Mostly, the calendar. In Ontario, three things eat into a sweet corn crop: cold soil in spring, a fall frost that arrives before a late variety is done, and a neighbouring type that tassels at the same moment as yours and drains the sweetness from the kernels. Insect and animal pests are a separate subject. No pest calendar is published for this page, so none is invented here. What follows is the timing that decides whether the seed sprouts, the ears fill, and the sugar stays.
The short answer
Soil reaches 60 F (about 16 °C) later than the last spring frost, not on the same day. At Toronto Pearson airport, the reference station for this page, Environment and Climate Change Canada puts the last spring frost on average at April 30. “Average” matters here. It is the midpoint of a window, and in one year out of two the final frost lands after it. Soil also lags behind air. A mild night in early spring tells you nothing about the ground at the depth where the seed sits. How much later the soil gets to 60 F is not published here, and it changes from one year to the next. A thermometer beats a date.
Clemson Cooperative Extension gives two soil thresholds, and they are not interchangeable. Its wording: “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.” (Clemson Cooperative Extension, Sweet Corn). So 50 F (10 °C) is the floor for ordinary sweet corn, and 65 F (about 18 °C) is the floor for extra-sweet types. The 60 F figure from Utah State University Extension sits between them. Keep the depth in mind too: four inches, about 10 cm. A reading taken at the surface runs warmer and would send you out too early. The sowing side of this is covered in When to Sow Sweet Corn in Ontario.
Then comes the second question: do 60 to 90 days from planting fit before autumn? The first fall frost averages October 16 at the same station. Count the days between the two average dates and you get about 169. That is simple arithmetic on two midpoints, not a published growing season, but it shows the shape of things. Even a 90-day variety fits on paper, with roughly eleven weeks to spare before the soil’s slow warming is subtracted. Those 60 to 90 days run from planting, not from the last frost, and that difference is exactly where gardeners lose weeks.
Three cautions keep that picture honest. The October 16 date is also a mean, so in one year out of two the first frost arrives earlier. The figures come from one airport station, and a province is not one climate: northern and upland districts run weeks behind it. And Canadian normals are a 30-year average that stops about a decade before the one used for the US pages on this site, so the two are not strictly comparable.
Heat is not the limiting side of the equation. Pearson averages 15.8 days a year above 30 °C and 6.08 days above 32 °C. Those are counts of scorching days, not a measure of the growing season. What decides the outcome for corn here is the length of the frost-free stretch. For the wider picture, see Growing Sweet Corn in Ontario.
Does a late variety fit here?
Short answer for a Pearson-like climate: yes, with a margin that depends on how long you wait for warm soil. The longer answer is in the arithmetic below. Both examples use the same two averages, April 30 and October 16, and the same wait for warm soil that nobody can predict in advance.
A 60-day variety
Take the 169 days between the two mean frost dates and subtract 60. That leaves about 109 days of slack, a little over 15 weeks. The soil would have to stay below 60 F for more than 15 weeks past the average last frost before this variety ran out of season. That does not happen in a normal year. A 60-day type fits comfortably, even in a cold, late spring, and it leaves room to stagger plantings so that ears do not all arrive in one weekend.
Staggering has a second payoff, covered below: plantings that tassel at different times are the simplest way to keep sweet corn types from crossing. The early variety is the forgiving choice, and for a first season it is the one to pick. It also suits cooler districts where the frost-free stretch is shorter than at the airport.
A 90-day variety
Subtract 90 from 169 and the slack shrinks to about 79 days, roughly 11 weeks. On a mean year, then, a 90-day type fits here. But the soil has a vote. If you hold out for 65 F because the variety is extra-sweet, the wait comes out of those 11 weeks. Add the fact that the first fall frost arrives before October 16 in one year out of two, and the margin looks thinner than 79 days suggests. A cool spring followed by an early frost can leave ears unfinished.
In northern or upland districts, running weeks behind the airport, the late variety does not fit reliably. Take the same sum with your own local dates: if soil warming and the first frost leave fewer than 90 days, pick a shorter type. The question of whether ears will finish is picked apart further in Will Sweet Corn Ripen in Ontario?. When in doubt, shorter wins, because a variety that finishes a week early costs you nothing and one that finishes a week late costs you the crop.
What goes wrong here
Three failures account for most disappointing ears in Ontario. Two are caused by the climate. The third has nothing to do with it.
- Seed sown into cold soil. The air feels like spring, but the ground at 4 inches is still under the threshold. Ordinary sweet corn needs at least 50 F (10 °C) there, and extra-sweet types need 65 F (about 18 °C). A supersweet seed sown at 50 F rots rather than germinates, which is why a gardener can succeed one year and fail the next after changing variety alone. Remedy: push a thermometer 4 inches into the soil and wait for the number that matches your type. Do not use the last frost date as a substitute, because soil warms well after it.
- A variety too late for the season. Sixty to ninety days from planting must fit before the first fall frost, and that frost comes before the October 16 average in one year out of two. The late type that works at Toronto Pearson may not work in a cooler district. Remedy: count back from your local frost, subtract the days the soil keeps you waiting, and choose a variety whose days to maturity sit inside what is left. If the sum is close, choose the shorter one.
- Sweetness lost to crossing. This one is not climate. Sweetness is lost if su, se and sh2 types tassel at the same time, and the starchy, tough kernels that result look like a failed crop. Utah State University Extension publishes no isolation distance, so none is given here. The condition is timing: the types must not tassel together. Remedy: grow one type, or sow the different types at separate times so they tassel apart. And plant in blocks of three or more rows rather than one long row, which is a shape of garden, not a date.
Notice what is not on the list. Hot weather did not make it. A count of 6.08 days a year above 32 °C at the airport station describes a handful of extreme afternoons, not a season hostile to corn. The real hazards are the ones above, and each of them can be avoided with a thermometer, a calendar and a plan for who tassels when.
Common questions
Can I plant sweet corn once the last frost has passed?
Not on that signal alone. The average last frost at Toronto Pearson is April 30, but the soil reaches 60 F later, and extra-sweet types want 65 F at the 4-inch depth. Plant by soil temperature, not by the frost date.
What insects and animals eat sweet corn in Ontario, and when?
No pest calendar is published on this page, so no dates or species timings are given here. Your local extension office or a nearby garden centre can tell you which pests are active in your district this season. What this page covers is the timing that costs you ears before any pest gets to them.
How fast does sweet corn lose its sweetness after picking?
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.” Five days is a ceiling under optimal conditions, not a promise. Type matters too: su loses sugar fastest, se holds it longer, and sh2 holds it longest, which is a practical reason to choose one over another.
How do I know when to pick?
Two clocks answer two different questions. The 60 to 90 days from planting tell you whether a variety fits the season. The 17 to 24 days from silk emergence tell you which week to pick. Then check three signs together: dry brown silks, husks still moist and green, and milky juice when you puncture a kernel. One sign alone can mislead you.