In Montana, sweet corn is ready 60 to 90 days after it goes into soil that has reached 60 F, and that soil arrives well after the last frost, not on it. At Billings, the reference station for the figures below, the season has room for that full range. The ear itself has a narrow window, and once it is picked the sugar starts leaving the kernel. The wider picture is in Growing Sweet Corn in Montana.
The short answer
Soil reaches 60 F later than the last frost, and that gap decides when corn can go in. At Billings, the last spring frost falls between April 29 and May 10, according to NOAA NCEI Climate Normals. That span is the band where the chance of a 32 F freeze sits between 30 and 70 percent. Soil is slower than air. It holds winter’s cold for weeks after the nights stop freezing, so a bed can be frost-free and still too cold for a seed.
Utah State University Extension sets the floor for sweet corn at soil above 60 F. Clemson Extension splits it by type: “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. A reading at the surface runs warmer than one 4 inches down and would send you out too early. No calendar date for any of these thresholds is published here, so the thermometer gives the go-ahead. The sowing side is covered in When to Sow Sweet Corn in Montana.
Do 60 to 90 days fit before fall?
At Billings, yes. The first fall frost lands between October 1 and October 14. Count from May 10, the late edge of the spring band, to October 1, the early edge of the fall one, and you get 144 days. Soil will not be ready on May 10, of course. Give it a full month more and about 114 days remain, still above the 90-day top of the range.
How fast an ear fades
Two clocks run here, and they answer different questions. The 60 to 90 days from planting tell you whether a variety fits the season. Roughly 17 to 24 days from silk emergence tell you which week to pick. Use three signs together: dry brown silks, husks still moist and green, and milky juice when you puncture a kernel. Any one alone can mislead you.
After picking, the fade begins. Clemson Extension puts it this way: “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.” Five days is a ceiling under ideal conditions, not a promise. Type matters too. The su type loses sugar fastest, se holds it longer, and sh2 holds it longest.
Does a late variety fit here?
At Billings, a late variety fits, but with a thinner cushion than the arithmetic first suggests. Elsewhere in Montana it may not fit at all. The numbers below use the same NOAA NCEI Climate Normals frost dates, and the 144-day span from the last section.
A 60-day variety
This one fits easily. Against the generous 144 days, it leaves 84 to spare. Even with soil a month late, about 54 days of slack remain. That slack lets you sow more than once and spread the harvest instead of facing every ear in the same week. Write down each sowing date and each variety’s days to maturity, because those two numbers decide when each planting tassels. That matters for the sweetness problem covered below.
A 90-day variety
On paper it fits at Billings. From the earliest conceivable start, 54 days are left over. With soil a month behind the last frost, only 24 remain, measured against the earliest fall frost in the band. That is thin. A cool stretch slows growth, and a 90-day figure is a count within a range, not a guarantee. If the variety is extra-sweet, the soil must reach 65 F first, which pushes the sowing later and eats more of that cushion.
Billings is one of the milder valleys, and that caution carries weight. Mountain valleys elsewhere in Montana can frost in any month, and no frost dates for them are published here. In those places a 90-day variety does not fit as a safe bet. A 60-day one is the sensible choice until you have your own frost records.
What goes wrong here
Three failures show up most often. Two come from the climate. The third has nothing to do with it.
- Soil sown too cold. Seed in soil under its threshold sits there and rots instead of germinating. An extra-sweet seed sown at 50 F is the classic case, which is why a gardener can succeed one year and fail the next after changing variety. The remedy is a thermometer at the 4-inch depth: 50 F for ordinary sweet corn, 65 F for extra-sweet, and Utah State’s 60 F as the working floor in between. Never use the last frost date as a stand-in. Soil warms well after it.
- A variety too late for the season. The 60 to 90 days must fit between planting and the first fall frost, which at Billings falls between October 1 and October 14. The remedy is to count forward from the day you actually sow, not from the last frost, and to drop to a shorter variety if the count comes up tight. In valleys that can frost in any month, shorter is the safer starting point.
- Sweetness lost to crossing. Su, se and sh2 types lose their sweetness if they tassel at the same time. Climate has no part in this, and neither does a spacing figure. The Utah State source gives no isolation distance, so none is offered here. The condition is timing: the types must not tassel together. The remedy is to grow one type per season, or to space sowings and choose maturities so that tasselling does not overlap. Plant in blocks of three or more rows, which is a shape for the garden and not a date.
If the ears are sweet but chewed or damaged, that is a separate problem, covered in What Eats Corn in Montana, and When.
Common questions
How do I know an ear is ready to pick?
Check three signs together: dry brown silks, husks still moist and green, and milky juice from a punctured kernel. A rough guide to the week is 17 to 24 days from silk emergence. That clock is separate from the 60 to 90 days from planting, which only tells you whether the variety fits the season.
Does Montana’s summer heat change how fast corn fades?
It can. Billings averages 28.3 days a year at 90 F or more, per NOAA NCEI Climate Normals. That figure measures summer heat load, not the average summer temperature. Clemson Extension says sugar leaves the kernel rapidly at high temperatures, so an ear left in a hot kitchen or a warm car loses sweetness faster than one brought in from the garden and kept out of the heat.
Can I grow two types of sweet corn in the same garden?
Yes, but not if they tassel at the same time. Su, se and sh2 types lose their sweetness when they do. Utah State University Extension publishes no separating distance, so the advice rests on timing: different sowings with different maturities, and blocks of three or more rows within each planting.
Do these frost dates apply across all of Montana?
No. They come from the Billings station, which sits in one of the milder valleys. Mountain valleys elsewhere in the state can frost in any month, and their dates are not published here. The soil thermometer and the 60 to 90 day count travel with you to any garden. The Billings frost window does not.