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Frost dates and planting calendar for Denver, Colorado

Updated October 30, 2026. Based on USDA, NOAA and Colorado State University Extension data. Published by Humblebee.

USDA hardiness zone
6a[1]
Median last spring frost
May 4[2]
Median first fall frost
Oct 8[3]
Growing season, median
156 days[4]

Denver sits in USDA hardiness zone 6a on the 2023 map[1]. At the nearby NOAA station, the median last spring frost is May 4[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after Apr 20, half of years May 4, 1 in 10 years after May 16Last frostmedian May 4First fall frost: 9 in 10 years before Oct 20, half of years Oct 8, 1 in 10 years before Sep 25First frostmedian Oct 8Frost riskHardyHardy vegetables: plant Apr 6 to Apr 20Apr 6 to Apr 20Semi-hardySemi-hardy vegetables: plant Apr 20 to May 4Apr 20 to May 4TenderTender vegetables: plant around May 4around May 4Very tenderVery tender vegetables: plant from May 18from May 18
Shaded: the span between the dates frost passes in 9 of 10 years and 1 of 10 years[2][3]. Green: planting windows from the CSU rules applied to the median last frost (GardenFacts calculation)[7]. Exact dates are in the tables below.

These frost dates are drawn from NOAA climate normals at the Denver-Stapleton station near the city center, based on records from 1991 to 2020[5]. They are odds, not a forecast. In half of years the last spring frost comes after May 4, while in 1 year out of 10 it comes after May 16[2]. In the fall, the first frost arrives before October 8 in half of years and before September 25 in 1 of 10 years[3]. A hard freeze of 28°F has a median last spring date of April 22 and a median first fall date of October 18[6].

Frost Likely (9 in 10 years) Even odds (5 in 10) Unlikely (1 in 10)
Last spring frost (32°F)[2] after April 20 after May 4 after May 16
First fall frost (32°F)[3] before September 25 before October 8 before October 20
  • Hard freeze (28°F): median last spring date April 22, median first fall date October 18[6].
  • Frost-free growing season: at least 138 days in 9 of 10 years, 156 days in half of years and 175 days in 1 of 10 years[4].
  • The low reached 32°F or colder in 100% of years and 28°F or colder in 100% of years[8].
  • Weather station: Denver-Stapleton (USW00023062), 0.6 miles from the city center, 5,286 feet, 28 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension sorts vegetables into groups by how much cold they can take[9]. Our GardenFacts calculation uses the median last frost as the average date[7]. Hardy crops like peas, lettuce and spinach[9] can go in as early as April 6 to April 20[7]. Semi-hardy crops like beets, carrots and potatoes[10] fit from April 20 up to May 4[11]. Tender crops like snap beans and sweet corn[12] go in around May 4[13]. Very tender crops like tomatoes and peppers[14] wait until about May 18[15].

