green, and milky juice when you puncture a kernel. Any one alone misleads. Silks can brown on an ear that is not full, and juice can run clear on one that is still too young.
After picking, the fade begins. Clemson Cooperative Extension says: “Sweet corn loses sugar from the kernel rapidly at high temperatures… Sweet corn can be stored under optimal conditions for about five days, but will lose sweetness with each day of storage.” Read “about five days” as a ceiling under ideal conditions, not a promise. That is the mechanism behind the old advice to run from the garden to the pot: sugar leaves the kernel, and heat speeds it up.
The type you plant sets how fast that happens. The su type loses sugar fastest, se holds it longer, and sh2 holds it longest. Minneapolis averages 10.6 days a year at 90 F or higher, so heat is not the usual limit on growing corn here. But a warm afternoon on the counter works against you. Chill ears soon after picking.
Does a late variety fit here?
Counting works backward from the first fall frost. The Twin Cities window runs from the last spring frost (April 17 to April 30) to the first fall frost (October 11 to October 23). At the tight end, April 30 to October 11 is 164 days. At the generous end, April 17 to October 23 is 189. Warm soil arrives well into that stretch, so the real planting window is shorter. Northern Minnesota runs two to three weeks tighter at both ends, which trims the stretch by about 28 to 42 days and leaves roughly 122 to 136 days at the tight end.
A 60-day variety
It fits, and comfortably. Sixty days is a little over a third of the 164-day stretch in the Twin Cities. In the north, it is still only about half of the shortened one. Counting back from October 11, a 60-day planting would have to go in by about mid-August to finish before the earliest frost in the band. That is a paper figure, though. Day counts assume warm weather, and September growth is slower than July growth, so I would not push a planting that late.
The early variety also lets you stagger plantings. Sowing a second block two weeks after the first spreads the harvest across weeks instead of one weekend. That is helpful given how fast ripe ears fade.
A 90-day variety
In the Twin Cities, it fits. Ninety days from a 164-day stretch leaves 74 days of slack, and the wait for 60 F soil eats only part of it. To miss the frost, planting would have to slip past about July 13, and soil warms long before that.
The far north is different. Take the 122 to 136 days left there, subtract 90, and you have 32 to 46 days of slack. Then subtract the wait for warm soil, which is longer and later up there, and add one cool summer slowing growth. The late variety does not leave a margin I would trust. Ears could still be filling when frost arrives. An extra-sweet type that needs 65 F soil only tightens things. Choose a shorter count there.
These dates are bands of probability, not guarantees. A frost can land before or after them in any given year.
What goes wrong here
Three failures account for most disappointing corn in Minnesota. Two come from the climate. The third has nothing to do with it.
- Seed sown into cold soil. Seed that sits in cold, wet ground rots rather than sprouts. This hits extra-sweet types hardest, because they need 65 F at 4 inches, not the 50 F that ordinary corn tolerates. It explains why a gardener can succeed one year and fail the next after switching variety. Remedy: take the soil reading at 4 inches, not at the surface, and wait. Utah State’s advice is soil above 60 F, and I would aim for that, since spring nights swing cold.
- A variety too late for the season. A 90-day type planted into a short northern season can meet frost while the kernels are still forming. Remedy: count back from the first fall frost, shift that date two to three weeks earlier in the north, and choose a shorter day count. A 60-day variety sidesteps the problem.
- Sweetness lost to crossing. Climate plays no part here. Sweetness is lost if su, se and sh2 types tassel at the same time. Utah State Extension publishes no separating distance, so none is given here. The condition is timing. Remedy: grow one type, or stagger plantings so the types are not tassel-ing together. Plant each type in blocks of three or more rows, which is a shape for the garden, not a date.
A failure of a different kind, raiders arriving at ripening, is covered in What Eats Corn in Minnesota, and When.
Common questions
Is 60 F soil the same as the last frost date?
No. Frost is a measure of air, and soil warms well after it. The Twin Cities last frost falls between April 17 and April 30, but no date for 60 F soil is published here. Read a thermometer at 4 inches deep instead of trusting the calendar.
How long does picked corn keep its sweetness?
Clemson Extension says sweet corn “can be stored under optimal conditions for about five days, but will lose sweetness with each day of storage.” Five days is the best case. Heat shortens it, and su types fade faster than se or sh2.
Can I plant one long row?
Plant in blocks of three or more rows. Utah State Extension is the source for that shape, which matters for pollination. Corn is wind-pollinated, so a single row pollinates poorly and leaves ears with gaps.
Do the frost dates apply in northern Minnesota?
They describe the Twin Cities, which are the mild south of the state. Northern Minnesota runs two to three weeks tighter at both ends, so frost comes later in spring and earlier in fall. Count your own window before choosing a day count.