When to Plant Spinach: Sow Early, Beat the Long Days

Sow spinach as soon as the soil can be worked, usually 4 to 6 weeks before your average last spring frost, and stop about six weeks before daytime highs normally reach 80°F. Sow again once late-summer air cools below 70°F. In Chicago that is March 17 to May 2 in spring and September 4 to 24 for fall; in Dallas it is January 5 to March 15 and October 13 to 18.

Spinach also bolts on day length, which most guides skip. We tested the published rules against NOAA 1991–2020 normals¹ for 37,563 lower-48 ZIP codes and added the date daylight reaches 14 hours. Air temperature stands in for soil temperature, because our NOAA data has no soil readings.

Spring spinach planting dates for 22 cities

Penn State Extension says to plant 4 to 8 weeks before the average last frost,² Bonnie Plants says 4 to 6,¹¹ Colorado State 2 to 4⁶ and Utah State 2 to 3.⁴ Each window below opens on the later of two dates: eight weeks before the median last frost, or the day the normal daily mean air temperature reaches 40°F, the soil minimum Colorado State and Utah State give.^6,4 It closes 42 days before normal daily highs reach 80°F: the Old Farmer’s Almanac says spinach needs six weeks of cool weather from seeding to harvest,³ and Penn State says it wants to bolt at 80°F.²

The median window is 52 days, and 98.6% of ZIP codes with 80°F highs get at least 30. Fargo has 39 (April 10 to May 19). Gulf and Atlantic coast gardens start in December: Houston from December 8 to March 8, Charleston from December 10 to April 9.

Cold: how early is too early

Across the lower 48, eight weeks before the median last frost falls before the normal air mean reaches 40°F in 64.3% of ZIP codes; four weeks before does so in only 9.6%. So the outer end of Penn State’s range is too early for most gardens. Frost is not the obstacle: Penn State says young seedlings tolerate 15 to 20°F.²

Day length and heat: why spring spinach ends early

Day length is the first clock, heat the second. The Almanac ties bolting to about 14 hours of daylight or more, with warmth,³ and Penn State calls spinach day-length sensitive:²

It will bolt in June even if the temperatures are cool.

Daylight reaches 14 hours on April 27 in Minneapolis, May 2 in Chicago and May 23 in Macon, a date set only by latitude.¹⁷ Where highs reach 80°F (91% of ZIP codes), daylight arrives first in 76.1%, and 42 days after the median last frost the days are already 14 hours or longer in 74.8%. Sowing on the last-frost date also runs past the 80°F deadline in 19.8% of those (4.5% two weeks before, 1.0% four weeks before). The earliest sowing matters most.

The Organic Seed Alliance says most modern European varieties do not initiate bolting until day length reaches close to 16 hours;¹³ 98.4% of lower-48 ZIP codes never get that long a day. Clemson stresses slow-bolting varieties for spring and overwintered crops.⁵

How hot is too hot: ten temperature rules compared

Every source agrees spinach is a cool-season crop, but the germination ceiling runs from 68°F to 100°F. Maryland rates days to maturity at 28 to 55.⁷

Fall spinach planting dates: wait for the cool, then count back

The Almanac says to sow about two months before the first hard frost,¹² and Penn State and Bonnie say 6 to 8 weeks before the first frost.^2,11 A count back from frost ignores summer. Among ZIP codes that reach a 70°F summer mean, eight weeks before the median first frost is still 70°F or warmer in 67.6%, and six weeks before in 18.5%. The Almanac also says to sow in fall when soil is 70°F or cooler.³ Pam Dawling, who writes the Sustainable Market Farming blog from central Virginia, puts it simply:¹⁵

Spinach does not germinate in hot soil!

Our fall window opens on the first day after the summer peak that the normal air mean falls below 70°F, an air stand-in for the 68–70°F ceilings from Penn State and Colorado State, and closes 42 days before the median first 32°F frost. Illinois Extension suggests chilling seed in the refrigerator for 1 or 2 weeks before fall planting.⁸ About 15% of ZIP codes never reach a 70°F mean and need no gate.

The median fall window is 8 days, and 18.5% of ZIP codes have none, because the air cools only just before the six-week deadline. Utah State is more generous: plant 50 to 75 days before maturity, with maturity 1 to 2 weeks after first frost.⁴ That puts the last sowing 36 days before first frost, which gives a median of 14 days and 2.9% with none.

Overwintering spinach: sow small, mulch, wait

Utah State says:⁴

Spinach will easily over-winter for an early spring crop

That holds if the plant has 4 to 5 true leaves formed late in the year and is heavily mulched.⁴ A Fairfax County Master Gardener who overwinters spinach in northern Virginia sows around early October, says two to six leaves by Thanksgiving is enough, and begins harvesting in late February or early March.¹⁴

Across the lower 48, the coldest month’s normal average low is 20°F or warmer in 59.1% of ZIP codes, and 15°F or warmer, the Almanac’s limit,³ in 78.0%. Single nights run colder, so treat it as a sign, not a guarantee.

How published local calendars compare

Six places have a published window we could convert to dates, ten windows in all. Penn State’s fall count in Gettysburg nearly matches ours. In the South the published fall calendars open earlier than our heat gate: Clemson’s Piedmont window by 48 days, UGA’s middle Georgia window by 33 days (ours is empty there).^5,9 Clemson’s coastal window ends October 15; ours runs to November 18.

How we calculated these dates

Daily temperatures are NOAA’s monthly normals placed on the 15th and interpolated, for the station nearest each ZIP code.¹ Frost dates are medians for 32°F. Day length uses the standard sunrise equation with 0.833° of refraction, as NOAA’s solar calculator does.¹⁷ Day counts and temperature lines come from the sources above, applied to air, not soil. Years vary by weeks.

Next steps

Look up your own dates with our frost date lookup. Spinach shares its early window with carrots, lettuce and peas; see fall vegetable planting dates, frost covers and winter sowing, or all vegetable guides.