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Frost dates and planting calendar for Flagstaff, Arizona

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
Jun 4[2]
Median first fall frost
Sep 28[3]
Growing season, median
115 days[4]

Flagstaff is in USDA hardiness zone 6a at the city center[1]. NOAA records at the Flagstaff Pulliam Airport station show a median last spring frost of June 4[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after May 21, half of years Jun 4, 1 in 10 years after Jun 19Last frostmedian Jun 4First fall frost: 9 in 10 years before Oct 9, half of years Sep 28, 1 in 10 years before Sep 16First frostmedian Sep 28Frost riskHardyHardy vegetables: plant May 7 to May 21May 7 to May 21Semi-hardySemi-hardy vegetables: plant May 21 to Jun 4May 21 to Jun 4TenderTender vegetables: plant around Jun 4around Jun 4Very tenderVery tender vegetables: plant from Jun 18from Jun 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)[6]. Exact dates are in the tables below.

These frost dates come from NOAA climate normals for the Flagstaff Pulliam Airport station, using 26 years of records from 1991 to 2020[5]. The last spring frost comes after May 21 in 9 of 10 years, after June 4 in half of years, and after June 19 in 1 of 10 years[2]. The first fall frost comes before September 16 in 1 of 10 years, before September 28 in half of years, and before October 9 in 9 of 10 years[3]. These are chances from past records, not a forecast, so any year can come earlier or later[2][3].

Frost Likely (9 in 10 years) Even odds (5 in 10) Unlikely (1 in 10)
Last spring frost (32°F)[2] after May 21 after June 4 after June 19
First fall frost (32°F)[3] before September 16 before September 28 before October 9
  • Hard freeze (28°F): median last spring date May 20, median first fall date October 9[7].
  • Frost-free growing season: at least 96 days in 9 of 10 years, 115 days in half of years and 133 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: Flagstaff Pulliam Airport (USW00003103), 3.8 miles from the city center, 7,003 feet, 26 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension's planting guide was written for Colorado, so treat these dates as a starting point[6]. Hardy vegetables such as lettuce, peas and spinach can go in as early as 2 to 4 weeks before the median last frost, which works out to May 7 to May 21[6]. Semi-hardy vegetables such as carrots and potatoes can go in as early as 2 weeks before the median last frost, from May 21 up to the median date[9]. Tender vegetables such as snap beans and sweet corn can be seeded around June 4[10]. Very tender vegetables such as tomatoes and peppers typically wait 2 or more weeks after the median last frost, so the earliest start is June 18[11].

  • Hardy (broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips): as early as 2 to 4 weeks before the median last frost (June 4) is May 7 to May 21[6]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (June 4) is May 21, up to the median date itself[9]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, June 4[10]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (June 4), is June 18[11]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] May 7 Jul 11 (from transplants)
Cabbage (hardy)[12] May 7 Jul 31 (from transplants)
Kohlrabi (hardy)[12] May 7 Jun 26
Lettuce (hardy)[12] May 7 Jul 6
Onions (hardy)[12] May 7 Aug 25
Peas (hardy)[12] May 7 Jul 11
Radishes (hardy)[12] May 7 Jun 6
Spinach (hardy)[12] May 7 Jun 16
Turnips (hardy)[12] May 7 Jun 26
Beets (semi-hardy)[12] May 21 Jul 20
Carrots (semi-hardy)[12] May 21 Jul 30
Cauliflower (semi-hardy)[12] May 21 not listed
Parsnips (semi-hardy)[12] May 21 Jul 30
Potatoes (semi-hardy)[12] May 21 Sep 23
Swiss chard (semi-hardy)[12] May 21 Jul 20
Snap beans (tender)[12] Jun 4 Aug 3
Sweet corn (tender)[12] Jun 4 Aug 3
Cucumbers (tender)[12] Jun 4 Jul 29
Summer squash (tender)[12] Jun 4 Jul 24
Cantaloupe (very tender)[12] Jun 18 Sep 11
Eggplant (very tender)[12] Jun 18 Aug 17 (from transplants)
Peppers (very tender)[12] Jun 18 Aug 27 (from transplants)
Tomatoes (very tender)[12] Jun 18 Aug 22 (from transplants)
Winter squash (very tender)[12] Jun 18 Sep 26
Watermelon (very tender)[12] Jun 18 Sep 11
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 2023 USDA map puts the center of Flagstaff in zone 6a, which means an average yearly coldest night of -10°F to -5°F[1]. The zone comes from the coldest winter night, not from summer heat or frost dates[14]. Flagstaff covers 3 ZIP code areas, with 54% of the city's land area in zone 6a areas and 46% in zone 6b areas[15].

Zone 6a: average annual extreme minimum temperature -10°F to -5°F (1991 to 2020)[1]. Modeled value at the city center: -5.9°F[1].

Share of the city's land area by ZIP code zone: zone 6a 54%, zone 6b 46%[15].

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

Zone by ZIP code
ZIP code Zone Coldest night range
86004[15] 6b -5 to 0°F
86005[15] 6a -10 to -5°F
86001[15] 6a -10 to -5°F

Questions people ask

What growing zone is Flagstaff?
The center of Flagstaff is in USDA zone 6a[1]. About 46% of the city's land area is in ZIP code areas rated zone 6b[15].
When is the last frost in Flagstaff?
The median last spring frost is June 4, so in half of years the last frost comes after that date[2]. In 1 of 10 years it comes after June 19[2].
When is the first frost in Flagstaff?
The median first fall frost is September 28[3]. In 1 of 10 years it comes before September 16[3].
When to plant tomatoes in Flagstaff?
CSU Extension typically says to wait 2 or more weeks after the average last spring frost, which gives an earliest start of June 18[11]. CSU's guide was written for Colorado[11].
How long is the growing season in Flagstaff?
The season between the last spring and first fall frost is at least 115 days in half of years[4]. It is at least 96 days in 9 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Flagstaff, Arizona 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 5.9°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 May 21, in half of years after June 4, and in 1 of 10 years after June 19 (NOAA 1991 to 2020 normals). The median last frost date is June 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 16, in half of years before September 28, and in 9 of 10 years before October 9 (NOAA 1991 to 2020 normals). The median first frost date is September 28. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 96 days in 9 of 10 years, at least 115 days in half of years, and at least 133 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Flagstaff Pulliam Airport weather station (USW00003103), 3.8 miles from the center of Flagstaff at 7,003 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 May 20 and the median first fall date is October 9. 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. Flagstaff, Arizona overlaps 3 ZIP code areas. By land area, 54% lies in ZIP code areas rated zone 6a, 46% lies in ZIP code areas rated zone 6b. 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.