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Frost dates and planting calendar for Helena, Montana

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

USDA hardiness zone
4b[1]
Median last spring frost
May 9[2]
Median first fall frost
Sep 30[3]
Growing season, median
142 days[4]

Helena, Montana sits in USDA hardiness zone 4b[1]. The median last spring frost at the Helena airport station is May 9, so in half of years the last frost comes after that date[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after Apr 29, half of years May 9, 1 in 10 years after May 25Last frostmedian May 9First fall frost: 9 in 10 years before Oct 11, half of years Sep 30, 1 in 10 years before Sep 17First frostmedian Sep 30Frost riskHardyHardy vegetables: plant Apr 11 to Apr 25Apr 11 to Apr 25Semi-hardySemi-hardy vegetables: plant Apr 25 to May 9Apr 25 to May 9TenderTender vegetables: plant around May 9around May 9Very tenderVery tender vegetables: plant from May 23from May 23
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 dates come from NOAA climate normals for the Helena Regional Airport station, using records from 1991 to 2020[5]. They are odds, not promises. In 9 of 10 years the last spring frost comes after April 29, and in 1 of 10 years it comes after May 25[2]. In the fall, the first frost comes before September 30 in half of years, and before October 11 in 9 of 10 years[3]. At this station the low reached 32°F or colder in 100% of years[6].

Frost Likely (9 in 10 years) Even odds (5 in 10) Unlikely (1 in 10)
Last spring frost (32°F)[2] after April 29 after May 9 after May 25
First fall frost (32°F)[3] before September 17 before September 30 before October 11
  • Hard freeze (28°F): median last spring date April 29, median first fall date October 7[8].
  • Frost-free growing season: at least 121 days in 9 of 10 years, 142 days in half of years and 159 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: Helena Regional Airport (USW00024144), 2.8 miles from the city center, 3,828 feet, 25 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension wrote its vegetable planting guide for Colorado, so these dates are a GardenFacts calculation, not a Montana guide[7]. We took the NOAA median last frost of May 9 as the average last frost[7]. Hardy vegetables such as peas, lettuce and spinach can go in as early as 2 to 4 weeks before that, which is April 11 to April 25[7]. Semi-hardy vegetables such as carrots and potatoes can go in as early as April 25, up to the median date itself[9]. Tender vegetables such as snap beans and sweet corn can be sown from seed around May 9[10]. Very tender vegetables such as tomatoes and peppers typically start 2 or more weeks after the median last frost, with May 23 as 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 9) is April 11 to April 25[7]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (May 9) is April 25, up to the median date itself[9]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, May 9[10]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (May 9), is May 23[11]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] Apr 11 Jun 15 (from transplants)
Cabbage (hardy)[12] Apr 11 Jul 5 (from transplants)
Kohlrabi (hardy)[12] Apr 11 May 31
Lettuce (hardy)[12] Apr 11 Jun 10
Onions (hardy)[12] Apr 11 Jul 30
Peas (hardy)[12] Apr 11 Jun 15
Radishes (hardy)[12] Apr 11 May 11
Spinach (hardy)[12] Apr 11 May 21
Turnips (hardy)[12] Apr 11 May 31
Beets (semi-hardy)[12] Apr 25 Jun 24
Carrots (semi-hardy)[12] Apr 25 Jul 4
Cauliflower (semi-hardy)[12] Apr 25 not listed
Parsnips (semi-hardy)[12] Apr 25 Jul 4
Potatoes (semi-hardy)[12] Apr 25 Aug 28
Swiss chard (semi-hardy)[12] Apr 25 Jun 24
Snap beans (tender)[12] May 9 Jul 8
Sweet corn (tender)[12] May 9 Jul 8
Cucumbers (tender)[12] May 9 Jul 3
Summer squash (tender)[12] May 9 Jun 28
Cantaloupe (very tender)[12] May 23 Aug 16
Eggplant (very tender)[12] May 23 Jul 22 (from transplants)
Peppers (very tender)[12] May 23 Aug 1 (from transplants)
Tomatoes (very tender)[12] May 23 Jul 27 (from transplants)
Winter squash (very tender)[12] May 23 Aug 31
Watermelon (very tender)[12] May 23 Aug 16
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 4b means an average coldest night of -25°F to -20°F over 1991 to 2020[1]. The zone is based on the coldest winter night, not on summer heat or frost dates[14]. Helena overlaps 3 ZIP code areas, and by land area 100% of the city lies in ZIP code areas rated zone 4b[15].

Zone 4b: average annual extreme minimum temperature -25°F to -20°F (1991 to 2020)[1]. Modeled value at the city center: -20.2°F[1].

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

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

Zone by ZIP code
ZIP code Zone Coldest night range
59601[15] 4b -25 to -20°F
59602[15] 4b -25 to -20°F
59623[15] 5a -20 to -15°F

Questions people ask

What growing zone is Helena, Montana?
The 2023 USDA map puts the center of Helena in zone 4b[1].
When is the last frost in Helena?
The median last spring frost is May 9, so in half of years the last frost comes after that date[2]. In 1 of 10 years it comes after May 25[2].
When to plant tomatoes in Helena?
CSU Extension says very tender vegetables like tomatoes typically start 2 or more weeks after the average last frost, which gives May 23 as the earliest start here[11].
When is the first frost in Helena?
The median first fall frost is September 30, so in half of years it comes before that date[3].
How long is the growing season in Helena?
The growing season is at least 142 days in half of years[4]. It is at least 121 days in 9 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Helena, Montana in zone 4b. Zone 4b means an average annual extreme minimum temperature of -25°F to -20°F over 1991 to 2020. The map's modeled value at the city center is minus 20.2°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 29, in half of years after May 9, and in 1 of 10 years after May 25 (NOAA 1991 to 2020 normals). The median last frost date is May 9. 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 17, in half of years before September 30, and in 9 of 10 years before October 11 (NOAA 1991 to 2020 normals). The median first frost date is September 30. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 121 days in 9 of 10 years, at least 142 days in half of years, and at least 159 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Helena Regional Airport weather station (USW00024144), 2.8 miles from the center of Helena at 3,828 feet, using 25 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 April 29 and the median first fall date is October 7. 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. Helena, Montana overlaps 3 ZIP code areas. By land area, 100% lies in ZIP code areas rated zone 4b. 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.