Sweet corn is ready in Manitoba roughly 60 to 90 days after the seed goes into soil that has reached 60 F, and that soil warms well after the last frost. At Winnipeg, the last spring frost averages May 23 and the first fall frost September 22, which leaves 122 days between them. Warming soil eats the front of that window, so early varieties fit comfortably and late ones barely. Picked ears fade fast. The full picture is in our guide to Growing Sweet Corn in Manitoba.
The short answer
Soil reaches 60 F later than the last frost. The two are different events, and mixing them up is the commonest early mistake. Air can be frost-free for days while the top few inches of the garden are still too cold for seed to wake up. May 23 is also only a mean date from Environment and Climate Change Canada, the midpoint of a window. In about one year out of two, the last frost comes after it.
The soil rule is a measurement, not a date. Clemson Extension says: “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.” Utah State University Extension puts the working figure for sweet corn at soil above 60 F. Read the thermometer at 4 inches, because a surface reading runs warmer and sends you out too early. The thresholds are not interchangeable either. A supersweet (sh2) seed sown at 50 F tends to rot rather than sprout, which is why a gardener can succeed one year and fail the next after switching varieties. For the sowing side of this, see When to Sow Sweet Corn in Manitoba.
Two clocks, two questions
The 60 to 90 days run from planting and answer one question only: does this variety fit the season? A second clock, roughly 17 to 24 days from silk emergence, tells you which week to pick. Keep them apart. The first is for choosing seed in spring, the second for standing in the rows in late summer.
Ripeness takes three signs together: silks dry and brown, husks still moist and green, and milky juice when you puncture a kernel. One sign alone misleads. Brown silks on a dry-husked ear can mean it is already past its best.
How fast it fades once picked
Clemson Extension puts it plainly: “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.” Note that “about five days” is a ceiling under optimal conditions, not a promise. Sugar leaves the kernel, and heat speeds it up, which is the real reason behind the old advice to run from the garden to the pot.
Genotype changes the speed. Standard su types lose sugar fastest, se types hold it longer, and sh2 holds it longest. If your picking happens whenever you get to it, that is a good argument for sh2. It needs the warmest soil of the three before sowing, though, so you pay for the slower fade with a later start.
Does a late variety fit here?
Start from the 122 days between the two mean frost dates, both read from the Winnipeg Richardson Int’l A station. A province is not one climate. Northern and upland districts run weeks behind the airport, and Canadian normals cover an older 30-year stretch than the US figures used elsewhere on this site. Treat 122 as the best case, not a guarantee.
A 60-day variety
This one fits with room to spare. Subtract 60 from 122 and about two months of slack remain. That slack absorbs a slow spring, a sh2 seed waiting for 65 F soil, and a September frost that arrives before the mean. It also leaves space for a second sowing a couple of weeks after the first, which spreads the harvest instead of dumping it all on one weekend.
Heat is not the worry. Winnipeg Richardson Int’l A averages 13.3 days a year above 30 °C, and 4.54 days above 32 °C. Those are counts of hot days, not a measure of the growing season, and they say nothing against warm-season crops. The limit for corn here is the calendar between the frosts.
A 90-day variety
On paper this fits: 122 minus 90 leaves 32 days. In practice that margin is thin, and several things take it away. Waiting for 60 F soil after the last frost spends a good part of it. If the variety is extra-sweet and needs 65 F, more goes. Then the fall frost comes before September 22 in roughly half of years.
So no, a 90-day variety does not fit comfortably in Manitoba. It works only in a warm spring, sown the day the soil allows, followed by a late autumn. That is a gamble, not a plan. Pick something at the shorter end of the range and let the long summer days do the rest. Early-sown corn also meets different problems from late corn, covered in What Eats Corn in Manitoba, and When.
What goes wrong here
Three failures account for most disappointing patches. Two come from the climate and one has nothing to do with it.
- Soil sown too cold. Seed goes in on the strength of a warm week of air, and the ground at 4 inches is still below what the type needs. Ordinary sweet corn needs at least 50 F at that depth, extra-sweet needs 65 F, and Utah State’s figure for sweet corn in general is above 60 F. The remedy is a soil thermometer pushed in to 4 inches, read in the morning, and patience with sh2 seed in particular. Cold-sown supersweet seed rots, and the gaps in the row show it two weeks later.
- A variety too late for the season. A 90-day type sown after a slow spring runs into the first fall frost with ears still filling. The ears are small, the kernels watery, and the frost ends the argument. The remedy is arithmetic done in winter: count the days from your realistic sowing date, not from the last frost, and choose a variety whose maturity leaves a few weeks of slack. Treat September 22 as a coin flip, not a deadline you can trust.
- Sweetness lost to neighbouring types. No weather is involved here. When su, se and sh2 types tassel at the same time, sweetness is lost, and the kernels turn starchy instead of sugary. The university source gives no separation distance, so none appears here. The rule is about timing: the types must not tassel together. The remedy is to grow one type, or to stagger sowings so their tassels do not overlap. Plant in blocks of three or more rows rather than one long single row, because a block is a shape of garden that lets pollen land where you want it.
Common questions
Is the last frost date the day I can plant sweet corn?
No. May 23 is a mean date at the Winnipeg station, so frost can still come after it in about half of years. Soil at 4 inches also warms later than the air. Plant when the thermometer says the soil is ready for your type, not when the calendar says the frost has passed.
Do the Winnipeg dates apply to my part of Manitoba?
Only roughly. Both frost dates come from the Winnipeg Richardson Int’l A station, which sits at an airport outside the city. Northern and upland districts run weeks behind it, and so does any low spot where cold air pools. If you garden away from Winnipeg, shorten your expectations and favour the quicker varieties.
How do I know an ear is ready to pick?
Look for three signs together: dry brown silks, husks that are still moist and green, and milky juice when you puncture a kernel. Counting roughly 17 to 24 days from silk emergence tells you which week to start checking. The 60 to 90 days from planting is a different clock and does not tell you when to pick.
How long will picked corn stay sweet?
Less long than most people hope. Clemson Extension says corn can be stored under optimal conditions for about five days but “will lose sweetness with each day of storage.” Heat speeds the loss. Standard su types fade fastest, se types hold sugar longer, and sh2 holds it longest, so the variety you chose in spring decides how forgiving your kitchen schedule will be in August.