GardenFacts

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Frost dates and planting calendar for Boulder, 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 6[2]
Median first fall frost
Oct 5[3]
Growing season, median
151 days[4]

Boulder, Colorado sits in USDA hardiness zone 6a on the 2023 map[1]. The median last spring frost at the Boulder weather station is May 6, which means a last frost after that date in half of years[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after Apr 24, half of years May 6, 1 in 10 years after May 18Last frostmedian May 6First fall frost: 9 in 10 years before Oct 18, half of years Oct 5, 1 in 10 years before Sep 21First frostmedian Oct 5Frost riskHardyHardy vegetables: plant Apr 8 to Apr 22Apr 8 to Apr 22Semi-hardySemi-hardy vegetables: plant Apr 22 to May 6Apr 22 to May 6TenderTender vegetables: plant around May 6around May 6Very tenderVery tender vegetables: plant from May 20from May 20
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)[6]. Exact dates are in the tables below.

These frost dates are odds from NOAA records for 1991 to 2020, not a forecast[5]. At the Boulder station, the last spring frost comes after April 24 in 9 of 10 years, after May 6 in half of years, and after May 18 in 1 of 10 years[2]. The first fall frost comes before September 21 in 1 of 10 years, before October 5 in half of years, and before October 18 in 9 of 10 years[3]. Any year can come earlier or later than these dates[5].

Frost Likely (9 in 10 years) Even odds (5 in 10) Unlikely (1 in 10)
Last spring frost (32°F)[2] after April 24 after May 6 after May 18
First fall frost (32°F)[3] before September 21 before October 5 before October 18
  • Hard freeze (28°F): median last spring date April 27, median first fall date October 15[7].
  • Frost-free growing season: at least 131 days in 9 of 10 years, 151 days in half of years and 170 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: Boulder (USC00050848), 2.4 miles from the city center, 5,484 feet, 26 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension's planting guide was written for Colorado, and it groups vegetables by how they handle frost[6]. The GardenFacts calculation treats the median last frost, May 6, as the average last frost[6]. Hardy vegetables such as lettuce, peas and spinach can go in as early as 2 to 4 weeks before that date, which is April 8 to April 22[6]. Tender vegetables such as snap beans and sweet corn go in from seed around May 6[9]. Very tender vegetables such as tomatoes and peppers typically wait 2 or more weeks after the median last frost, so the earliest start is May 20[10].

  • Hardy (broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips): as early as 2 to 4 weeks before the median last frost (May 6) is April 8 to April 22[6]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (May 6) is April 22, up to the median date itself[11]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, May 6[9]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (May 6), is May 20[10]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] Apr 8 Jun 12 (from transplants)
Cabbage (hardy)[12] Apr 8 Jul 2 (from transplants)
Kohlrabi (hardy)[12] Apr 8 May 28
Lettuce (hardy)[12] Apr 8 Jun 7
Onions (hardy)[12] Apr 8 Jul 27
Peas (hardy)[12] Apr 8 Jun 12
Radishes (hardy)[12] Apr 8 May 8
Spinach (hardy)[12] Apr 8 May 18
Turnips (hardy)[12] Apr 8 May 28
Beets (semi-hardy)[12] Apr 22 Jun 21
Carrots (semi-hardy)[12] Apr 22 Jul 1
Cauliflower (semi-hardy)[12] Apr 22 not listed
Parsnips (semi-hardy)[12] Apr 22 Jul 1
Potatoes (semi-hardy)[12] Apr 22 Aug 25
Swiss chard (semi-hardy)[12] Apr 22 Jun 21
Snap beans (tender)[12] May 6 Jul 5
Sweet corn (tender)[12] May 6 Jul 5
Cucumbers (tender)[12] May 6 Jun 30
Summer squash (tender)[12] May 6 Jun 25
Cantaloupe (very tender)[12] May 20 Aug 13
Eggplant (very tender)[12] May 20 Jul 19 (from transplants)
Peppers (very tender)[12] May 20 Jul 29 (from transplants)
Tomatoes (very tender)[12] May 20 Jul 24 (from transplants)
Winter squash (very tender)[12] May 20 Aug 28
Watermelon (very tender)[12] May 20 Aug 13
Spacing, depth and soil temperature (CSU Extension)
Vegetable Min. soil temp Depth Spacing Days to harvest
Broccoli[13] 40°F 1/2 in 18 in 65
Cabbage[13] 40°F 1/2 in 18 in 85
Kohlrabi[13] 40°F 1/2 in 7-9 in 50
Lettuce[13] 35°F 1/4 in 7-9 in 60
Onions[13] 35°F 1/4 in (seed), 1-2 in (sets) 4-6 in 110
Peas[13] 40°F 1 in 4-6 in 65
Radishes[13] 40°F 1/2 in 2-3 in 30
Spinach[13] 40°F 1/2 in 4-6 in 40
Turnips[13] 40°F 1/2 in 4-6 in 50
Beets[13] 40°F 3/4-1 in 4-6 in 60
Carrots[13] 40°F 1/4 in 2-3 in 70
Cauliflower[13] 40°F 1/2 in 18 in –
Parsnips[13] 35°F 1/2 in 5-6 in 70
Potatoes[13] 45°F – – 125
Swiss chard[13] 40°F 1 in 7-9 in 60
Snap beans[13] 55°F 1-1 1/2 in 6 in 60
Sweet corn[13] 50°F 1-1 1/2 in 12 x 30 in or 9 x 36 in 60-90
Cucumbers[13] 60°F 1 in 6 in trellised, 24-36 in untrellised 55
Summer squash[13] 60°F 1-1 1/2 in 36-48 in 50
Cantaloupe[13] 60°F 1-1 1/2 in 36-48 in 85
Eggplant[13] 60°F 1/4 in 18-24 in 60
Peppers[13] 60°F 1/4 in 15-18 in 70
Tomatoes[13] 50°F 1/4 in 24 in trellised 65
Winter squash[13] 60°F 1-1 1/2 in 36-48 in 100
Watermelon[13] 60°F 1-1 1/2 in 36-48 in 85

