Frost dates Colorado Aurora

Frost dates in Aurora, Colorado

Last spring frost May 4 First fall frost Oct 8 NOAA 1991-2020

The average last spring frost in Aurora is May 4 and the average first fall frost is Oct 8, measured at DENVER-STAPLETON, about 3 miles from the city centre. Below: how sure those dates are, and what to start, sow and set out each week.

Average last spring frostMay 432°F, 50% probability
Average first fall frostOct 832°F, 50% probability
Days between them157average frost-free season
How sure is that date? NOAA probability levels for a 32°F freeze
ProbabilityFreeze still comes after (spring)First freeze has come by (fall)
90%Apr 20Oct 20
50%May 4Oct 8
10%May 16Sep 25

Read it like this: in half of years the last 32°F freeze of spring comes after May 4, and in 1 year in 10 there is still a freeze after May 16. In 1 year in 10 the first fall freeze arrives before Sep 25.

Measured at: DENVER-STAPLETON, CO (NOAA station USW00023062), about 3 miles away at 5,286 ft, 28 years of record in 1991-2020.

Other stations nearby, for comparison:

  • CHERRY CREEK DAM, CO (NOAA station USC00051547), about 7 miles away at 5,647 ft, 13 years of record in 1991-2020: last spring frost May 7, first fall frost Oct 8
  • DENVER WATER DEPT, CO (NOAA station USC00052223), about 9 miles away at 5,228 ft, 22 years of record in 1991-2020: last spring frost Apr 30, first fall frost Oct 12

Seed-starting schedule for Aurora, CO

Counted from a last frost date of May 4 (the average: a freeze comes later than this in half of years).

  1. Week of Jan 2614 weeks before last frost
    • Start seeds indoorsLeekJan 26 to Feb 9
    • Start seeds indoorsOnionJan 26 to Feb 9
  2. Week of Feb 213 weeks before last frost
    • Start seeds indoorsParsleyFeb 2 to Feb 9
  3. Week of Feb 912 weeks before last frost
    • Start seeds indoorsCeleryFeb 9 to Feb 23
    • Start seeds indoorsSpinachFeb 9 to Feb 23
  4. Week of Feb 2310 weeks before last frost
    • Start seeds indoorsBroccoliFeb 23 to Mar 9
    • Start seeds indoorsBrussels sproutFeb 23 to Mar 9
    • Start seeds indoorsCabbageFeb 23 to Mar 9
    • Start seeds indoorsKaleFeb 23 to Mar 9
    • Start seeds indoorsKohlrabiFeb 23 to Mar 9
  5. Week of Mar 29 weeks before last frost
    • Start seeds indoorsLettuceMar 2 to Mar 9
  6. Week of Mar 98 weeks before last frost
    • Start seeds indoorsCauliflowerMar 9 to Mar 23
    • Start seeds indoorsEggplantMar 9 to Mar 23
    • Start seeds indoorsPepperMar 9 to Mar 23
    • Start seeds indoorsSwiss chardMar 9 to Mar 23
  7. Week of Mar 167 weeks before last frost
    • Start seeds indoorsTomatoMar 16 to Mar 30
  8. Week of Mar 236 weeks before last frost
    • Start seeds indoorsBasilMar 23 to Apr 6
    • Set transplants outSpinachMar 23 to Apr 13
  9. Week of Apr 64 weeks before last frost
    • Sow seed outdoorsRadishApr 6 to Apr 20
    • Sow seed outdoorsSwiss chardfrom Apr 6
    • Sow seed outdoorsTurnipApr 6 to Apr 20
    • Set transplants outBroccoliApr 6 to Apr 20
    • Set transplants outBrussels sproutApr 6 to Apr 20
    • Set transplants outCabbagefrom Apr 6
    • Set transplants outKalefrom Apr 6
    • Set transplants outKohlrabifrom Apr 6
    • Set transplants outLeekfrom Apr 6
    • Set transplants outLettuceApr 6 to Apr 13
    • Set transplants outOnionfrom Apr 6
  10. Week of Apr 133 weeks before last frost
    • Start seeds indoorsCucumberApr 13 to Apr 20
    • Set transplants outParsleyApr 13 to Apr 20
  11. Week of Apr 202 weeks before last frost
    • Start seeds indoorsMelonApr 20 to Apr 27
    • Start seeds indoorsPumpkinApr 20 to Apr 27
    • Start seeds indoorsSummer squashApr 20 to Apr 27
    • Start seeds indoorsWinter squashApr 20 to Apr 27
    • Start seeds indoorsZucchiniApr 20 to Apr 27
    • Plant seed potatoes outdoorsPotatoApr 20 to Apr 27
    • Set transplants outCauliflowerfrom Apr 20
    • Set transplants outSwiss chardfrom Apr 20
  12. Week of May 4Last frost week
    • Sow seed outdoorsBeanfrom May 4
    • Sow seed outdoorsParsnipfrom May 4
    • Set transplants outCeleryfrom May 4
  13. Week of May 111 week after last frost
    • Plant slips outdoorsSweet potatoMay 11 to May 18
    • Set transplants outCucumberMay 11 to May 18
    • Set transplants outTomatoMay 11 to May 18
  14. Week of May 182 weeks after last frost
    • Set transplants outBasilfrom May 18
    • Set transplants outEggplantMay 18 to May 25
    • Set transplants outMelonfrom May 18
    • Set transplants outPepperfrom May 18
    • Set transplants outPumpkinfrom May 18
    • Set transplants outSummer squashfrom May 18
    • Set transplants outWinter squashfrom May 18
    • Set transplants outZucchinifrom May 18

