Sweet corn in New Brunswick is ready 60 to 90 days after planting, and that count starts only once the soil has warmed, which happens well after the last spring frost. Once the ears are ripe, the sugar starts leaving the kernel, and it leaves faster in warmth and faster in older types. Below: when the soil is ready, whether a long-season variety beats the autumn frost, and where a New Brunswick patch tends to go wrong.
The short answer
Soil at 60 F comes later than the last frost, and the gap is real. Environment and Climate Change Canada’s climate normals for the Moncton station put the average last spring frost at May 23. That is a midpoint, not a safe date: in one year out of two, the last frost lands after it. Air warms faster than ground. A garden can be frost-free for weeks and still hold soil too cold for corn seed.
Utah State University Extension says sweet corn needs soil above 60 F. Clemson Cooperative Extension sets the floor and the extra-sweet threshold separately: “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.” Keep the depth. A reading taken at the surface runs warmer and will send you out too early. The two numbers are not interchangeable either. Ordinary sweet corn tolerates the lower one, while a supersweet (sh2) seed sown at 50 F tends to rot instead of sprouting. That explains the gardener who succeeds one year and fails the next after switching varieties. No calendar date is given here, because the thermometer is the instruction and a date would be a different claim.
Then the 60 to 90 days have to fit before autumn. The same station puts the average first fall frost at October 2, with the same caveat: half of years it comes earlier. From May 23 to October 2 is about 132 days between the two mean dates. Subtract the wait for warm soil (how long that wait runs is not published here) and the usable season shrinks. A 60-day type fits comfortably. A 90-day type is another matter, covered in the next section. Our When to Sow Sweet Corn in New Brunswick page goes deeper on the sowing side.
Two limits on these numbers. Moncton is one station, and a province is not one climate: northern and upland districts run weeks behind it. The Canadian normals also cover a different thirty-year period than the US figures used elsewhere on this site, so don’t line them up side by side. And heat is not the constraint. Moncton A averages 1.0 day a year reaching 32 °C and 4.6 days reaching 30 °C. Those count extreme days, not the length of the season, and season length is what decides corn here.
Picking at the right week
Two separate clocks run here, and they answer different questions. The 60 to 90 days from planting tells you whether a variety fits the season. A second count, 17 to 24 days from the day silks first appear, tells you which week to pick. Use the three ripeness signs together: dry brown silks, husks still moist and green, and milky juice when you puncture a kernel. One sign alone misleads.
After picking, the fade starts. 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. The type matters too: su loses sugar fastest, se holds it longer, sh2 holds it longest. If you pick for the table and eat the same day, su is fine. If ears will sit a while, choose se or sh2, mindful that sh2 wants the warmest soil.
Does a late variety fit here?
It depends on which end of the range you pick, and on how long the soil takes to warm in your own garden. The only fixed points are the two mean frost dates and the 60-to-90-day range from Utah State. No variety is named in either source, so this is arithmetic on the range, not a recommendation of a product.
A 60-day variety
Roughly 132 days separate the average last spring frost from the average first fall frost. A 60-day type uses less than half of that. Even if the soil needs a few weeks past the frost date to reach 60 F, the crop still has room, and room is what you need when half of all autumns bring frost earlier than the mean. It also leaves space for a second sowing a couple of weeks later, which spreads the harvest and avoids a single week of too much corn. For most New Brunswick gardeners, especially away from the warmest southeast, this is the dependable choice.
A 90-day variety
The late one does not fit with any margin. Take the same 132 days and subtract 90: 42 days of slack on paper. Now take out the wait for 60 F soil, which runs past the last frost, and the wait for 65 F if the seed is extra-sweet, which runs later still. Most of the slack goes. What remains is a bet that spring is early and autumn is late, and the October 2 figure is a midpoint, so in about half of years frost arrives before it. Ears caught unfilled by frost do not finish. In an upland or northern garden running weeks behind Moncton, a 90-day variety will not make it. Anyone determined to try could use warmed soil and an early start, but I would not count on a full crop. For a plain answer: in this territory, pick the shorter end of the range.
What goes wrong here
Three failures account for most disappointing patches. Two come from the climate, and the third has nothing to do with it. Pests are a separate list, covered in What Eats Corn in New Brunswick, and When.
- Seed sown into cold soil. A frost-free week feels like permission, but the soil lags. Seed sitting in cold ground rots, and extra-sweet types rot most readily at 50 F. The remedy is to measure at the 4-inch depth, wait for 60 F for ordinary corn and 65 F for extra-sweet, and skip the calendar. A patch sown a week late almost always beats one replanted twice.
- A variety too late for the season. A long-maturing type sown on warm soil can still be filling ears when the first fall frost arrives, and the average date of October 2 means half of years it arrives sooner. The remedy is to count the 60 to 90 days from the day your soil is ready, not from the last frost, and to choose the shorter end of the range. If your garden sits inland or north, assume the season is shorter than Moncton’s.
- Two types tasselling together. This one has no connection to weather. Utah State Extension warns that sweetness is lost when su, se and sh2 types tassel at the same time, because pollen from one type crosses the other’s ears. The published guidance gives no isolation distance, so none is offered here. The condition is timing. The remedy is to grow one type, or to stagger plantings so that different types do not tassel together. Plant in blocks of three or more rows, which is a shape of garden, not a date, so that pollen reaches the silks.
A cold spring explains the first, a short autumn the second, and the third happens in the best weather. That is why a patch can germinate perfectly, ripen in time, and still taste flat. For the bigger picture on the crop, start with Growing Sweet Corn in New Brunswick.
Common questions
Is the soil warm enough once the last frost has passed?
Not necessarily. The average last frost at Moncton is May 23, but soil warms later than air. Check the 4-inch depth: Utah State says above 60 F, and Clemson gives 50 F as the minimum for ordinary corn and 65 F for extra-sweet.
Can I grow a 90-day variety in New Brunswick?
Only with luck. After the wait for warm soil, a 90-day crop has little room before an autumn frost that arrives earlier than October 2 in about half of years. Inland and northern gardens run weeks behind Moncton, so the odds are worse there.
How long does picked corn stay sweet?
Clemson Extension says sweet corn can be stored under optimal conditions for about five days, “but will lose sweetness with each day of storage.” Treat five days as the maximum. Heat speeds the loss, and su loses sugar fastest while sh2 holds it longest.
How do I know an ear is ready to pick?
Look for all three signs together: dry brown silks, husks that are still moist and green, and milky juice when you puncture a kernel. As a guide to timing, ripeness arrives 17 to 24 days after silks first emerge. That count is separate from the 60 to 90 days from planting, which only tells you whether the variety fits your season.