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Frost dates and planting calendar for Broomfield, 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 2[2]
Median first fall frost
Oct 11[3]
Growing season, median
160 days[4]

Broomfield, Colorado is in USDA hardiness zone 6a at the city center[1]. The median last spring frost is May 2, which means a last frost after that date in half of years[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after Apr 18, half of years May 2, 1 in 10 years after May 14Last frostmedian May 2First fall frost: 9 in 10 years before Oct 24, half of years Oct 11, 1 in 10 years before Sep 26First frostmedian Oct 11Frost riskHardyHardy vegetables: plant Apr 4 to Apr 18Apr 4 to Apr 18Semi-hardySemi-hardy vegetables: plant Apr 18 to May 2Apr 18 to May 2TenderTender vegetables: plant around May 2around May 2Very tenderVery tender vegetables: plant from May 16from May 16
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.

NOAA frost dates are odds from 1991 to 2020 records, not a forecast[5]. In 9 of 10 years the last spring frost comes after April 18, and in 1 of 10 years it comes after May 14[2]. In the fall, the first frost comes before October 11 in half of years, and before October 24 in 9 of 10 years[3]. These dates come from the Northglenn weather station, which is near the center of Broomfield[5]. Any year can be earlier or later than the median.

Frost Likely (9 in 10 years) Even odds (5 in 10) Unlikely (1 in 10)
Last spring frost (32°F)[2] after April 18 after May 2 after May 14
First fall frost (32°F)[3] before September 26 before October 11 before October 24
  • Hard freeze (28°F): median last spring date April 20, median first fall date October 21[7].
  • Frost-free growing season: at least 142 days in 9 of 10 years, 160 days in half of years and 180 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: Northglenn (USC00055984), 4.3 miles from the city center, 5,407 feet, 26 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension sorts vegetables into four groups, and its guide was written for Colorado[6]. Hardy vegetables such as lettuce, peas and spinach can go in as early as 2 to 4 weeks before the average last frost, which works out to April 4 to April 18 here[6]. Semi-hardy vegetables such as carrots and potatoes can go in as early as 2 weeks before it, from April 18 up to the median date[9]. Tender vegetables such as snap beans and sweet corn go in from seed around the median last frost date, May 2[10]. Very tender vegetables such as tomatoes and peppers typically wait 2 or more weeks after it, so the earliest start is May 16[11]. These dates are simple arithmetic from the CSU rules and the NOAA median[6].

  • Hardy (broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips): as early as 2 to 4 weeks before the median last frost (May 2) is April 4 to April 18[6]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (May 2) is April 18, up to the median date itself[9]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, May 2[10]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (May 2), is May 16[11]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] Apr 4 Jun 8 (from transplants)
Cabbage (hardy)[12] Apr 4 Jun 28 (from transplants)
Kohlrabi (hardy)[12] Apr 4 May 24
Lettuce (hardy)[12] Apr 4 Jun 3
Onions (hardy)[12] Apr 4 Jul 23
Peas (hardy)[12] Apr 4 Jun 8
Radishes (hardy)[12] Apr 4 May 4
Spinach (hardy)[12] Apr 4 May 14
Turnips (hardy)[12] Apr 4 May 24
Beets (semi-hardy)[12] Apr 18 Jun 17
Carrots (semi-hardy)[12] Apr 18 Jun 27
Cauliflower (semi-hardy)[12] Apr 18 not listed
Parsnips (semi-hardy)[12] Apr 18 Jun 27
Potatoes (semi-hardy)[12] Apr 18 Aug 21
Swiss chard (semi-hardy)[12] Apr 18 Jun 17
Snap beans (tender)[12] May 2 Jul 1
Sweet corn (tender)[12] May 2 Jul 1
Cucumbers (tender)[12] May 2 Jun 26
Summer squash (tender)[12] May 2 Jun 21
Cantaloupe (very tender)[12] May 16 Aug 9
Eggplant (very tender)[12] May 16 Jul 15 (from transplants)
Peppers (very tender)[12] May 16 Jul 25 (from transplants)
Tomatoes (very tender)[12] May 16 Jul 20 (from transplants)
Winter squash (very tender)[12] May 16 Aug 24
Watermelon (very tender)[12] May 16 Aug 9
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

The USDA zone is based on the average coldest night of each winter from 1991 to 2020, not on summer heat or frost dates[14]. Zone 6a means an average yearly extreme low of -10°F to -5°F[1]. Broomfield overlaps 6 ZIP code areas[15]. By land area, 89% of the city lies in zone 6a ZIP code areas and 11% lies in zone 5b ZIP code areas[15].

Zone 6a: average annual extreme minimum temperature -10°F to -5°F (1991 to 2020)[1]. Modeled value at the city center: -7.9°F[1].

Share of the city's land area by ZIP code zone: zone 6a 89%, zone 5b 11%[15].

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

Zone by ZIP code
ZIP code Zone Coldest night range
80020[15] 6a -10 to -5°F
80023[15] 6a -10 to -5°F
80021[15] 6a -10 to -5°F
80516[15] 5b -15 to -10°F
80234[15] 6a -10 to -5°F
80026[15] 6a -10 to -5°F

Questions people ask

What growing zone is Broomfield, Colorado?
The center of Broomfield is in USDA zone 6a on the 2023 map[1]. By land area, 89% of the city is in zone 6a ZIP code areas and 11% is in zone 5b ZIP code areas[15].
When is the last frost in Broomfield?
The median last spring frost is May 2, so in half of years the last frost comes after that date[2]. In 1 of 10 years it comes after May 14[2].
When to plant tomatoes in Broomfield?
CSU Extension says tomatoes typically go in 2 or more weeks after the average last spring frost[11]. Using the NOAA median last frost of May 2, the earliest start is May 16[11].
When is the first frost in Broomfield?
The median first fall frost is October 11, so in half of years the first frost comes before that date[3]. In 1 of 10 years it comes before September 26[3].
How long is the growing season in Broomfield?
The growing season is at least 160 days in half of years[4]. It is at least 142 days in 9 of 10 years and at least 180 days in 1 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Broomfield, 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 7.9°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 18, in half of years after May 2, and in 1 of 10 years after May 14 (NOAA 1991 to 2020 normals). The median last frost date is May 2. 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 26, in half of years before October 11, and in 9 of 10 years before October 24 (NOAA 1991 to 2020 normals). The median first frost date is October 11. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 142 days in 9 of 10 years, at least 160 days in half of years, and at least 180 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Northglenn weather station (USC00055984), 4.3 miles from the center of Broomfield at 5,407 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 20 and the median first fall date is October 21. 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 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
  10. 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
  11. 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
  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. Broomfield, Colorado overlaps 6 ZIP code areas. By land area, 89% lies in ZIP code areas rated zone 6a, 11% lies in ZIP code areas rated zone 5b. 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.