USDA Hardiness Zone 4b
Frost Dates for 59601 — Helena, Montana
NOAA 1991–2020 normals from Helena Regional Airport, MT (3,828 ft), 4 miles east.
Updated
In Helena, the last spring frost (32°F) is around May 9 in half of years and has passed by May 25 in nine years out of ten. The first fall frost comes around September 30, as early as September 17 one year in ten, leaving a typical freeze-free season of 142 days (NOAA, Helena Regional Airport, MT).
- Last spring frost
- May 9
- First fall frost
- September 30
- Growing season
- 142 days
- Hardiness zone
- 4b
50% probability
50% probability
freeze-free, 50%
-25 to -20°F (-31.7 to -28.9°C) coldest night
- First snow
- October 26
- Last snow
- April 16
- Winters with snow
- 100%
- Snow per year
- 37.2"
median, 1991–2020
median, 1991–2020
21 of 21 observed
NOAA normal
Frost risk by date in Helena
Half of springs at Helena Regional Airport, MT see their last 32°F night by May 9; only one in ten is past it by April 29, and one in ten still has one after May 25. Planting tender crops after May 15 leaves a 30% risk of a frost on them, after May 19 20%, and after May 25 10%. The 16 days between the even-odds date and the one-in-ten date is narrower than at 40% of the 68 Montana stations behind this site's ZIP pages.
In fall the first 32°F night comes by September 30 in half of years, as early as September 17 one year in ten, and holds off past October 11 one year in ten. A hard freeze of 28°F follows 7 days later, around October 7; in spring the one-in-ten date for 28°F is May 11, 14 days before the 32°F one. The freeze-free season lasts 142 days in a typical year, longer than at 78% of the 68 Montana stations; it reaches 159 days one year in ten and falls short of 121 one year in ten. On average 170 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 16 days longer, to about May 25, and return around September 19. Severe cold of 16°F or below comes every winter, usually between November 7 and March 22.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 10 | May 15 | May 18 | May 21 | May 25 | May 28 | Jun 1 | Jun 6 | Jun 13 |
| 32°F | Apr 29 | May 2 | May 4 | May 7 | May 9 | May 12 | May 15 | May 19 | May 25 |
| 28°F | Apr 15 | Apr 19 | Apr 22 | Apr 26 | Apr 29 | May 1 | May 3 | May 6 | May 11 |
| 24°F | Apr 1 | Apr 5 | Apr 10 | Apr 13 | Apr 16 | Apr 20 | Apr 24 | Apr 28 | May 1 |
| 20°F | Mar 13 | Mar 21 | Mar 28 | Mar 31 | Apr 4 | Apr 6 | Apr 10 | Apr 15 | Apr 21 |
| 16°F | Mar 3 | Mar 8 | Mar 12 | Mar 17 | Mar 22 | Mar 27 | Mar 31 | Apr 4 | Apr 11 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 4 | Sep 8 | Sep 12 | Sep 16 | Sep 19 | Sep 22 | Sep 25 | Sep 28 | Oct 1 |
| 32°F | Sep 17 | Sep 22 | Sep 25 | Sep 27 | Sep 30 | Oct 1 | Oct 4 | Oct 7 | Oct 11 |
| 28°F | Sep 25 | Sep 30 | Oct 2 | Oct 5 | Oct 7 | Oct 10 | Oct 14 | Oct 17 | Oct 22 |
| 24°F | Oct 1 | Oct 6 | Oct 11 | Oct 15 | Oct 18 | Oct 21 | Oct 25 | Oct 28 | Nov 2 |
| 20°F | Oct 11 | Oct 19 | Oct 24 | Oct 27 | Oct 30 | Nov 1 | Nov 5 | Nov 9 | Nov 13 |
| 16°F | Oct 22 | Oct 28 | Nov 1 | Nov 4 | Nov 7 | Nov 10 | Nov 14 | Nov 17 | Nov 23 |
Freeze probabilities 1991–2020 at Helena Regional Airport, MT, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 59601 →Compare Helena with another place →
Weather by month in Helena
July is the warmest month at Helena Regional Airport, MT, averaging 86°F by day and 55°F at night; January is the coldest at 32°F and 14°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 84% of years.
Warmth for growing adds up to 2,130 growing degree days a year (base 50°F), more than at 74% of the 68 Montana stations. 26% of them (562) have built up by the end of June, and July adds the most, 638. Precipitation totals 11.5 inches a year, most in June (2.2") and least in January (0.4"); May to September bring 63% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 32 | 14 | 0.4 | 100% | 29 | 0 |
| Feb | 37 | 17 | 0.4 | 100% | 26 | 0 |
| Mar | 48 | 25 | 0.5 | 100% | 25 | 5 |
| Apr | 57 | 32 | 1.0 | 100% | 15 | 33 |
| May | 66 | 42 | 2.0 | 84% | 3 | 168 |
| Jun | 75 | 49 | 2.2 | 2% | <1 | 355 |
| Jul | 86 | 55 | 1.1 | 0% | 0 | 638 |
| Aug | 85 | 53 | 1.0 | 1% | 0 | 583 |
| Sep | 73 | 45 | 1.0 | 53% | 2 | 287 |
| Oct | 58 | 34 | 0.8 | 100% | 14 | 57 |
| Nov | 43 | 23 | 0.6 | 100% | 25 | 3 |
| Dec | 33 | 14 | 0.5 | 100% | 30 | 0 |
Monthly normals 1991–2020 at Helena Regional Airport, MT (3,828 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, Tizer Gardens, MT, 15 miles south of Helena and 844 feet higher, has a last frost 40 days later (June 18), a first frost 25 days earlier (September 5), a season 61 days shorter.
When it snows in Helena, Montana
The first measurable snow (0.1 inch or more) in Helena usually arrives around October 26, and the last of the season around April 16. Those are the middle dates across the winters of 1991–2020 at Helena Regional Airport, MT, 3.9 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 23 days after the last snow.
Planting calendar for 59601
| When | What to do |
|---|---|
| February 28 – March 14 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 14 – March 28 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| April 11 – April 25 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| May 9 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 8 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 16 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| September 30 onward | Plant garlic, mulch beds, lift tender bulbsGarlic wants a cold spell; bare soil loses structure over winter. |
The weeks before and after the 50% dates are general rules of thumb, not NOAA figures; a seed packet gives the timing for each variety.
Where these figures come from
ZIP 59601 is 4 miles from Helena Regional Airport, MT, which stands at 3,828 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 2 other ZIP codes share the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59601 in zone 4b, the band where the coldest night of the year averaged -25 to -20°F (-31.7 to -28.9°C) over 1991–2020; a zone describes winter lows, not frost dates.
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.