GardenFacts

All states › Wyoming › Casper

Frost dates and planting calendar for Casper, Wyoming

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

USDA hardiness zone
5a[1]
Median last spring frost
May 10[2]
Median first fall frost
Sep 29[3]
Growing season, median
138 days[4]

The USDA map puts the center of Casper in zone 5a[1]. In half of years the last spring frost comes after May 10, the NOAA median[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after Apr 30, half of years May 10, 1 in 10 years after May 24Last frostmedian May 10First fall frost: 9 in 10 years before Oct 12, half of years Sep 29, 1 in 10 years before Sep 14First frostmedian Sep 29Frost riskHardyHardy vegetables: plant Apr 12 to Apr 26Apr 12 to Apr 26Semi-hardySemi-hardy vegetables: plant Apr 26 to May 10Apr 26 to May 10TenderTender vegetables: plant around May 10around May 10Very tenderVery tender vegetables: plant from May 24from May 24
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[2]. They come from the Casper Wwtp weather station, 2 miles from the city center[5]. In 9 of 10 years the last spring frost comes after April 30, and in 1 of 10 years it comes after May 24[2]. In half of years the first fall frost comes before September 29[3]. In 1 of 10 years it comes before September 14[3].

Frost Likely (9 in 10 years) Even odds (5 in 10) Unlikely (1 in 10)
Last spring frost (32°F)[2] after April 30 after May 10 after May 24
First fall frost (32°F)[3] before September 14 before September 29 before October 12
  • Hard freeze (28°F): median last spring date May 1, median first fall date October 6[7].
  • Frost-free growing season: at least 118 days in 9 of 10 years, 138 days in half of years and 155 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: Casper Wwtp (USC00481569), 2 miles from the city center, 5,097 feet, 18 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension wrote its planting guide for Colorado, so treat it as a guide and not a Casper rule[6]. Hardy vegetables such as peas, spinach and lettuce can go in as early as 2-4 weeks before the average last spring frost, which is April 12 to April 26 using the median[6]. Semi-hardy vegetables such as carrots and potatoes can go in as early as 2 weeks before the median, which is April 26[9]. Tender vegetables such as snap beans and sweet corn can be seeded around May 10[10]. Very tender vegetables such as tomatoes and peppers typically wait 2 or more weeks after the median, so May 24 is the earliest start[11].

  • Hardy (broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips): as early as 2 to 4 weeks before the median last frost (May 10) is April 12 to April 26[6]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (May 10) is April 26, up to the median date itself[9]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, May 10[10]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (May 10), is May 24[11]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] Apr 12 Jun 16 (from transplants)
Cabbage (hardy)[12] Apr 12 Jul 6 (from transplants)
Kohlrabi (hardy)[12] Apr 12 Jun 1
Lettuce (hardy)[12] Apr 12 Jun 11
Onions (hardy)[12] Apr 12 Jul 31
Peas (hardy)[12] Apr 12 Jun 16
Radishes (hardy)[12] Apr 12 May 12
Spinach (hardy)[12] Apr 12 May 22
Turnips (hardy)[12] Apr 12 Jun 1
Beets (semi-hardy)[12] Apr 26 Jun 25
Carrots (semi-hardy)[12] Apr 26 Jul 5
Cauliflower (semi-hardy)[12] Apr 26 not listed
Parsnips (semi-hardy)[12] Apr 26 Jul 5
Potatoes (semi-hardy)[12] Apr 26 Aug 29
Swiss chard (semi-hardy)[12] Apr 26 Jun 25
Snap beans (tender)[12] May 10 Jul 9
Sweet corn (tender)[12] May 10 Jul 9
Cucumbers (tender)[12] May 10 Jul 4
Summer squash (tender)[12] May 10 Jun 29
Cantaloupe (very tender)[12] May 24 Aug 17
Eggplant (very tender)[12] May 24 Jul 23 (from transplants)
Peppers (very tender)[12] May 24 Aug 2 (from transplants)
Tomatoes (very tender)[12] May 24 Jul 28 (from transplants)
Winter squash (very tender)[12] May 24 Sep 1
Watermelon (very tender)[12] May 24 Aug 17
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

Zone 5a means the average coldest night of the winter is -20°F to -15°F, based on 1991 to 2020[1]. The zone is not about summer heat or frost dates[14]. All 4 ZIP code areas that overlap Casper are rated zone 5a, by land area[15].

Zone 5a: average annual extreme minimum temperature -20°F to -15°F (1991 to 2020)[1]. Modeled value at the city center: -17°F[1].

Share of the city's land area by ZIP code zone: zone 5a 100%[15].

All towns in zone 5a and what zone 5a means

Zone by ZIP code
ZIP code Zone Coldest night range
82601[15] 5a -20 to -15°F
82604[15] 5a -20 to -15°F
82609[15] 5a -20 to -15°F
82644[15] 5a -20 to -15°F

Questions people ask

What growing zone is Casper, Wyoming?
The 2023 USDA map puts the center of Casper in zone 5a[1]. By land area, 100% of the city lies in ZIP code areas rated 5a[15].
When is the last frost in Casper?
The median last spring frost is May 10 at the NOAA station, so in half of years it comes after that date[2]. In 1 of 10 years it comes after May 24[2].
When to plant tomatoes in Casper?
CSU Extension says very tender vegetables like tomatoes typically go in 2 or more weeks after the average last frost[11]. Using the median, the earliest start is May 24[11].
When is the first frost in Casper?
The median first fall frost is September 29, so in half of years it comes before that date[3]. In 1 of 10 years it comes before September 14[3].
How long is the growing season in Casper?
The season between the last and first 32°F frost is at least 138 days in half of years[4]. It is at least 118 days in 9 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Casper, Wyoming in zone 5a. Zone 5a means an average annual extreme minimum temperature of -20°F to -15°F over 1991 to 2020. The map's modeled value at the city center is minus 17°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 30, in half of years after May 10, and in 1 of 10 years after May 24 (NOAA 1991 to 2020 normals). The median last frost date is May 10. 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 14, in half of years before September 29, and in 9 of 10 years before October 12 (NOAA 1991 to 2020 normals). The median first frost date is September 29. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 118 days in 9 of 10 years, at least 138 days in half of years, and at least 155 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Casper Wwtp weather station (USC00481569), 2 miles from the center of Casper at 5,097 feet, using 18 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 1 and the median first fall date is October 6. 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. Casper, Wyoming overlaps 4 ZIP code areas. By land area, 100% lies in ZIP code areas rated zone 5a. 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.