  • Hardy: 2 to 4 weeks before the median last frost (May 4) is April 6 to April 20[7][9]
  • Semi-hardy: 2 weeks before the median last frost (May 4) is April 20, up to the median date itself[11][10]
  • Tender: around the median last frost date, May 4[13][12]
  • Very tender: two weeks after the median last frost (May 4) is May 18[15][14]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[16] Apr 6 Jun 10 (from transplants)
Cabbage (hardy)[16] Apr 6 Jun 30 (from transplants)
Kohlrabi (hardy)[16] Apr 6 May 26
Lettuce (hardy)[16] Apr 6 Jun 5
Onions (hardy)[16] Apr 6 Jul 25
Peas (hardy)[16] Apr 6 Jun 10
Radishes (hardy)[16] Apr 6 May 6
Spinach (hardy)[16] Apr 6 May 16
Turnips (hardy)[16] Apr 6 May 26
Beets (semi-hardy)[16] Apr 20 Jun 19
Carrots (semi-hardy)[16] Apr 20 Jun 29
Cauliflower (semi-hardy)[16] Apr 20 not listed
Parsnips (semi-hardy)[16] Apr 20 Jun 29
Potatoes (semi-hardy)[16] Apr 20 Aug 23
Swiss chard (semi-hardy)[16] Apr 20 Jun 19
Snap beans (tender)[16] May 4 Jul 3
Sweet corn (tender)[16] May 4 Jul 3
Cucumbers (tender)[16] May 4 Jun 28
Summer squash (tender)[16] May 4 Jun 23
Cantaloupe (very tender)[16] May 18 Aug 11
Eggplant (very tender)[16] May 18 Jul 17 (from transplants)
Peppers (very tender)[16] May 18 Jul 27 (from transplants)
Tomatoes (very tender)[16] May 18 Jul 22 (from transplants)
Winter squash (very tender)[16] May 18 Aug 26
Watermelon (very tender)[16] May 18 Aug 11
Spacing, depth and soil temperature (CSU Extension)
Vegetable Min. soil temp Depth Spacing Days to harvest
Broccoli[17] 40°F 1/2 in 18 in 65
Cabbage[17] 40°F 1/2 in 18 in 85
Kohlrabi[17] 40°F 1/2 in 7-9 in 50
Lettuce[17] 35°F 1/4 in 7-9 in 60
Onions[17] 35°F 1/4 in (seed), 1-2 in (sets) 4-6 in 110
Peas[17] 40°F 1 in 4-6 in 65
Radishes[17] 40°F 1/2 in 2-3 in 30
Spinach[17] 40°F 1/2 in 4-6 in 40
Turnips[17] 40°F 1/2 in 4-6 in 50
Beets[17] 40°F 3/4-1 in 4-6 in 60
Carrots[17] 40°F 1/4 in 2-3 in 70
Cauliflower[17] 40°F 1/2 in 18 in –
Parsnips[17] 35°F 1/2 in 5-6 in 70
Potatoes[17] 45°F – – 125
Swiss chard[17] 40°F 1 in 7-9 in 60
Snap beans[17] 55°F 1-1 1/2 in 6 in 60
Sweet corn[17] 50°F 1-1 1/2 in 12 x 30 in or 9 x 36 in 60-90
Cucumbers[17] 60°F 1 in 6 in trellised, 24-36 in untrellised 55
Summer squash[17] 60°F 1-1 1/2 in 36-48 in 50
Cantaloupe[17] 60°F 1-1 1/2 in 36-48 in 85
Eggplant[17] 60°F 1/4 in 18-24 in 60
Peppers[17] 60°F 1/4 in 15-18 in 70
Tomatoes[17] 50°F 1/4 in 24 in trellised 65
Winter squash[17] 60°F 1-1 1/2 in 36-48 in 100
Watermelon[17] 60°F 1-1 1/2 in 36-48 in 85

Hardiness zone

Zone 6a means the coldest winter night of the year averaged -10°F to -5°F over 1991 to 2020[1]. The zone is about winter cold only. It says nothing about frost dates or summer heat[18]. All of Denver's 38 ZIP code areas fall in zone 6a by land area[19].

Zone 6a: average annual extreme minimum temperature -10°F to -5°F (1991 to 2020)[1]. Modeled value at the city center: -8.1°F[1].

Share of the city's land area by ZIP code zone: zone 6a 100%[19].

All towns in zone 6a and what zone 6a means

Zone by ZIP code
ZIP code Zone Coldest night range
80249[19] 6a -10 to -5°F
80239[19] 6a -10 to -5°F
80219[19] 6a -10 to -5°F
80216[19] 6a -10 to -5°F
80238[19] 6a -10 to -5°F
80210[19] 6a -10 to -5°F
80204[19] 6a -10 to -5°F
80223[19] 6a -10 to -5°F
80220[19] 6a -10 to -5°F
80205[19] 6a -10 to -5°F
80211[19] 6a -10 to -5°F
80207[19] 6a -10 to -5°F
80237[19] 6a -10 to -5°F
80209[19] 6a -10 to -5°F
80222[19] 6a -10 to -5°F
80224[19] 6a -10 to -5°F
80236[19] 6a -10 to -5°F
80231[19] 6a -10 to -5°F
80212[19] 6a -10 to -5°F
80230[19] 6a -10 to -5°F
80206[19] 6a -10 to -5°F
80123[19] 6a -10 to -5°F
80218[19] 6a -10 to -5°F
80247[19] 6a -10 to -5°F
80227[19] 6a -10 to -5°F
80202[19] 6a -10 to -5°F
80246[19] 6a -10 to -5°F
80203[19] 6a -10 to -5°F
80221[19] 6a -10 to -5°F
80235[19] 6a -10 to -5°F

