USDA Hardiness Zone 4b
Frost Dates for 59405 — Great Falls, Montana
NOAA 1991–2020 normals from Great Falls 16st, MT (3,505 ft), 1 miles northwest.
Updated
In Great Falls, the last spring frost (32°F) is around May 6 in half of years and has passed by May 19 in nine years out of ten. The first fall frost comes around October 1, as early as September 17 one year in ten, leaving a typical freeze-free season of 147 days (NOAA, Great Falls 16st, MT).
- Last spring frost
- May 6
- First fall frost
- October 1
- Growing season
- 147 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 5
- Last snow
- May 11
- Winters with snow
- 100%
- Snow per year
- 66.1"
median, 1991–2020
median, 1991–2020
30 of 30 observed
NOAA normal
Frost risk by date in Great Falls
Half of springs at Great Falls 16st, MT see their last 32°F night by May 6; only one in ten is past it by April 23, and one in ten still has one after May 19. Planting tender crops after May 11 leaves a 30% risk of a frost on them, after May 14 20%, and after May 19 10%. The 13 days between the even-odds date and the one-in-ten date is narrower than at 74% of the 68 Montana stations behind this site's ZIP pages.
In fall the first 32°F night comes by October 1 in half of years, as early as September 17 one year in ten, and holds off past October 16 one year in ten. A hard freeze of 28°F follows 10 days later, around October 11; in spring the one-in-ten date for 28°F is May 8, 11 days before the 32°F one. The freeze-free season lasts 147 days in a typical year, longer than at 88% of the 68 Montana stations; it reaches 166 days one year in ten and falls short of 129 one year in ten. On average 143 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 13 days longer, to about May 19, and return around September 21. Severe cold of 16°F or below comes every winter, usually between November 8 and March 25.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 6 | May 11 | May 14 | May 16 | May 19 | May 21 | May 24 | May 27 | Jun 1 |
| 32°F | Apr 23 | Apr 28 | May 1 | May 3 | May 6 | May 8 | May 11 | May 14 | May 19 |
| 28°F | Apr 7 | Apr 13 | Apr 17 | Apr 21 | Apr 24 | Apr 27 | Apr 30 | May 3 | May 8 |
| 24°F | Mar 28 | Apr 2 | Apr 6 | Apr 9 | Apr 13 | Apr 16 | Apr 21 | Apr 25 | Apr 30 |
| 20°F | Mar 15 | Mar 21 | Mar 28 | Apr 1 | Apr 3 | Apr 7 | Apr 10 | Apr 14 | Apr 21 |
| 16°F | Mar 4 | Mar 10 | Mar 15 | Mar 20 | Mar 25 | Mar 30 | Apr 2 | Apr 6 | Apr 11 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 7 | Sep 11 | Sep 14 | Sep 18 | Sep 21 | Sep 23 | Sep 25 | Sep 29 | Oct 3 |
| 32°F | Sep 17 | Sep 22 | Sep 25 | Sep 28 | Oct 1 | Oct 4 | Oct 7 | Oct 11 | Oct 16 |
| 28°F | Sep 25 | Oct 1 | Oct 4 | Oct 7 | Oct 11 | Oct 15 | Oct 19 | Oct 23 | Oct 29 |
| 24°F | Oct 2 | Oct 8 | Oct 15 | Oct 20 | Oct 24 | Oct 27 | Oct 30 | Nov 2 | Nov 8 |
| 20°F | Oct 8 | Oct 19 | Oct 26 | Oct 29 | Nov 1 | Nov 3 | Nov 7 | Nov 12 | Nov 19 |
| 16°F | Oct 23 | Oct 29 | Nov 1 | Nov 5 | Nov 8 | Nov 12 | Nov 16 | Nov 21 | Nov 26 |
Freeze probabilities 1991–2020 at Great Falls 16st, 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 59405 →Compare Great Falls with another place →
Weather by month in Great Falls
July is the warmest month at Great Falls 16st, MT, averaging 86°F by day and 55°F at night; January is the coldest at 37°F and 18°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 71% of years.
Warmth for growing adds up to 2,214 growing degree days a year (base 50°F), more than at 81% of the 68 Montana stations. 28% of them (628) have built up by the end of June, and July adds the most, 632. Precipitation totals 15.4 inches a year, most in May (2.7") and least in January (0.6"); May to September bring 61% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 37 | 18 | 0.6 | 100% | 24 | 1 |
| Feb | 39 | 19 | 0.6 | 100% | 24 | 5 |
| Mar | 48 | 25 | 0.6 | 100% | 22 | 12 |
| Apr | 58 | 33 | 1.8 | 100% | 14 | 50 |
| May | 67 | 42 | 2.7 | 71% | 3 | 181 |
| Jun | 75 | 50 | 2.7 | 1% | <1 | 379 |
| Jul | 86 | 55 | 1.3 | 0% | 0 | 632 |
| Aug | 84 | 53 | 1.3 | 0% | 0 | 578 |
| Sep | 73 | 45 | 1.4 | 47% | 1 | 281 |
| Oct | 59 | 36 | 1.1 | 100% | 11 | 81 |
| Nov | 45 | 26 | 0.7 | 100% | 19 | 13 |
| Dec | 37 | 20 | 0.6 | 100% | 26 | 3 |
Monthly normals 1991–2020 at Great Falls 16st, MT (3,505 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, Great Falls International Airport, MT, 5 miles west of Great Falls and 159 feet higher, has a last frost 13 days later (May 19), a first frost 7 days earlier (September 24), a season 21 days shorter.
When it snows in Great Falls, Montana
The first measurable snow (0.1 inch or more) in Great Falls usually arrives around October 5, and the last of the season around May 11. Those are the middle dates across the winters of 1991–2020 at Great Falls International Airport, MT, 5.3 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 5 days before the last snow.
Planting calendar for 59405
| When | What to do |
|---|---|
| February 25 – March 11 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 11 – March 25 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| April 8 – April 22 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| May 6 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 9 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 17 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| October 1 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 59405 is 1 mile from Great Falls 16st, MT, which stands at 3,505 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59405 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.