Hardiness zone

A zone describes the average coldest winter night from 1991 to 2020, not summer heat or frost dates[14]. Zone 6a means an average yearly extreme low of -10°F to -5°F[1]. All of Boulder's land area lies in ZIP code areas rated zone 6a[15].

Zone 6a: average annual extreme minimum temperature -10°F to -5°F (1991 to 2020)[1]. Modeled value at the city center: -7.7°F[1].

Share of the city's land area by ZIP code zone: zone 6a 100%[15].

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

Zone by ZIP code
ZIP code Zone Coldest night range
80301[15] 6a -10 to -5°F
80304[15] 6a -10 to -5°F
80302[15] 6a -10 to -5°F
80303[15] 6a -10 to -5°F
80305[15] 6a -10 to -5°F
80503[15] 6a -10 to -5°F
80310[15] 6a -10 to -5°F

Questions people ask

What growing zone is Boulder, Colorado?
The 2023 USDA map puts the center of Boulder in zone 6a[1]. By land area, 100% of the city lies in ZIP code areas rated 6a[15].
When is the last frost in Boulder?
The median last spring frost is May 6, so in half of years the last frost comes after that date[2]. In 1 of 10 years it comes after May 18[2].
When is the first frost in Boulder?
The median first fall frost is October 5, so in half of years it comes before that date[3]. In 1 of 10 years it comes before September 21[3].
When to plant tomatoes in Boulder?
CSU Extension says very tender vegetables like tomatoes typically go in 2 or more weeks after the average last frost[10]. Using the median last frost, the earliest start is May 20[10].
How long is the growing season in Boulder?
The season between the last spring and first fall frost is at least 151 days in half of years[4]. It is at least 131 days in 9 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Boulder, 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 7.7°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 24, in half of years after May 6, and in 1 of 10 years after May 18 (NOAA 1991 to 2020 normals). The median last frost date is May 6. 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 21, in half of years before October 5, and in 9 of 10 years before October 18 (NOAA 1991 to 2020 normals). The median first frost date is October 5. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 131 days in 9 of 10 years, at least 151 days in half of years, and at least 170 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Boulder weather station (USC00050848), 2.4 miles from the center of Boulder at 5,484 feet, using 26 years of records from 1991 to 2020. NOAA Climate Normals 1991-2020
  6. Colorado State University Extension's vegetable planting guide (written for Colorado) groups broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips as hardy vegetables, with this rule: as early as 2-4 weeks before the average last... CSU Extension GardenNotes #720
  7. Hard freeze (28°F) at this station: the median last spring date is April 27 and the median first fall date is October 15. 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 (written for Colorado) groups snap beans, sweet corn, cucumbers, summer squash as tender vegetables, with this rule: from seed around the average last spring frost; cucumber and summer squash... CSU Extension GardenNotes #720
  10. Colorado State University Extension's vegetable planting guide (written for Colorado) groups cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon as very tender vegetables, with this rule: typically 2 or more weeks after the average last... CSU Extension GardenNotes #720
  11. Colorado State University Extension's vegetable planting guide (written for Colorado) groups beets, carrots, cauliflower, parsnips, potatoes, Swiss chard as semi-hardy vegetables, with this rule: as early as 0-2 weeks before the average last spring frost.... CSU Extension GardenNotes #720
  12. 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
  13. CSU Extension vegetable table: soil temperature, depth, spacing, days to harvest (table on this page). CSU Extension GardenNotes #720
  14. 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
  15. Boulder, Colorado overlaps 7 ZIP code areas. By land area, 100% lies in ZIP code areas rated 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.