Sweet corn is ready roughly 60 to 90 days after planting, and the exact day comes from the plant: pick when the silks are dry and brown, the husks are still moist and green, and a punctured kernel gives milky juice. The sugar then starts draining out of the kernels almost immediately. This page covers when to expect that window, whether a late variety fits, and what ruins a crop even when the calendar works. For planting itself, see Growing Sweet Corn in British Columbia.
The short answer
Soil reaches 60 F (about 16 °C) later than the last spring frost. Sometimes much later. At Vancouver Int’l A, the Environment and Climate Change Canada station used as the reference here, the last spring frost falls on average on March 18. That is a mean, the midpoint of a window. In one year out of two the frost lands after it, so it is not a safe date. And the soil date is not published here at all, because frost and soil warmth are two different events that happen to share a spring.
Soil first, calendar second
Utah State University Extension says sweet corn needs soil above 60 F and 60 to 90 days from planting. Clemson Extension gives the finer print: “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 in mind. A thermometer pushed 4 inches into the bed reads cooler than a surface reading on a sunny afternoon, and that cooler number is the honest one. Ordinary sweet corn has a floor of 50 F, extra-sweet types need 65 F, and 60 F is the general benchmark in between. Use the thermometer, not a date.
The first fall frost at the same station falls on average on November 10, and half of all years it arrives earlier. Between the two means sit roughly 237 days. Even after soil warms well past the last frost, a 60 to 90 day crop has room at the reference station. Airports run warmer or cooler than the neighbourhoods around them, though, and a province is not one climate: northern and upland districts run weeks behind Vancouver. The Canadian normals also use an older 30-year period than the US figures elsewhere on this site, so the two are not a like-for-like comparison. For sowing dates tied to local conditions, see When to Sow Sweet Corn in British Columbia.
Two clocks, two questions
The 60 to 90 days from planting tell you whether a variety fits the season. A separate clock, 17 to 24 days from the first silk emergence, tells you which week to pick. Do not merge them. The first is a purchasing decision made in spring, the second a daily walk down the row in summer.
How fast it fades
Clemson Extension puts the mechanism 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.” Read “about five days” as a ceiling under ideal conditions, not a promise. Heat speeds the loss, which is the real reason behind the old advice to run from the garden to the pot. Genotype matters too: su types lose sugar fastest, se types hold it longer, sh2 types longest.
Does a late variety fit here?
Fitting means one thing: the number of days from planting must land before the first fall frost. The arithmetic below uses mean dates from the reference station, so treat the margins as rough, not guaranteed.
A 60-day variety
Counting back 60 days from the November 10 mean puts the outer edge of the calendar around the second week of September. That is a ceiling, not a recommendation. A crop sown that late would be racing cooling soil and shortening days, so the useful reading is the other direction. Sown once the soil has passed 60 F, a 60-day variety has plenty of slack at Vancouver Int’l A. That slack lets you stagger several sowings, which stretches the harvest and also lets you keep different sweetness types from tasselling together (more on that below). It fits comfortably.
Summer heat is not what limits this crop. The same station counts only 0.29 days a year above 30 °C, and 0.07 days a year above 32 °C. Those are counts of extreme days, not a measure of the growing season. They say nothing about whether corn finishes. What decides that is the length of the frost-free period and the summer means, and the frost-free stretch above is long.
A 90-day variety
Counting back 90 days from November 10 lands around the middle of August. A 90-day variety must be in the ground by then to finish before the mean first frost, and at this station soil is warm enough for corn well before that. So the late variety fits at the reference station, with weeks to spare if you sow once the soil allows.
The answer changes with the district. Where spring comes weeks later and fall frost arrives earlier, the margin that looks generous at Vancouver Int’l A can shrink to nothing, and a 90-day variety may simply not fit. If your garden sits in a northern or upland district, check local frost dates before buying seed, or choose a shorter variety. And remember that half of all years bring frost before the mean, so a variety that finishes with only days to spare is a gamble.
What goes wrong here
Three failures account for most disappointment. Two come from the season and one has nothing to do with climate. The pests that arrive once the ears fill are covered separately in What Eats Corn in British Columbia, and When.
- Soil sown too cold. A mild March frost date tempts people outdoors, but frost-free is not the same as warm. Seed in cold soil sits and rots rather than germinating. Extra-sweet (sh2) seed is the least forgiving: Clemson’s threshold of 65 °F at the 4-inch depth is 15 degrees above the 50 °F floor for ordinary corn. That is why a gardener can succeed one year and fail the next after switching variety, with the weather unchanged. The remedy is a soil thermometer at 4 inches, taken in the morning, and a plan to wait until it holds the right number for the type you chose.
- A variety too late for the season. A 90-day label with a sowing date near the middle of August leaves no room for a cool, slow summer or an early frost. Count forward from planting to the first fall frost mean for your district, not for Vancouver, and subtract a margin. If the count is tight, switch to a shorter variety, and sow once the soil is warm rather than on the first mild weekend.
- Sweetness lost to the neighbours. This one is not about climate at all. Utah State University Extension warns that sweetness is lost if su, se and sh2 types tassel at the same time, and that corn should go in blocks of three or more rows so pollen can reach the silks. The source gives no isolation distance, so none is offered here. The remedy rests on timing and layout: keep different types from tasselling together by sowing them in separate plantings, and plant each type in a block rather than a single long row. If two types already share a bed, expect the sweetness to fall short of what the seed packet promised.
Common questions
Is the last frost date the day to plant sweet corn?
No. The mean last frost at Vancouver Int’l A is March 18, but soil reaches 60 F well after that, and extra-sweet types want 65 °F at 4 inches. Frost is about air temperature, while germination depends on soil. Plant by the thermometer, not the date.
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. Any one alone can mislead. As a guide to timing, the harvest window opens roughly 17 to 24 days after silks first appear.
How long does picked corn keep its sweetness?
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 upper limit, not a guarantee. Heat speeds the loss, and su types fade fastest, se types more slowly, sh2 types slowest.
Can I plant different sweet corn types side by side?
Only if they will not tassel at the same time. The condition from Utah State University Extension is about timing, and no separation distance is published. Sowing the types in different plantings, and growing each in a block of three or more rows, protects the sweetness.