Sweet corn is ready 60 to 90 days after planting, and the planting day is set by the soil, not the frost calendar. The soil has to climb above 60 F, and in the Central Valley, Northern California and the Sierra that happens well after the last freeze. Along the coast and in Southern California there is no freeze to wait out at all. Once the ear is picked, a second clock starts. Sugar drains from the kernel, and heat speeds the drain.
The short answer
Soil temperature is the start line
Utah State University Extension puts the threshold for sweet corn at soil above 60 F. That day arrives later than the last frost, never on it. Warm air in March can fool you, but the soil is slower than the air, and corn seed sits in the soil. No calendar date for 60 F soil is published here, and any date would be a guess. The instruction is to measure.
Clemson Cooperative Extension tells you where to measure: “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.” Those are two different thresholds. A surface reading runs warmer than the 4-inch depth, so push the thermometer down before you trust it. The When to Sow Sweet Corn in California page works through the sowing decision in more detail.
Does 60 to 90 days fit before autumn?
Across much of the state, yes, and easily. NOAA’s climate normals for the Los Angeles reference station list a frost-free season of 365 days, and its daily records show zero days at or below 32 F in 30 years. No freeze occurs there in an average year (NOAA NCEI data). Coastal and southern California has no first fall frost to beat, so the season never closes on you. The limits there are heat, drought and water restrictions. The same station averages 2.4 days a year at 90 F or more, a mild heat load, though it is a coastal reading and says little about inland valleys.
That reading cannot stand in for the whole state. Northern California, the Central Valley and the Sierra all freeze, so there the arithmetic is real: count 60 to 90 days forward from the day your soil crosses the threshold and see whether you land before the first fall freeze. That frost date is not published here.
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 silk emergence, tells you which week to pick. Don’t merge them into one number.
On the day, trust three signs together: dry brown silks, husks still moist and green, and milky juice when you puncture a kernel. One sign alone misleads.
Then the fade begins. Clemson says it 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.” Five days is a ceiling under ideal conditions, not a promise. Genotype changes the speed. Standard su types lose sugar fastest, se holds it longer, and sh2 holds it longest.
Does a late variety fit here?
A 60-day variety
Almost everywhere in California, yes. In the frost-free coast and south, nothing ends the season, so 60 days fits with room to spare. Succession sowings can run on, with the soil temperature as the only gate. In the Central Valley, Northern California and the Sierra, 60 days is the safest bet because it leaves the most slack before the first fall freeze. Work the count from your own soil date, not from a seed packet’s optimism.
A short crop also suits a state where water is the scarce input. Fewer days in the ground means fewer weeks of irrigation. Corn is a thirsty crop, and a 60-day planting asks for a month less of it than a 90-day one. The Growing Sweet Corn in California overview covers the water side in more depth.
A 90-day variety
It depends on where you garden, and in one place the answer is no. On the frost-free coast and in Southern California, a 90-day variety fits. No freeze cuts it short, and the cost is three more months of watering and a longer window for pests (see What Eats Corn in California, and When).
In the Central Valley and Northern California, it fits only if your count works. Take the day your soil reaches the threshold, add 90 days, and compare the result with your first fall freeze. If you grow an extra-sweet type that needs 65 F soil, start the count later, because that soil warms later. Where the sum overshoots the freeze, the late variety does not fit, and you should drop to an earlier one.
At high Sierra elevations, the late variety does not fit. The soil warms late and the freeze comes early, so 90 days will almost certainly run out of season. Choose the earliest variety you can find, and expect a tight finish even then.
What goes wrong here
Three failures account for most disappointing corn in California. Two come from the climate and the calendar. The third has nothing to do with either.
- Soil sown too cold. Mild air tempts gardeners to plant while the 4-inch soil is still below the threshold. An extra-sweet seed sown at 50 F rots rather than germinates, which is why the same gardener can succeed one year and fail the next after switching varieties. Remedy: push a thermometer 4 inches down, wait for 50 F for ordinary corn and 65 F for extra-sweet, and plant on the reading, not the date.
- A variety too late for the season. This hits freeze zones, not the frost-free coast. A 90-day variety started when the soil finally warms can run into the first fall freeze before the ears fill. Remedy: count forward from your soil date to your first fall freeze before you buy seed, and choose a shorter variety if the count fails.
- Sweetness lost because two types tasselled together. Sweetness is decided by what tassels nearby, not by climate. Utah State University Extension warns that sweetness is lost if su, se and sh2 types tassel at the same time. The source gives no isolation distance, so none is offered here. Remedy: do not let different types shed pollen in the same stretch. Stagger the sowings so the tassel windows do not overlap, or grow one type per season.
Planting so the types stay apart
The timing fix sounds fussy but is simple in practice. If you want a standard sweet corn and an extra-sweet one, sow them far enough apart in time that one has finished shedding pollen before the other starts. Keep a note of each planting date and watch for tassels.
Plant in blocks of at least three rows. That is Utah State’s guidance, and it is a shape for the garden, not a date. Corn is wind-pollinated, so one long single row pollinates poorly and leaves gaps in the ears. A square or rectangular block puts the tassels upwind of the silks and fills the ears out.
Common questions
Can I plant sweet corn in winter in Southern California because there is no frost?
Absence of frost does not mean warm soil. The instruction is the 4-inch soil temperature: at least 50 F for ordinary sweet corn, 65 F for extra-sweet, and above 60 F per Utah State. No calendar date is published here. A winter planting in cold soil can rot even where the air never freezes.
How do I know an ear is ready to pick?
Look for three signs at once: dry brown silks, husks still moist and green, and milky juice when you puncture a kernel. Counting 17 to 24 days from silk emergence tells you which week to start checking. The 60 to 90 days from planting is a different clock and only says whether the variety fits your season.
How long does picked corn stay sweet?
Less long than most people hope. Clemson says it can be stored “for about five days” under optimal conditions, and that it “will lose sweetness with each day of storage.” Heat speeds the sugar loss. Among the genotypes, su fades fastest, se holds longer, and sh2 holds longest, which is the practical reason to choose an extra-sweet type if you can’t eat the corn right away.
Do I need blocks, or can I plant one long row?
Plant blocks of three or more rows, per Utah State University Extension. This is advice about the shape of the garden. It has no bearing on when to sow, and it works alongside the timing rule that keeps su, se and sh2 types from tasselling together.