Sweet corn is ready 60 to 90 days after planting, depending on the variety, and in Alabama that count does not start on the last frost date. It starts when the soil warms. Picking starts a second clock: sugar drains from the kernels, and heat speeds the loss. The long Alabama season leaves room even for a late variety. Its summers decide how sweet the ear tastes by dinner. The full growing picture is in Growing Sweet Corn in Alabama.
The short answer
Soil reaches 60 F well after the last frost, and the two are not the same day. For the reference city, NOAA NCEI Climate Normals put the last spring freeze between March 19 and April 3 (the 30-70% probability band for 32 F, from an airport weather station). Gardens in suburbs and the countryside usually see that last frost later, and the first fall frost earlier, often by one to two weeks. Elevation widens the gap. Treat those dates as the reference-city figure, not a state-wide promise, and check your own county.
Soil first, calendar second
Utah State University Extension says sweet corn needs soil above 60 F. Clemson Extension gives two thresholds instead of one: “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 depth matters. A reading at the surface is warmer than the seed will ever feel, and it sends people out too early.
The two numbers are not interchangeable. Ordinary sweet corn will start at 50 F. An extra-sweet (sh2) seed sown into soil at that temperature tends to rot instead of germinating. That explains a common puzzle: the same gardener has a perfect stand one year and a patchy row the next, after nothing changed but the seed packet. No date for when Alabama soil crosses these marks is published here, and a date would be a different claim anyway. A soil thermometer pushed 4 inches down is the instruction. The sowing window itself is covered in When to Sow Sweet Corn in Alabama.
Two clocks, two questions
The 60 to 90 days run from planting. They tell you whether a variety fits the season at all. A different count, 17 to 24 days from silk emergence, tells you which week to pick. Keep them apart. One is a planning number, the other a harvest number.
When the week arrives, look for three signs together: dry brown silks, husks that are still moist and green, and milky juice when you puncture a kernel. Brown silks alone can fool you, and so can a plump-looking husk. All three at once is the test.
How fast it fades
Clemson Cooperative Extension spells out the mechanism: “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 the last clause twice. About five days is a ceiling under ideal conditions, not a guarantee, and sweetness slips every day along the way.
Heat is the accelerator. The reference station at Huntsville averages 58.5 days a year at 90 F or more, so an ear picked on a summer afternoon starts losing sugar the moment it comes off the stalk. The genotype changes the pace. Standard su types lose sugar fastest, se types hold it longer, and sh2 types hold it longest. That is the practical reason to choose one over another, especially if the corn will not go straight from garden to pot.
Does a late variety fit here?
On the calendar, yes, with one condition. The gap between the last spring freeze and the first fall freeze at the reference city runs well over six months. The first fall frost there falls between November 2 and November 16, again per NOAA NCEI, and again earlier in many gardens. The soil needs to warm first, but even after that wait, both ends of the 60 to 90 day range have room.
A 60-day variety
An early type fits with plenty to spare. Sown after the soil passes its threshold, it finishes long before the first fall frost, even when your county’s frost comes one to two weeks ahead of the airport’s. The spare weeks are useful. They allow a second planting, provided it is timed so the two plantings do not tassel together (more on that below).
The catch is heat, not frost. The days at 90 F or more arrive in summer, and later sowings will be filling ears right through them. Sugar loss at high temperatures is the same mechanism Clemson describes for stored corn, so pick promptly and cool the ears fast.
A 90-day variety
A late type fits too, if it goes in when the soil warms in spring. From that point, ninety days still ends far ahead of a November frost. Extra-sweet types start a little later still, because they wait for 65 F soil, but the margin is generous.
The late variety does not fit a sowing made in the back half of summer, at least not without arithmetic. Count back 90 days from your own county’s first fall frost, then subtract another one to two weeks, since your garden likely freezes before the airport does. If today’s date falls after that, the 90-day type will not make it. The reference-city dates are a starting point, and the numbers that count are the ones from your own county. A longer reading of the threats during those weeks is in What Eats Corn in Alabama, and When.
What goes wrong here
Three failures account for most disappointing rows. Two come from the climate and one has nothing to do with it.
- Seed sown into cold soil. The last frost has passed, so the bed feels safe, but the soil is not warm yet. Seed sits, rots, and leaves gaps. The remedy is a thermometer at the 4-inch depth: at least 50 F for ordinary sweet corn, 65 F for extra-sweet. If you changed variety this year, change the threshold too.
- A variety too late for the planting date. Spring sowings give a 90-day type plenty of room, but a sowing made late in summer can run into a frost that arrives early in your garden. The remedy is to count 90 days back from your county’s first fall frost and leave the extra week or two as a margin. If the count fails, pick a type with a shorter clock. Remember that the 60 to 90 days measure fit with the season, not the picking week.
- Sweetness lost to the neighbors. No weather causes this one. Sugar is lost when su, se and sh2 types tassel at the same time, so the problem is timing, not climate. Utah State University Extension also says corn belongs in blocks of three or more rows, which is a shape of garden rather than a date. The source gives no isolation distance, so none is offered here. Instead, keep different types from tasseling together by separating plantings in time, and grow each type in its own block.
The third failure is the most frustrating because it looks like bad luck. The corn grows well and the ears fill, then the first bite is starchy. Check what was tasseling nearby, including a neighbor’s patch, before blaming the weather.
Common questions
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 a kernel is punctured. Picking time runs 17 to 24 days from silk emergence, a different count from the 60 to 90 days that start at planting.
Can I plant once the last frost has passed?
Not on that signal alone. Soil at 60 F arrives well after the last frost. Check the temperature at the 4-inch depth: at least 50 F for ordinary types, and 65 F before sowing extra-sweet varieties.
How long does picked corn stay sweet?
Clemson Extension says it can be stored under optimal conditions for about five days, “but will lose sweetness with each day of storage.” Five days is a maximum, not a promise. Su types lose sugar fastest, se hold it longer, and sh2 hold it longest.
Can I plant different sweet corn types side by side?
The published problem is su, se and sh2 types tassel at the same time. No isolation distance is given, so avoid overlap by separating plantings in time, and plant each type in blocks of three or more rows.