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Frost dates and planting calendar for Bismarck, North Dakota

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

USDA hardiness zone
4a[1]
Median last spring frost
May 14[2]
Median first fall frost
Sep 27[3]
Growing season, median
136 days[4]

Bismarck, North Dakota is in USDA hardiness zone 4a[1]. The median last spring frost at the Bismarck weather station is May 14[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after May 1, half of years May 14, 1 in 10 years after May 27Last frostmedian May 14First fall frost: 9 in 10 years before Oct 9, half of years Sep 27, 1 in 10 years before Sep 15First frostmedian Sep 27Frost riskHardyHardy vegetables: plant Apr 16 to Apr 30Apr 16 to Apr 30Semi-hardySemi-hardy vegetables: plant Apr 30 to May 14Apr 30 to May 14TenderTender vegetables: plant around May 14around May 14Very tenderVery tender vegetables: plant from May 28from May 28
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 come from NOAA climate normals for the Bismarck weather station, using records from 1991 to 2020[5]. They are odds, not promises. In half of years the last spring frost comes after May 14, and in 1 of 10 years it comes after May 27[2]. In the fall, the median first frost date is September 27, and in 1 of 10 years the first frost comes before September 15[3]. The low reached 32°F or colder in 100% of years at this station[6]. Any year can be earlier or later.

Frost Likely (9 in 10 years) Even odds (5 in 10) Unlikely (1 in 10)
Last spring frost (32°F)[2] after May 1 after May 14 after May 27
First fall frost (32°F)[3] before September 15 before September 27 before October 9
  • Hard freeze (28°F): median last spring date May 3, median first fall date October 5[8].
  • Frost-free growing season: at least 117 days in 9 of 10 years, 136 days in half of years and 153 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[6].
  • Weather station: Bismarck (USW00024011), 3.1 miles from the city center, 1,658 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[7]. Hardy vegetables like broccoli, lettuce, peas and spinach can go in as early as 2 to 4 weeks before the median last frost, which works out to April 16 to April 30[7]. Semi-hardy vegetables like carrots, beets and potatoes can go in as early as 2 weeks before the median last frost, which is April 30[9]. Tender vegetables like snap beans and sweet corn can be planted from seed around May 14[10]. Very tender vegetables like tomatoes, peppers and watermelon typically start 2 or more weeks after the median last frost, so the earliest start is May 28[11].

  • Hardy (broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips): as early as 2 to 4 weeks before the median last frost (May 14) is April 16 to April 30[7]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (May 14) is April 30, up to the median date itself[9]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, May 14[10]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (May 14), is May 28[11]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] Apr 16 Jun 20 (from transplants)
Cabbage (hardy)[12] Apr 16 Jul 10 (from transplants)
Kohlrabi (hardy)[12] Apr 16 Jun 5
Lettuce (hardy)[12] Apr 16 Jun 15
Onions (hardy)[12] Apr 16 Aug 4
Peas (hardy)[12] Apr 16 Jun 20
Radishes (hardy)[12] Apr 16 May 16
Spinach (hardy)[12] Apr 16 May 26
Turnips (hardy)[12] Apr 16 Jun 5
Beets (semi-hardy)[12] Apr 30 Jun 29
Carrots (semi-hardy)[12] Apr 30 Jul 9
Cauliflower (semi-hardy)[12] Apr 30 not listed
Parsnips (semi-hardy)[12] Apr 30 Jul 9
Potatoes (semi-hardy)[12] Apr 30 Sep 2
Swiss chard (semi-hardy)[12] Apr 30 Jun 29
Snap beans (tender)[12] May 14 Jul 13
Sweet corn (tender)[12] May 14 Jul 13
Cucumbers (tender)[12] May 14 Jul 8
Summer squash (tender)[12] May 14 Jul 3
Cantaloupe (very tender)[12] May 28 Aug 21
Eggplant (very tender)[12] May 28 Jul 27 (from transplants)
Peppers (very tender)[12] May 28 Aug 6 (from transplants)
Tomatoes (very tender)[12] May 28 Aug 1 (from transplants)
Winter squash (very tender)[12] May 28 Sep 5
Watermelon (very tender)[12] May 28 Aug 21
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 4a means the coldest night of the winter averages -30°F to -25°F, based on 1991 to 2020[1]. The zone is about winter lows, not summer heat or frost dates[14]. All 4 ZIP code areas that overlap Bismarck are rated zone 4a by land area, so the city does not span more than one zone[15].

Zone 4a: average annual extreme minimum temperature -30°F to -25°F (1991 to 2020)[1]. Modeled value at the city center: -25.5°F[1].

Share of the city's land area by ZIP code zone: zone 4a 100%[15].

All towns in zone 4a and what zone 4a means

Zone by ZIP code
ZIP code Zone Coldest night range
58504[15] 4a -30 to -25°F
58503[15] 4a -30 to -25°F
58501[15] 4a -30 to -25°F
58505[15] 4a -30 to -25°F

Questions people ask

What growing zone is Bismarck, North Dakota?
The center of Bismarck is in USDA zone 4a on the 2023 map[1]. All of the city's land area lies in ZIP code areas rated 4a[15].
When is the last frost in Bismarck?
The median last spring frost is May 14, so in half of years the last frost comes after that date[2]. In 1 of 10 years it comes after May 27[2].
When is the first frost in Bismarck?
The median first fall frost is September 27[3]. In 1 of 10 years the first frost comes before September 15[3].
When to plant tomatoes in Bismarck?
CSU Extension says very tender vegetables like tomatoes typically start 2 or more weeks after the average last frost, which gives May 28 as the earliest start here[11]. CSU's guide was written for Colorado[11].
How long is the growing season in Bismarck?
In half of years the growing season is at least 136 days[4]. It is at least 117 days in 9 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Bismarck, North Dakota in zone 4a. Zone 4a means an average annual extreme minimum temperature of -30°F to -25°F over 1991 to 2020. The map's modeled value at the city center is minus 25.5°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 1, in half of years after May 14, and in 1 of 10 years after May 27 (NOAA 1991 to 2020 normals). The median last frost date is May 14. 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 15, in half of years before September 27, and in 9 of 10 years before October 9 (NOAA 1991 to 2020 normals). The median first frost date is September 27. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 117 days in 9 of 10 years, at least 136 days in half of years, and at least 153 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Bismarck weather station (USW00024011), 3.1 miles from the center of Bismarck at 1,658 feet, using 26 years of records from 1991 to 2020. NOAA Climate Normals 1991-2020
  6. 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
  7. 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
  8. Hard freeze (28°F) at this station: the median last spring date is May 3 and the median first fall date is October 5. 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. Bismarck, North Dakota overlaps 4 ZIP code areas. By land area, 100% lies in ZIP code areas rated zone 4a. 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.