Not in the city centre? Look up your own ZIP code

More in Colorado

Where the numbers come from

Sources and method

Frost dates. U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), NOAA National Centers for Environmental Information (Palecki, Durre, Applequist, Arguez and Lawrimore, 2021). These are 30-year station statistics for 1991 to 2020. "Frost" here means an air temperature of 32°F or lower at the station. We use 6,539 stations that have at least 10 years of record and a published date; 176 more are stations where a freeze is too rare for NOAA to date, and 180 are stations cold enough to have frost risk all year.

Probability level. The headline dates are NOAA's 50% level: in half of years the last spring freeze comes later than that date, and in half of years the first fall freeze comes earlier. The 10% and 90% levels are shown next to them. For tender plants, the 10% spring date is the safer one to plan from: a freeze follows it in only 1 year in 10.

Which station. Your ZIP code is matched to the nearest of those stations by straight-line distance from the ZIP code's centre point (GeoNames, geonames.org, CC BY 4.0; 40,979 ZIP codes in the 50 states and DC). Nearest is not always most alike: a station on a hill, in a valley or by the water can differ from your garden by a week or more. That is why the station's name, distance and elevation are shown with every result, along with two more stations nearby. NOAA makes the same point: the normals are limited between stations as distance grows horizontally and vertically.

Seed-starting offsets. Growing Vegetables: When to Plant Your Vegetable Garden, a fact sheet from University of New Hampshire Extension. It gives each crop's weeks from seed to transplant and when it is safe to set out or sow relative to the last frost. The method is theirs too: take the planting date and count back the seed-to-transplant weeks. Where the fact sheet gives a range of planting dates, the start-indoors dates count back from the earliest one. The figures used, as the fact sheet states them:

  • Basil: Seed to transplant 6-8 weeks; set out 2 weeks after last frost.
  • Bean: Plant seed in garden after last frost.
  • Broccoli: Seed to transplant 4-6 weeks; set out 2-4 weeks before last frost.
  • Brussels sprout: Seed to transplant 4-6 weeks; set out 2-4 weeks before last frost.
  • Cabbage: Seed to transplant 4-6 weeks; set out 4 weeks before last frost.
  • Cauliflower: Seed to transplant 4-6 weeks; set out 2 weeks before last frost.
  • Celery: Seed to transplant 10-12 weeks; set out after last frost.
  • Cucumber: Seed to transplant 3-4 weeks; set out 1-2 weeks after last frost.
  • Eggplant: Seed to transplant 8-10 weeks; set out 2-3 weeks after last frost.
  • Kale: Seed to transplant 4-6 weeks; set out 4 weeks before last frost.
  • Kohlrabi: Seed to transplant 4-6 weeks; set out 4 weeks before last frost.
  • Leek: Seed to transplant 8-10 weeks; set out 4 weeks before last frost.
  • Lettuce: Seed to transplant 4-5 weeks; set out 3-4 weeks before last frost.
  • Melon: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
  • Onion: Seed to transplant 8-10 weeks; set out 4 weeks before last frost.
  • Parsley: Seed to transplant 9-10 weeks; set out 2-3 weeks before last frost.
  • Parsnip: Plant seed in garden after last frost.
  • Pepper: Seed to transplant 8-10 weeks; set out 2 weeks after last frost.
  • Potato: Plant seed potatoes in garden 1-2 weeks before last frost.
  • Pumpkin: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
  • Radish: Plant seed in garden 2-4 weeks before last frost.
  • Spinach: Seed to transplant 4-6 weeks; set out 3-6 weeks before last frost.
  • Summer squash: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
  • Sweet potato: Plant slips in garden 1-2 weeks after last frost.
  • Swiss chard: Seed to transplant 4-6 weeks; set out 2 weeks before last frost; or plant seed in garden 4 weeks before last frost.
  • Tomato: Seed to transplant 6-8 weeks; set out 1-2 weeks after last frost.
  • Turnip: Plant seed in garden 2-4 weeks before last frost.
  • Winter squash: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
  • Zucchini: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.

Not on the calendar. The same fact sheet times these by soil temperature, not by frost date, so they get no date here:

  • Pea: as soon as the soil thaws.
  • Beet: when the soil warms to 45°F.
  • Carrot: when the soil warms to 60°F.
  • Corn: when the soil warms to 60°F.
  • Cucumber, pumpkin, summer squash, winter squash, zucchini: direct-sow when the soil warms to 70°F.

Rutabaga is left out: the fact sheet gives it New Hampshire calendar dates, not a frost offset.

The fact sheet was written for New Hampshire. The offsets are relative to the last frost, so they carry to other places with a spring frost, but where winters are mild many cool-season crops are grown in fall and winter instead. Your state's extension service has the local calendar. Dates are 30-year statistics, not a forecast: check the forecast before setting tender plants out.

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