When to Plant Broccoli: Transplant Early, Finish Before the Heat

Set broccoli transplants in the garden 2 to 4 weeks before your average last spring frost, and stop about 60 days before your normal daily temperature reaches 70°F. For fall, plant so heads mature once the air has cooled below 70°F, and finish about 74 days before your first frost. In Chicago that is March 17 to April 12 in spring and July 6 to August 23 for fall; in Dallas it is February 2 to 25 and August 14 to September 16.

Most guides give a count of weeks before frost. We tested those counts against NOAA 1991–2020 normals¹ for 37,563 lower-48 ZIP codes, set twelve published local windows beside them, and built dates for 22 cities in spring and 18 in fall. Everything uses air temperature as a stand-in for soil temperature, because our NOAA data has no soil readings.

Spring broccoli transplant dates for 22 cities

Published counts cluster around 2 to 4 weeks before the last frost: the Old Farmer’s Almanac says to transplant 2 to 3 weeks before it, once soil reaches 40°F,² Maryland Extension says four weeks,⁷ and Colorado State says 2 to 4.¹¹ Each window below opens on the later of two dates: four weeks before the median last frost, or the day the normal daily mean air temperature reaches 40°F.

It closes 60 days before the daily mean reaches 70°F. Iowa State says broccoli is ready 60 to 75 days after transplanting and grows best at 60 to 70°F,³ and Ohio State and the Almanac put the optimum at 65 to 70°F.^4,2 The Almanac states the goal this way:

to get broccoli to mature before or after high temperatures are expected

The window is short almost everywhere: the median across ZIP codes with a 70°F summer mean is 23 days, and 9.5% have less than two weeks. Denver and Macon have 13 days, Austin 6. Gulf and Atlantic coast gardens start in winter: Houston January 5 to February 14. Portland and Seattle never reach a 70°F normal mean, so they have no heat deadline.

Cold: frost, buttoning and how early is too early

Frost is rarely the problem. Utah State says broccoli is not seriously damaged by temperatures down to 28°F,⁶ and our frost-cover guide groups it with the hardy crops that take a mid-20s freeze. Long cold is the risk. Iowa State lists prolonged temperatures below 50°F among the stresses that make plants form heads too early, called buttoning,³ and Clemson says 10 or more consecutive days of 35 to 50°F can make a plant bolt.⁸ Older plants are more exposed:

Larger, more mature transplants are prone to form small, early heads

That is why spring dates mean young transplants with 4 to 6 mature leaves.⁶ Three weeks before the median last frost, the normal daily high is below 50°F in 4.0% of ZIP codes and the daily mean is below 50°F in 74.1%, so a cool start is the usual condition. Four weeks before it, the normal mean has not yet reached 40°F in 9.6% of ZIP codes.

Heat: the spring deadline

Warmth ends the crop. Iowa State says hot weather can stop heads forming:³

can prevent broccoli from forming heads or crowns

It puts that at 86°F or more by day and 77°F or more at night. A normal monthly high of 86°F occurs somewhere in the year in 51.8% of ZIP codes, but a month that also has a normal low of 77°F is rare (0.7%), so we plan with the gentler 70°F mean. Transplanting on the median last-frost date is too late for a 60-day crop in 67.8% of the ZIP codes that reach a 70°F mean. The Almanac’s window fails less: 9.5% at its late end, two weeks before the last frost. At 75°F, the line Utah State gives for growing best,⁶ almost half of ZIP codes (47.8%) never reach it, and among the 19,615 that do, the median window grows to 44 days.

Temperature rules compared across nine sources

Every source calls broccoli a cool-season crop, but the lines differ. The table sets them side by side and the chart puts them on one scale.

