In north Florida, sweet corn is ready 60 to 90 days after planting, and it can only be planted once the soil holds above 60 F, a point that arrives well after the last frost. Then the clock turns against you. Sugar starts leaving the kernel the moment the ear is picked, and heat speeds that up. The wider picture is in Growing Sweet Corn in Florida. This page covers the soil, the calendar and the mistakes that cost sweetness.
The short answer
Soil first, calendar second
Soil at 60 F comes later than the last frost. Not the same day, and not a safe guess to treat them as the same. At the Jacksonville airport station, NOAA NCEI Climate Normals put the last spring 32 F freeze in a window running from February 16 to March 7 (that is the 30 to 70 percent probability band). The airport is an inland site, and three of the thirty fall seasons in that record saw no freeze at all. Frost tells you when cold air stops killing seedlings. It says little about how warm the ground is, because soil gives up winter’s chill slowly.
Utah State University Extension puts the corn requirement at soil above 60 F. Clemson Cooperative Extension adds two thresholds worth keeping straight: “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 two are not interchangeable. Ordinary sweet corn tolerates the cooler floor, while a supersweet seed sown at 50 F tends to rot rather than sprout. That is why a gardener can succeed one year and fail the next after changing nothing but the seed packet.
No calendar date goes with any of these numbers. A soil thermometer pushed in to 4 inches is the instruction. A surface reading runs warmer than the depth where the seed sits, and it would send you out too early.
Do 60 to 90 days fit before fall?
Yes, on frost alone, with room to spare. The first fall freeze window at the same station runs from November 26 to December 13. From the latest spring frost date to the earliest fall one is well over eight months, and a 90-day crop needs about three. Count back 90 days from the fall window and you land between late August and mid September, so a spring planting that waits for warm soil is nowhere near the limit. Frost is not what restricts corn here. The sowing window gets its own treatment in When to Sow Sweet Corn in Florida.
Ready, then fading
Two clocks run here, and they answer different questions. The 60 to 90 days from planting tells you whether a variety fits the season. A separate count, 17 to 24 days from the first silks appearing, tells you which week to start checking ears. Do not merge them into one number.
Once the ear is off the plant, Clemson Extension is blunt: “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. The type matters too. The su type loses sugar fastest, se holds it longer, and sh2 longest.
Everything above describes Jacksonville and north Florida. Central and south Florida are effectively frost-free, and there the calendar inverts: the main season runs through winter, and summer is the hard part. Never lay these windows over Orlando or Miami.
Does a late variety fit here?
The answer depends on which end of the 60-to-90 range you pick, and on what the summer does to each. At the Jacksonville reference station, 73.2 days a year reach 90 F or more. That is more than ten weeks of hot days, mostly stacked into summer, and it is the number that drives water demand and heat stress.
A 60-day variety
An early type is the easy case. Sown after the soil passes 60 F, it is ready roughly two months later, and in a spring planting that tends to land ahead of the worst of the summer. On the frost arithmetic alone, a fall sowing also fits: count 60 days back from the November 26 to December 13 window and you reach late September to mid October. That is only the frost math, a limit on how late you can sow, and not an instruction to sow then.
The short season has a second payoff. Corn that matures quickly spends fewer days under water stress, and it gets picked before heat has much chance to speed the sugar loss Clemson describes.
A 90-day variety
A late type does fit north Florida, with a condition. In spring there are well over eight months between the last frost window and the first fall one, so 90 days fits comfortably if you sow when the soil is ready. In fall it fits only if the seed is in the ground between late August and mid September, which means sowing into hot soil and hoping the ears fill before the freeze window opens.
The real problem is a late spring sowing. Push a 90-day type into the back half of the planting season and it silks and fills in the hottest weeks of the year, when every day above 90 F adds stress and the picking window shrinks. If you want a long-season type, plant it as early as the soil allows, and lean toward a type that holds sugar (se or sh2, remembering sh2 wants 65 F at 4 inches).
None of this transfers south. In central and south Florida the main season is winter, so a 90-day crop is judged against summer heat rather than against a fall freeze.
What goes wrong here
Three failures account for most disappointing corn in this climate, and one of them has nothing to do with weather.
- Soil sown too cold. Seed goes in on the strength of a mild week or a frost date, and the ground at 4 inches is still below the line. Ordinary sweet corn needs at least 50 F by Clemson’s floor, and extra-sweet types need 65 F, or the seed sits and rots. The remedy is to read the soil at 4 inches, not the air, and to hold off longer if the packet says supersweet. Keep in mind the last frost date does not mean the soil is warm.
- A variety too late for the season. A 90-day type sown late in spring ripens into the hottest stretch, and one sown late in summer races a freeze that may or may not come. The remedy is to count forward from the sowing date against the first fall frost window of November 26 to December 13, and to choose the shorter type when the count is tight. In central and south Florida, run the same count against summer heat instead.
- Sweetness lost to the neighbor. This failure has no climate behind it. Sweetness is lost if su, se and sh2 types tassel at the same time. The source publishes no separation distance, so do not trust any figure you have heard. The fix is timing: grow one type per planting, or stagger sowings so two types never tassel together. Plant in blocks of three or more rows rather than one long single row, since the shape of the patch matters for pollination.
Insects and birds are a different kind of loss, covered in What Eats Corn in Florida, and When. They take ears the soil and the tassels already got right.
Common questions
How do I know an ear is ready to pick?
Use three signs together, never one alone: the silks are dry and brown, the husks are still moist and green, and a punctured kernel leaks milky juice. For timing, expect the check to start 17 to 24 days after the silks first emerge.
Is the last frost date the day to plant?
No. At the Jacksonville airport station the last freeze window runs February 16 to March 7, but soil reaches 60 F later. Plant by the thermometer at 4 inches, using 50 F as the floor for ordinary sweet corn and 65 F for extra-sweet.
Can I use these dates in Orlando or Miami?
No. These frost windows describe Jacksonville, in north Florida. Central and south Florida are effectively frost-free, so the calendar inverts there: the main season runs through winter, and summer is the hard part.
Is sh2 worth the extra care?
For holding sugar after picking, yes, since sh2 keeps its sweetness longest and su loses it fastest. The cost is a warmer seedbed (65 F at 4 inches) and strict timing against any other type tasselling nearby. A 65 F reading is easy to miss, and a cold-sown sh2 row is the commonest way to lose a whole planting.