Frost dates Colorado Denver

Frost dates in Denver, Colorado

Last spring frost Apr 30 First fall frost Oct 12 NOAA 1991-2020

The average last spring frost in Denver is Apr 30 and the average first fall frost is Oct 12, measured at DENVER WATER DEPT, about 1 mile from the city centre. Below: how sure those dates are, and what to start, sow and set out each week.

Average last spring frostApr 3032°F, 50% probability
Average first fall frostOct 1232°F, 50% probability
Days between them165average 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 15Oct 26
50%Apr 30Oct 12
10%May 13Sep 28

Read it like this: in half of years the last 32°F freeze of spring comes after Apr 30, and in 1 year in 10 there is still a freeze after May 13. In 1 year in 10 the first fall freeze arrives before Sep 28.

Measured at: DENVER WATER DEPT, CO (NOAA station USC00052223), about 1 mile away at 5,228 ft, 22 years of record in 1991-2020.

Other stations nearby, for comparison:

  • DENVER-STAPLETON, CO (NOAA station USW00023062), about 6 miles away at 5,286 ft, 28 years of record in 1991-2020: last spring frost May 4, first fall frost Oct 8
  • LAKEWOOD, CO (NOAA station USC00054762), about 7 miles away at 5,640 ft, 24 years of record in 1991-2020: last spring frost May 3, first fall frost Oct 7

Seed-starting schedule for Denver, CO

Counted from a last frost date of Apr 30 (the average: a freeze comes later than this in half of years).

  1. Week of Jan 2214 weeks before last frost
    • Start seeds indoorsLeekJan 22 to Feb 5
    • Start seeds indoorsOnionJan 22 to Feb 5
  2. Week of Jan 2913 weeks before last frost
    • Start seeds indoorsParsleyJan 29 to Feb 5
  3. Week of Feb 512 weeks before last frost
    • Start seeds indoorsCeleryFeb 5 to Feb 19
    • Start seeds indoorsSpinachFeb 5 to Feb 19
  4. Week of Feb 1910 weeks before last frost
    • Start seeds indoorsBroccoliFeb 19 to Mar 5
    • Start seeds indoorsBrussels sproutFeb 19 to Mar 5
    • Start seeds indoorsCabbageFeb 19 to Mar 5
    • Start seeds indoorsKaleFeb 19 to Mar 5
    • Start seeds indoorsKohlrabiFeb 19 to Mar 5
  5. Week of Feb 269 weeks before last frost
    • Start seeds indoorsLettuceFeb 26 to Mar 5
  6. Week of Mar 58 weeks before last frost
    • Start seeds indoorsCauliflowerMar 5 to Mar 19
    • Start seeds indoorsEggplantMar 5 to Mar 19
    • Start seeds indoorsPepperMar 5 to Mar 19
    • Start seeds indoorsSwiss chardMar 5 to Mar 19
  7. Week of Mar 127 weeks before last frost
    • Start seeds indoorsTomatoMar 12 to Mar 26
  8. Week of Mar 196 weeks before last frost
    • Start seeds indoorsBasilMar 19 to Apr 2
    • Set transplants outSpinachMar 19 to Apr 9
  9. Week of Apr 24 weeks before last frost
    • Sow seed outdoorsRadishApr 2 to Apr 16
    • Sow seed outdoorsSwiss chardfrom Apr 2
    • Sow seed outdoorsTurnipApr 2 to Apr 16
    • Set transplants outBroccoliApr 2 to Apr 16
    • Set transplants outBrussels sproutApr 2 to Apr 16
    • Set transplants outCabbagefrom Apr 2
    • Set transplants outKalefrom Apr 2
    • Set transplants outKohlrabifrom Apr 2
    • Set transplants outLeekfrom Apr 2
    • Set transplants outLettuceApr 2 to Apr 9
    • Set transplants outOnionfrom Apr 2
  10. Week of Apr 93 weeks before last frost
    • Start seeds indoorsCucumberApr 9 to Apr 16
    • Set transplants outParsleyApr 9 to Apr 16
  11. Week of Apr 162 weeks before last frost
    • Start seeds indoorsMelonApr 16 to Apr 23
    • Start seeds indoorsPumpkinApr 16 to Apr 23
    • Start seeds indoorsSummer squashApr 16 to Apr 23
    • Start seeds indoorsWinter squashApr 16 to Apr 23
    • Start seeds indoorsZucchiniApr 16 to Apr 23
    • Plant seed potatoes outdoorsPotatoApr 16 to Apr 23
    • Set transplants outCauliflowerfrom Apr 16
    • Set transplants outSwiss chardfrom Apr 16
  12. Week of Apr 30Last frost week
    • Sow seed outdoorsBeanfrom Apr 30
    • Sow seed outdoorsParsnipfrom Apr 30
    • Set transplants outCeleryfrom Apr 30
  13. Week of May 71 week after last frost
    • Plant slips outdoorsSweet potatoMay 7 to May 14
    • Set transplants outCucumberMay 7 to May 14
    • Set transplants outTomatoMay 7 to May 14
  14. Week of May 142 weeks after last frost
    • Set transplants outBasilfrom May 14
    • Set transplants outEggplantMay 14 to May 21
    • Set transplants outMelonfrom May 14
    • Set transplants outPepperfrom May 14
    • Set transplants outPumpkinfrom May 14
    • Set transplants outSummer squashfrom May 14
    • Set transplants outWinter squashfrom May 14
    • Set transplants outZucchinifrom May 14

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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