Questions people ask

What growing zone is Denver?
Denver is in zone 6a on the 2023 USDA Plant Hardiness Zone Map[1]. By land area, the whole city falls in zone 6a[19].
When is the last frost in Denver?
The median last spring frost is May 4 at the Denver-Stapleton station[2]. In 1 year out of 10, the last frost is later than May 16[2].
When to plant tomatoes in Denver?
CSU Extension puts tomatoes in its very tender group, typically planted two weeks or more past the average last frost[14]. Using the May 4 median, our calculation gives May 18[15].
When is the first frost in Denver?
Half of years see the first fall frost before October 8[3]. In 1 of 10 years it comes before September 25[3].
How long is the growing season in Denver?
The time between the last spring and first fall frost lasts 156 days or more in half of years[4]. In 9 of 10 years it is at least 138 days[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Denver, Colorado in zone 6a. Zone 6a means an average annual extreme minimum temperature of -10°F to -5°F over 1991 to 2020. The map's modeled value at the city center is minus 8.1°F. USDA zone map 2023 (PRISM, OSU)
  2. Last spring frost (32°F) at this station: in 9 of 10 years the last frost comes after April 20, in half of years after May 4, and in 1 of 10 years after May 16 (NOAA 1991 to 2020 normals). The median last frost date is May 4. NOAA Climate Normals 1991-2020
  3. First fall frost (32°F) at this station: in 1 of 10 years the first frost comes before September 25, in half of years before October 8, and in 9 of 10 years before October 20 (NOAA 1991 to 2020 normals). The median first frost date is October 8. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 138 days in 9 of 10 years, at least 156 days in half of years, and at least 175 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Denver-Stapleton weather station (USW00023062), 0.6 miles from the center of Denver at 5,286 feet, using 28 years of records from 1991 to 2020. NOAA Climate Normals 1991-2020
  6. Hard freeze (28°F) at this station: the median last spring date is April 22 and the median first fall date is October 18. NOAA Climate Normals 1991-2020
  7. GardenFacts calculation for Denver: taking the NOAA median (50%) last frost date as the average last frost and applying the CSU rule for hardy vegetables, 2 to 4 weeks before the median last frost (May 4) is April 6 to April 20. NOAA Climate Normals 1991-2020
  8. At this station the low reached 32°F or colder in 100% of years from 1991 to 2020, and 28°F or colder in 100% of years. NOAA Climate Normals 1991-2020
  9. Colorado State University Extension's vegetable planting guide groups broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips as hardy vegetables. Its rule for planting them: as early as 2-4 weeks before the average last spring frost. CSU Extension GardenNotes #720
  10. Colorado State University Extension's vegetable planting guide groups beets, carrots, cauliflower, parsnips, potatoes, Swiss chard as semi-hardy vegetables. Its rule for planting them: as early as 0-2 weeks before the average last spring frost. CSU Extension GardenNotes #720
  11. GardenFacts calculation for Denver: taking the NOAA median (50%) last frost date as the average last frost and applying the CSU rule for semi-hardy vegetables, 2 weeks before the median last frost (May 4) is April 20, up to the median date itself. NOAA Climate Normals 1991-2020
  12. Colorado State University Extension's vegetable planting guide groups snap beans, sweet corn, cucumbers, summer squash as tender vegetables. Its rule for planting them: from seed around the average last spring frost; cucumber and summer squash transplants... CSU Extension GardenNotes #720
  13. GardenFacts calculation for Denver: taking the NOAA median (50%) last frost date as the average last frost and applying the CSU rule for tender vegetables, around the median last frost date, May 4. NOAA Climate Normals 1991-2020
  14. Colorado State University Extension's vegetable planting guide groups cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon as very tender vegetables. Its rule for planting them: typically 2 or more weeks after the average last spring frost. CSU Extension GardenNotes #720
  15. GardenFacts calculation for Denver: taking the NOAA median (50%) last frost date as the average last frost and applying the CSU rule for very tender vegetables, two weeks after the median last frost (May 4) is May 18. NOAA Climate Normals 1991-2020
  16. GardenFacts calculation: earliest planting date plus CSU days to harvest (table on this page), from the station's median last frost. NOAA Climate Normals 1991-2020
  17. CSU Extension vegetable table: soil temperature, depth, spacing, days to harvest (table on this page). CSU Extension GardenNotes #720
  18. USDA hardiness zones are based on the average annual extreme minimum winter temperature (the coldest night of each winter, averaged over 1991 to 2020 for the 2023 map), not on summer heat or frost dates. Each zone spans 10°F and each half zone (a or b)... USDA Plant Hardiness Zone Map
  19. Denver, Colorado overlaps 38 ZIP code areas. By land area, 100% in zone 6a. USDA zone map 2023 (PRISM, OSU)

Frost dates are probabilities from 1991 to 2020 weather records at one NOAA station, not a forecast. Any year can be earlier or later. The hardiness zone describes the average coldest winter night, not frost dates or summer heat. Spots inside the city can differ (ZIP table). Planting rules come from Colorado State University Extension's guide, written for Colorado. A local extension office may give dates tuned to the area. Planting and harvest dates marked GardenFacts calculation are simple arithmetic from the facts above, not advice from the source.

Hardiness zone data: USDA Plant Hardiness Zone Map (2023), USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu, accessed October 30, 2026. This is not the official USDA map. Frost data: NOAA NCEI U.S. Climate Normals 1991-2020. Planting rules: Colorado State University Extension.