Sweet corn in Maine is ready roughly 60 to 90 days after planting, and planting waits for soil near 60 F, which arrives well after the last frost. At Portland that frost falls between April 22 and May 1. The first fall frost follows between October 9 and October 21. A pot of water on the stove is a good idea for another reason: once picked, the sugar starts leaving the kernel right away.
The short answer
Soil sets the start, not the frost date
Soil at 60 F is a different milestone from the last frost, and it comes later. The frost date describes the air. Soil lags behind it, especially in Maine, where cold nights and cool spring rain keep the top few inches chilly for weeks after the freeze risk has passed. The Portland range of April 22 to May 1 comes from NOAA NCEI climate normals and marks the band where a 32 F freeze is 30 to 70 percent likely. No date for 60 F soil is published here, and that is fine. A soil thermometer gives a better answer than any calendar.
Clemson Cooperative Extension is specific about 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 two numbers are not interchangeable. Ordinary sweet corn tolerates the lower floor, while an extra-sweet (sh2) seed sown into 50 F soil tends to rot instead of sprouting. A surface reading runs warmer than the 4-inch depth, so taking it there would send you out too early. Utah State University Extension puts the general working figure at soil above 60 F, which sits between the two.
Two clocks, two questions
Once the seed is in, two separate clocks start. The first runs 60 to 90 days from planting and tells you whether a variety fits the season at all. The second runs about 17 to 24 days from silk emergence and tells you which week to pick. Don’t merge them. The first is a planning number, and the second is something you watch happen on the plant.
Ripeness takes three signs together: dry brown silks, husks still moist and green, and milky juice when you puncture a kernel. Any one alone misleads you. Silks can brown while the kernels are still watery, and a thumbnail test on a single ear tells you little about the rest of the row.
After picking, the fade begins. Clemson’s extension service 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. Portland averages only 3.8 days a year at 90 F or higher, so a warm car trunk or a sunny counter is a bigger threat than the weather. Standard su types lose sugar fastest, se types hold it longer, and sh2 types hold it longest. That gap is the practical reason to pick one genotype over another.
Does a late variety fit here?
It depends on how far from the coast you garden. The Portland dates come from the mild end of the state, and inland and northern Maine run substantially later in spring and earlier in fall. For the sowing calendar itself, see When to Sow Sweet Corn in Maine. Here the question is only whether the days add up.
A 60-day variety
This one fits almost anywhere in the state. Count back 60 days from the earliest first fall frost, October 9, and you land around August 10. That is the paper deadline at Portland, and it leaves a huge amount of room. Even a gardener who waits patiently for 4-inch soil to reach 60 F, or 65 F for an extra-sweet type, can plant well inside it.
The paper deadline is not a recommendation, though. Days-to-maturity figures on a seed packet assume decent warmth, and cool autumn nights stretch them. A corn crop planted at the far end of the window ripens slowly into shortening days. Treat the early part of the window as the target and the August date as the last-ditch limit.
A 90-day variety
At the coast, a 90-day variety fits with room to spare. Take May 1, the late end of the frost range, and add 90 days. You arrive near the end of July, about ten weeks before the earliest first fall frost. That cushion has to absorb the wait for warm soil, but it is generous.
Inland and in northern Maine, don’t assume it fits. Spring comes later there and fall comes earlier, so the cushion shrinks from both ends, and a 90-day variety may not mature before frost. Frost dates for those areas are not published here, so check the dates for your own town and subtract. If the margin is under a couple of weeks, choose something earlier. A shorter-season ear picked at its peak beats a late one frozen in the milk stage.
What goes wrong here
Three failures account for most disappointing sweet corn in Maine. The first two come from the climate, and the third has nothing to do with it. Cold soil and short seasons also leave the plants open to wildlife and insects, covered in What Eats Corn in Maine, and When.
- Seed sown into cold soil. A warm week in spring tempts people to plant, but the 4-inch soil may still be under 50 F. Seed sits, rots, or comes up thin and uneven. Remedy: push a thermometer 4 inches down and plant only at the right reading. That is 50 F at the very least for ordinary types, 60 F as the general working figure, and 65 F for extra-sweet. The frost date is not the signal.
- A variety too late for the season. A 90-day type planted after a slow spring can run into the first fall frost, which arrives between October 9 and October 21 at Portland and earlier inland. Remedy: add the days-to-maturity figure to your actual planting date, compare it with the frost date for your town, and drop to a shorter variety if the margin is thin.
- Sweetness lost to cross-tasselling. This failure is not about climate. If su, se and sh2 types tassel at the same time, pollen crosses between them and the sweetness is lost. The university source gives no separation distance, so none is offered here. The condition is timing: the types must not tassel together. Remedy: grow a single type, or stagger plantings so that different types shed pollen in different weeks. Plant in blocks of three or more rows instead of one long line, because a block pollinates itself far better than a single row does.
The third failure is the sneakiest, because the corn grows beautifully and the kernels come out starchy. Nothing looks wrong until the first bite. For a wider look at the crop in this state, start with Growing Sweet Corn in Maine.
Common questions
How do I know an ear is ready to pick?
Look for three signs together: the silks are dry and brown, the husk is still moist and green, and a punctured kernel gives milky juice. Picking usually falls about 17 to 24 days after the silks first appear. That window is a guide to when to start checking, and the three signs make the decision.
How long does picked corn stay sweet?
Clemson Cooperative Extension says sweet corn “can be stored under optimal conditions for about five days, but will lose sweetness with each day of storage.” Five days is the best case. Heat speeds the loss, so cool the ears fast. Standard su types fade quickest, and sh2 types hold sugar longest.
Can I plant as soon as the last frost has passed?
No. Soil reaches 60 F well after the last frost, which at Portland falls between April 22 and May 1. Measure at 4 inches deep and wait for the reading. Extra-sweet types need 65 F.
Can I grow two types of sweet corn in one garden?
Yes, as long as they do not tassel at the same time. The source names no safe distance, so plan on timing and on keeping each type in its own block of at least three rows. Mixing su, se and sh2 pollen is what costs you the sweetness. Corn from a neighbor’s garden is a separate matter, since the source gives no figure for outside pollen either.