When to Plant Corn: Match the Date to the Type, Not Just the Frost

Sow standard sweet corn outdoors once the soil reaches about 55°F, which in a typical US ZIP code is late April. Sugar-enhanced types wait for 60°F (about May 9) and supersweet types for 65°F (about May 25). In Chicago those dates are April 28, May 11 and May 26; in Dallas they are March 2, March 17 and April 7. For a late sowing, stop about 98 days before your median first frost for a 70-day variety: July 30 in Chicago, August 23 in Dallas.

Most guides give one rule for all corn: sow two to three weeks after the last frost. We tested that against NOAA 1991–2020 normals¹ for 37,563 lower-48 ZIP codes, built spring dates for 28 cities and fall dates for 21, and set the dates beside local dates published by eight extension services and the Nebraska Corn Board. Everything uses air temperature as a stand-in for soil temperature, because our NOAA data has no soil readings.

Spring sowing dates for 28 cities, by type of sweet corn

Sweet corn is not one crop for planting purposes. Iowa State lists five types and says standard (su), sugar-enhanced (se) and synergistic (syn) seed germinates well at soil temperatures of 55 to 60°F, while supersweet (sh2) and augmented supersweet seed is the least tolerant of cold soil.² K-State sets the lines at 55°F for standard, 60°F for sugar-enhanced and 65°F for supersweet seed.⁴ Illinois and the Old Farmer’s Almanac also put supersweet at 65°F,^3,12 and Tennessee says se and sh2 types prefer soil around 65°F.⁵ Each date in Table 1 is the later of our median last-frost date and the day the normal daily mean air temperature reaches the line for that type.

Choosing a type moves your date by a month

Across the 37,563 ZIP codes, the median standard-corn date is April 25, the sugar-enhanced date May 9 and the supersweet date May 25. The supersweet date falls a median 32 days after the standard one, and 14 or more days after it in 99.7% of ZIP codes. Supersweet seed goes in a median 36 days after the median last frost (middle half 32 to 42).

The usual rule does not fit any type. The University of Georgia says to wait until at least two weeks after the last average killing frost,⁷ and the Almanac says two to three weeks.¹² That is later than our standard-corn date in 88.2% of ZIP codes (by a week or more in 70.7%), but earlier than our supersweet date in 99.7% (by a week or more in 98.6%). Frost sets the standard-corn date in 38.0% of ZIP codes, as in Nashville, Atlanta and Dallas, and warmth sets it in the other 62.0%; for sugar-enhanced and supersweet seed, warmth sets it in 98.4% and 100%. Iowa State gives 60°F for supersweet soil;² if you prefer that line, use the sugar-enhanced column.

Corn takes a light frost better than beans or cucumbers

Corn seedlings recover from frost more often than most warm-season crops. Iowa State agronomists say the growing point stays below the soil until the six-leaf stage, and that after a moderate freeze, recovery:¹³

usually is rapid and almost complete

Recovery slows when air temperatures stay below normal afterward, and soft rot can follow. That is why published frost rules range from permissive to cautious. K-State puts the earliest sowing at

one week before the average frost date in your area

while Illinois says early standard corn goes in at or just before the mean frost-free date,³ Tennessee says some gardeners seed slightly before it and plan on seedlings emerging after the final frosts,⁵ and Georgia warns that too-early planting can leave weak stands and frost-killed seedlings.⁷ We use the median last frost as the floor. In 89.4% of ZIP codes our standard-corn date falls before the 10% last-frost date, which comes a median 16 days after the median one, so keep a cover handy; see our frost-cover guide.

Cold soil is the bigger risk. Purdue’s newsletter says corn needs about 115 growing degree days to emerge, so in soil averaging 50°F emergence can take upwards of 35 days, while in 65°F soil it can occur in 7 days or less.¹⁵ That is field-corn research, but seed in cold, wet soil rots the same way: Iowa State warns of seed rot and notes that most seed is treated and dyed pink.² Air is not soil, so check the soil at planting depth.

Temperature rules compared across ten sources

Table 2 lines up what each publication says. The soil lines cluster at 55, 60 and 65°F by type, the frost-date rules span a month, from a week before to two or three weeks after the last frost, and days to maturity run from 60 to 100.

Starting indoors, covers and seed treatment

Direct sowing is standard. The Almanac says starting corn indoors gives a three-to-four-week head start in the North, in 4-inch biodegradable pots, with seedlings at least 6 inches tall and soil warm before transplanting;¹² our seed-starting guide lists other crops. Illinois says early standard corn can go in at or just before the mean frost-free date unless you use clear polyethylene mulch to warm the soil,³ and the Almanac suggests black plastic in the North.¹² The dates in Table 1 assume no cover.

Succession sowing and keeping types apart

Iowa State suggests planting early, mid-season and late cultivars, or sowing the same cultivar in blocks every two weeks.² Illinois says to make the next planting when the previous one has three to four leaves,³ Georgia says successive two-week plantings,⁷ Clemson every 10 to 14 days⁹ and the Almanac every two to three weeks.¹² Between our standard-corn date and the last sowing for a 70-day variety, the median ZIP code has room for 7 sowings 14 days apart, or 5 at 21 days; the window is under 30 days in 4.9% of ZIP codes and over 100 days in 37.0%.