Most sources put the comfort zone at roughly 60 to 75°F, with trouble from long spells under 50°F and above 80°F; Bonnie Plants says broccoli flowers quickly in temperatures much above 80°F.¹³ Iowa State’s freeze guide adds that broccoli that survives cold often needs replacing, because it is unlikely to form usable heads.¹⁴

Fall broccoli: count back from frost, then check the heat

Clemson calls fall the best season for broccoli in South Carolina,⁸ and Grow Organic says fall crops usually see fewer heat spikes.¹² The counts differ. Illinois Extension says to count back from the first fall frost and add about 10 days to the days to harvest from transplants,⁵ Grow Organic says 10 to 12 weeks before the first frost,¹² and Utah State says to plant 50 to 75 days before maturity.⁶ Google’s AI Overview for “when to plant broccoli for fall harvest” said roughly 6 to 10 weeks when we checked on October 14, 2026; six weeks is 42 days, less than the 55 to 65 days Maryland gives from transplanting,⁷ so only the long end passes. Read literally, the Almanac’s “start seeds 6 to 8 weeks before the first fall frost” and transplanting “4 to 6 weeks later” leaves 0 to 4 weeks before frost, so we did not use it.²

Our fall window closes 74 days before the median first 32°F frost: 60 days to harvest plus 14 because fall growth slows, the same count-back as our fall vegetable planting guide. It opens 60 days before the day the normal mean falls under 70°F, so heads form in cool air. A July 1 transplant matures while the mean is still 70°F or higher in 61.3% of ZIP codes, an August 1 transplant in 16.5% and an August 15 transplant in 4.8%. Broccoli tolerates 28°F, and counting to the first 28°F freeze instead would add a median of 11 days.

Across the 31,789 ZIP codes with a 70°F summer mean, the median fall window is 36 days, 75.0% have 30 days or more, and 0.1% have under two weeks. About 15% of ZIP codes never reach a 70°F mean and have only a last date.

The last date arrives earlier than most calendars suggest. Across the ZIP codes with a 70°F mean the median is August 14, and by September 1 it has passed in 82.2%. Denver’s is July 30 and Minneapolis’s August 3, while Charleston’s runs to October 17 and Houston’s to October 12. Table 3’s longest window is Charleston (64 days) and its shortest Macon (23).

How published local calendars compare

Seven places have published windows we could convert to dates, twelve in all. Iowa State’s fall range (late June to early August) contains ours for Des Moines, and Utah State’s spring rule, Maryland’s fall date and Clemson’s coastal fall dates sit inside ours. In South Carolina the sources disagree: Clemson’s spring dates run weeks past our heat deadline, yet Clemson says spring temperature swings cause loose heads.⁸

UGA’s dates are for middle Georgia and shift about two weeks later in spring and earlier in fall for the north.⁹

Mild winters and frost-free ZIP codes

UF/IFAS lists transplants from mid-September through mid-February in north and central Florida and October through December in south Florida, and says broccoli takes 80 to 100 days from transplanting there.¹⁰ Our window for Gainesville is August 21 to September 19 in fall and February 3 to 21 in spring, so a 60-day crop counted back from the median first frost does not capture the Florida season; follow UF/IFAS there.

Frost-free ZIP codes have no frost count to use, so look at the months. The normal mean stays under 70°F from November through March in Phoenix (ZIP 85001), in all twelve months in San Francisco (94103), and only in January and February in Miami (33101). 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.

Seed or transplant, and how to stretch the window

The sources prefer transplants. Ohio State says it is better to use transplants than to seed directly,⁴ and the Almanac recommends them even for fall, because direct seeding in hot weather isn’t good for cole crops.² Ages run 4 to 6 weeks for the Almanac and Ohio State and 5 to 7 for Colorado State.^2,4,11 For indoor start dates, see our seed-starting guide; the dates here are for plants going into the ground.

Days to harvest set how late you can plant: Iowa State gives 60 to 75 after transplanting, Maryland 55 to 65 and longer for fall,⁷ Clemson 50 to 90 by variety⁸ and UF/IFAS 80 to 100.¹⁰ Our 60 is the low end. Clemson suggests slow-bolting varieties such as Green Magic,⁸ and Utah State says hot caps protect transplants from frost and to apply organic mulch above 80°F.⁶

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 32°F dates. The day counts and the 40°F and 70°F lines are the published rules above, applied to air temperature, not soil. Individual years vary by weeks.

Next steps

Look up your own frost dates with our frost date lookup and read what a last frost date means. Broccoli shares its early spring window with carrots, peas, lettuce and spinach. More crops are in our vegetable guides.