In New Brunswick, sweet corn is more often undone by the calendar than by hungry animals. Soil that is still too cold in spring, a season that closes before a late variety finishes, and two corn types tasselling together all cost the crop, the last by stripping out the sweetness. The sources behind this page carry no verified figures on insect or wildlife pressure, so what follows sticks to the timing problems that are measured.
The short answer
Soil reaches 60 F (about 16 °C) later than the last frost, and sweet corn waits for the soil. Utah State University Extension sets the clock at soil above 60 F and 60 to 90 days from planting. At the Moncton station, Environment and Climate Change Canada’s climate normals put the last spring frost on average on May 23 and the first fall frost on average on October 2. That leaves roughly 132 days between the two dates. Both a 60-day and a 90-day corn fit inside that stretch on paper, but the soil takes a bite out of it before the first seed goes in.
Why the frost date is the wrong starting gun
A frost date says when freezing air stops threatening seedlings. It says nothing about the seed bed. Clemson Cooperative Extension gives the number gardeners need: “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 Clemson factsheet is worth a read in full.) Keep the depth in mind. A thermometer pushed 4 inches down reads cooler than the surface on a sunny afternoon, and the surface reading would send you out too early.
The two thresholds do different jobs. Ordinary sweet corn can go in at 50 F, while extra-sweet (sh2) types want 65 F. A supersweet seed sown into 50 F soil tends to rot rather than sprout, which explains the gardener who succeeded one year and failed the next after switching varieties. No calendar date replaces the thermometer. The planting dates for the province are worked through in When to Sow Sweet Corn in New Brunswick.
Treat May 23 as the middle of a window, not a safe date. It is a mean, so in about one year out of two the last frost lands later. October 2 works the same way in the other direction: half the years, the first fall frost does not wait for it. Both figures come from Moncton, and a province is not one climate. Northern and upland districts run weeks behind that station. The Canadian normals also use an older averaging period than the US figures used elsewhere on this site, so don’t compare them date for date.
Summer heat is not the constraint. Moncton A averages 1.0 day a year reaching 32 °C, and 4.6 days a year above 30 °C. Those count extreme days only. What matters to corn is the length of the frost-free stretch and the warmth of the summer as a whole, and the stretch is what the next section measures.
Does a late variety fit here?
The test is simple arithmetic: take the roughly 132 days between the two mean frost dates, subtract the weeks the soil needs to reach its threshold, and see what remains. Moncton is the reference, so a garden in a cooler district should assume less.
A 60-day variety
Sixty days against 132 leaves about 72 days of slack on the averages. That slack absorbs a slow spring, a first sowing lost to cold, and a replant with time to spare. Even if the soil takes a month or more past the last frost to warm, an early type still finishes well before October. It is the safe choice for a cool garden and the sensible one for anyone planting a first block. Early corn gives up little in flavor if the genotype is right, and a few weeks of margin is worth more than a few extra inches of stalk.
A 90-day variety
Ninety days against 132 leaves about 42 days, and the soil spends them first. If it takes six weeks past the mean last frost to reach 60 F (65 F for extra-sweet), the margin is gone. Since the first fall frost comes before October 2 in roughly half the years, the cushion is thin even when everything goes right. At Moncton, a 90-day corn fits only if it is sown at the very start of the warm-soil window, with no room for a replant. In northern or upland districts, which run weeks behind that station, the late one does not fit. Whether a given ear will actually finish is a separate question, taken up in Will Sweet Corn Ripen in New Brunswick?
| Variety length | Days from planting | Days left between mean frosts | Verdict at the Moncton station |
|---|---|---|---|
| Early | 60 | About 72 | Fits, with room to replant |
| Late | 90 | About 42, before the soil wait | Fits only if sown early; not for cooler districts |
What goes wrong here
Three failures account for most of the lost crops, and only two of them come from the weather.
- Seed sown into cold soil. The seed sits, rots, and leaves gaps in the row. The remedy is a thermometer at 4 inches, not a date on the calendar: wait for at least 50 F for ordinary types and 65 F for extra-sweet. If a sowing fails, replant with an early variety, since the season has room for it.
- A variety too late for the season. A 90-day corn sown after a slow spring runs into the first fall frost before the ears fill. The remedy is to match length to the district: choose 60 days where springs are late or the frost comes early, and count back from October 2 as a midpoint, never a guarantee.
- Sweetness lost to neighbors. This one has nothing to do with climate. When su, se and sh2 types tassel at the same time, pollen crosses and the sugar is lost. The source publishes no separation distance, so no figure is offered here. The remedy is timing: grow one type, or sow types so they do not tassel together. Plant in blocks of three or more rows, not a single long row, so that pollen reaches the silks.
The third item is easy to miss because the plants look healthy. Stalks grow, ears fill, and the corn simply tastes starchy. Gardeners often blame the weather or the soil, when the cause was a neighbor’s planting date. Write down the genotype and the sowing date, and the pattern shows up the following year.
Common questions
Is the last frost date the day to plant sweet corn?
No. The mean last frost at the Moncton station is May 23, and soil at 60 F comes well after it. In about one year out of two, the last frost lands later than that mean. Plant by soil temperature at 4 inches. For the whole picture, see Growing Sweet Corn in New Brunswick.
Do hot summers hurt sweet corn in New Brunswick?
Rarely. Moncton A averages 1.0 day a year reaching 32 °C and 4.6 days above 30 °C. Those are counts of extreme days, not a measure of the season, and the length of the frost-free period matters far more.
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 ideal conditions, not a promise. Genotype matters too: su loses sugar fastest, se holds it longer, and sh2 holds it longest.
How do I know when to pick?
Look for three signs together: dry brown silks, husks still moist and green, and milky juice when you puncture a kernel. One sign alone misleads. Picking is also a different clock from planting. The 60 to 90 days run from planting and tell you whether a variety fits the season, while the days counted from silk emergence (17 to 24 of them) tell you which week to pick.