Timing also keeps types from crossing. Iowa State says supersweet types need isolation from standard, sugar-enhanced and synergistic types, that 250 feet is not practical for most home gardens, and that a minimum of 14 days should separate the tasseling time of different types.² Maryland and Clemson give two weeks or a month of planting time as the alternative to 400 yards of distance.^10,9 Because our supersweet date is 14 or more days after the standard date in 99.7% of ZIP codes, sowing each type on its own date produces a stagger in most places, but days to maturity matter too, so check the packet.

Heat at pollination

Corn is wind-pollinated, so plant blocks of at least four short rows rather than one long row.² Heat matters when the plants tassel and silk, about 50 days after sowing for a 70-day variety by our estimate, since silks to harvest takes 18 to 23 days.² Iowa State agronomists say:¹⁴

Pollen is no longer viable once temperatures reach the mid 90’s or greater

The same page adds that pollen usually sheds in the morning, and that high temperatures will not severely stress pollination if soil moisture is adequate; drought with heat is the problem.¹⁴ Heat rarely touches the first sowing: a standard-corn sowing on our date silks into a normal daily high of 90°F or more in 0.7% of ZIP codes. It touches mid-season sowings in the hot South and Southwest. A normal July high of 90°F or more occurs in 27.0% of ZIP codes, and a June 1 sowing silks into 90°F highs in 24.9%. A normal high of 90°F means many days reach the mid-90s, so treat Table 3 as a flag, not a ban, and water through silking.

Late sowings: the last date before fall frost

Frost ends the crop, so the last sowing date counts back from the median first 32°F frost. Our count is days to maturity plus 14 for slower fall growth plus 14 for a frost-tender crop, the same method as our fall vegetable planting guide, which uses 98 days for sweet corn: a 70-day variety needs 98 days and an 83-day variety, the main-crop example in the Illinois guide,³ needs 111. Days to maturity run from 60 to 100 in the Georgia and Almanac guides,^7,12 so find the number on your packet and add 28.

Across all ZIP codes the median last sowing date is July 18 for 70-day corn and July 5 for 83-day corn. It has passed by July 1 in 15.5% and 37.4% of ZIP codes, and by August 1 in 74.8% and 88.8%. A July 1 sowing fits the 70-day count in 84.5% of ZIP codes and the 83-day count in 62.6%. Late sowings ripen in cooling air: on the last sowing date the median normal daily mean is 74.8°F, but 70 days later it is 63.0°F, below 65°F in 80.2% of ZIP codes and below 60°F in 14.5%. Mississippi State warns that late-planted sweet corn has serious corn earworm problems,¹¹ and Illinois that later harvests usually have severe earworm damage unless timely control measures are followed;³ our dates are a frost limit, not a pest limit.

How published local calendars compare

Table 5 converts published dates to our cities. Spring dates mostly agree: Topeka’s standard date is inside K-State’s window and Des Moines’s inside Iowa State’s, though Memphis is 21 days earlier than Tennessee’s April 15 and Little Rock 9 days later than Arkansas’s mid-March. For fall, Iowa State’s July 1 and Illinois’s first week of July match our 83-day column, and Georgia’s July 20 is within two days of it.⁸ The Tennessee end dates, June 30 in the east and June 15 in the west, are 9 and 37 days earlier than our 83-day dates for Knoxville and Memphis; the publication does not say why, and our count is a frost limit only.⁵ Arkansas allows planting to early August,⁶ which matches our Little Rock 70-day date.

Field corn, popcorn and other kinds

Many search results are farm pages, which give earlier windows for yield: Purdue says Indiana’s optimum for maximum corn yield potential is April 20 to May 10, a week earlier in the south and a week later in the north,¹⁵ and the Nebraska Corn Board gives ideal soil as 50 to 55°F and the statewide average last freeze as April 23 to 25.¹⁶ Our Indianapolis standard date, April 25, is inside Purdue’s window, and Lincoln’s median last frost is April 24. Those are field-corn windows judged on yield, not on a home harvest of sweet corn, and field corn takes 95 to 115 days to mature, Nebraska says. Sweet corn should also be isolated from popcorn and field corn, Iowa State says, by 250 feet or by staggering the planting dates;² popcorn has its own timing, which this page does not cover.

Hot summers, mild winters and frost-free ZIP codes

Frost-free ZIP codes have no frost count, so use the months. In Phoenix (ZIP 85001) the normal mean stays at or above 55°F in all twelve months and at or above 65°F from March through November, but normal highs reach 90°F from May through September. San Francisco (94103) reaches 55°F from March to November and 65°F in no month, so supersweet seed never meets its line there. Miami (33101) is above 65°F all year. Our table has no frost dates for 104 of Hawaii’s 135 ZIP codes, so we give no Hawaii dates; ask your county extension office.

How we calculated these dates

Daily temperatures are NOAA’s monthly normals placed on the 15th of each month and interpolated linearly, for the station nearest each ZIP code.¹ Frost dates are the median and 10% 32°F dates. The 55, 60 and 65°F lines are the published rules above, applied to air temperature, not soil. Silking is 50 days after sowing for a 70-day variety. Individual years vary by weeks; a warm or cold spring will move every date, and a cold snap after sowing can set corn back even when it survives.

Next steps

Look up your own frost dates with our frost date lookup and read what a last frost date means. Sweet corn shares its warm soil with tomatoes, green beans and cucumbers, which are more frost-tender than corn, and the vegetable guides cover more crops.