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When to Plant Sweet Corn

The date to sow sweet corn outdoors for your ZIP code or zone, and when to sow again.

Tomato and pepper seedlings in trays under an LED grow light

Sweet Corn planting dates by zone

At the average last frost date for each USDA half-zone (the median across its ZIP codes, NOAA 1991 to 2020 normals). Enter your ZIP code below for your own dates.

ZoneLast frostSow outdoors
3aMay 28May 28
3bMay 17May 17
4aMay 13May 13
4bMay 8May 8
5aMay 4May 4
5bMay 1May 1
6aApr 29Apr 29
6bApr 23Apr 23
7aApr 16Apr 16
7bApr 6Apr 6
8aMar 29Mar 29
8bMar 18Mar 18
9aMar 3Mar 3
9bFeb 2Feb 2
10aJan 16Jan 16
10brareby season
Your location

Zone medians. Your ZIP code gives the dates of your nearest weather station.

Frost risk

Half of years still get a frost after the spring date, and half get the first fall frost before the fall date.

Frost dates at three risk levels
AverageSaferSafest
Last spring frostApr 23Apr 30May 9
First fall frostOct 22Oct 16Oct 7
Crops (1 selected)

First job: sow seeds outdoors

Apr 23sweet corn

Average frost dates: last Apr 23, first Oct 22 (zone 6b).
Season
Spring seed starting schedule
CropSow outdoorsNotes
Sweet CornTenderApr 23Sow outdoors at last frost. Sprouts in 5 to 10 days, best at 60 to 95°F soil.

How to start seeds indoors

Light
Give seedlings 14 to 18 hours of light a day on a timer, with fluorescent or LED fixtures 2 to 6 in above the leaves. Raise the lights as the plants grow.
Clemson HGIC: 16 to 18 hours, tubes 2 to 4 in above. UMD Extension: 14 to 16 hours, 2 to 6 in.
Seed starting mix
Use a sterile, soilless mix, never garden soil. To mix your own, combine equal parts by volume of fine pine bark, sphagnum peat moss and perlite.
NC State Extension Gardener Handbook; Clemson HGIC.
Sowing and warmth
Sow two or three seeds per cell and keep the strongest seedling. A mix temperature of 65 to 75°F germinates most vegetables; after they sprout, 55 to 60°F nights and 65 to 70°F days keep them stocky.
UMD Extension.
Potting up
Move seedlings into larger pots shortly after the first true leaves appear, while they are small and recover quickly.
UMD Extension; Clemson HGIC (one or two sets of true leaves).
Hardening off
Start two to three weeks before transplanting, once plants are about 3 in tall: set them outdoors by day, bring them in before sundown, and give them more direct sun each day. Skip days below 45°F or with strong wind.
Clemson HGIC: 2 to 3 weeks. NC State: at least 2 weeks, not below 45°F.

When to plant sweet corn

Sweet corn is sown straight into the garden, not started indoors. Colorado State University Extension and Utah State University Extension put the first sowing on your average last spring frost date, once the soil has warmed to at least 50°F. The table and planner above give that date for every zone and for your ZIP code.

Example dates for three zones

At the median last frost date for each half-zone, from NOAA's 1991 to 2020 normals:

ZoneLast frostSow outdoorsFirst fall frostDays until fall frost
5bMay 1May 1Oct 10162
7aApr 16Apr 16Oct 25192
9aMar 3Mar 3Nov 25267

Utah State gives 60 to 90 days from seeding to harvest, depending on the variety, so even zone 4b, sown May 8 with 146 days before its Oct 1 frost, has room for a late variety.

Soil temperature matters more than the calendar

Oregon State University Extension lists 50°F as the minimum for corn to germinate and 60 to 95°F as the best range, with seedlings up in 5 to 10 days in warm soil. Below 50°F it will not sprout at all, so if a soil thermometer still reads colder on your frost date, hold the seed back until it warms.

Sow 1 to 1.5 inches deep, 2 seeds per spot, in a block of at least three or four short rows so the ears pollinate well.

Sow every 10 to 14 days for a longer harvest

A single sowing ripens within a short window. Utah State suggests sowing again every 10 to 14 days. The last sowing depends on your first fall frost: count back the days to maturity of your variety. In zone 5b, with the average first frost on Oct 10, a 90 day variety has to be in the ground by Jul 12. The succession planting planner lays out every sowing for you.

Sowing different types at different dates also keeps standard, sugary enhanced and supersweet corn from tasseling at the same time and crossing.

Next steps