USDA Hardiness Zone 5a
Frost Dates for 59632 — Boulder, Montana
NOAA 1991–2020 normals from Boulder, MT (4,904 ft), 3 miles northwest.
Updated
In Boulder, the last spring frost (32°F) is around June 7 in half of years and has passed by June 25 in nine years out of ten. The first fall frost comes around September 8, as early as August 25 one year in ten, leaving a typical freeze-free season of 90 days (NOAA, Boulder, MT).
- Last spring frost
- June 7
- First fall frost
- September 8
- Growing season
- 90 days
- Hardiness zone
- 5a
50% probability
50% probability
freeze-free, 50%
-20 to -15°F (-28.9 to -26.1°C) coldest night
- First snow
- November 3
- Last snow
- April 16
- Winters with snow
- 81%
- Snow per year
- —
median, 1991–2020
median, 1991–2020
21 of 26 observed
NOAA normal
Frost risk by date in Boulder
Half of springs at Boulder, MT see their last 32°F night by June 7; only one in ten is past it by May 24, and one in ten still has one after June 25. Planting tender crops after June 14 leaves a 30% risk of a frost on them, after June 19 20%, and after June 25 10%. The 18 days between the even-odds date and the one-in-ten date is wider than at 68% of the 68 Montana stations behind this site's ZIP pages.
In fall the first 32°F night comes by September 8 in half of years, as early as August 25 one year in ten, and holds off past September 21 one year in ten. A hard freeze of 28°F follows 13 days later, around September 21; in spring the one-in-ten date for 28°F is June 5, 20 days before the 32°F one. The freeze-free season lasts 90 days in a typical year, shorter than at 93% of the 68 Montana stations; it reaches 110 days one year in ten and falls short of 68 one year in ten. On average 205 nights a year drop to 32°F or below. Severe cold of 16°F or below comes every winter, usually between October 19 and April 14.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | too rare for a date | ||||||||
| 32°F | May 24 | May 29 | Jun 2 | Jun 4 | Jun 7 | Jun 10 | Jun 14 | Jun 19 | Jun 25 |
| 28°F | May 7 | May 11 | May 14 | May 18 | May 21 | May 24 | May 27 | May 31 | Jun 5 |
| 24°F | Apr 24 | Apr 30 | May 2 | May 5 | May 8 | May 11 | May 13 | May 17 | May 22 |
| 20°F | Apr 9 | Apr 16 | Apr 20 | Apr 23 | Apr 27 | Apr 30 | May 2 | May 6 | May 12 |
| 16°F | Mar 24 | Apr 1 | Apr 5 | Apr 10 | Apr 14 | Apr 18 | Apr 22 | Apr 27 | May 2 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | too rare for a date | ||||||||
| 32°F | Aug 25 | Aug 30 | Sep 2 | Sep 5 | Sep 8 | Sep 10 | Sep 14 | Sep 17 | Sep 21 |
| 28°F | Sep 6 | Sep 11 | Sep 15 | Sep 18 | Sep 21 | Sep 23 | Sep 26 | Sep 30 | Oct 3 |
| 24°F | Sep 15 | Sep 21 | Sep 25 | Sep 27 | Sep 30 | Oct 2 | Oct 4 | Oct 8 | Oct 12 |
| 20°F | Sep 26 | Sep 30 | Oct 3 | Oct 6 | Oct 9 | Oct 12 | Oct 15 | Oct 18 | Oct 23 |
| 16°F | Oct 3 | Oct 8 | Oct 13 | Oct 16 | Oct 19 | Oct 23 | Oct 27 | Oct 30 | Nov 3 |
Freeze probabilities 1991–2020 at Boulder, 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 59632 →Compare Boulder with another place →
Weather by month in Boulder
July is the warmest month at Boulder, MT, averaging 83°F by day and 48°F at night; December is the coldest at 34°F and 12°F. A freezing night is all but certain in every month from September to May (a 90% chance or more). July is the only month with under a 10% chance of a 32°F night. In spring, June still brings one in 72% of years.
Warmth for growing adds up to 1,477 growing degree days a year (base 50°F), fewer than at 85% of the 68 Montana stations. 23% of them (335) have built up by the end of June, and July adds the most, 478. Precipitation totals 10.9 inches a year, most in June (2.3") and least in February (0.2"); May to September bring 72% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 35 | 12 | 0.3 | 100% | 30 | 0 |
| Feb | 38 | 14 | 0.2 | 100% | 27 | 0 |
| Mar | 47 | 21 | 0.4 | 100% | 28 | 1 |
| Apr | 55 | 27 | 0.8 | 100% | 23 | 13 |
| May | 64 | 35 | 1.9 | 100% | 10 | 90 |
| Jun | 72 | 43 | 2.3 | 72% | 2 | 232 |
| Jul | 83 | 48 | 1.4 | 4% | <1 | 478 |
| Aug | 82 | 46 | 1.1 | 24% | 1 | 440 |
| Sep | 72 | 38 | 1.1 | 98% | 6 | 193 |
| Oct | 57 | 28 | 0.6 | 100% | 21 | 31 |
| Nov | 42 | 19 | 0.5 | 100% | 27 | 1 |
| Dec | 34 | 12 | 0.3 | 100% | 30 | 0 |
Monthly normals 1991–2020 at Boulder, MT (4,904 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, 12 miles north of Boulder and 232 feet lower, has a last frost 11 days later (June 18), a first frost 3 days earlier (September 5), a season 9 days shorter.
When it snows in Boulder, Montana
The first measurable snow (0.1 inch or more) in Boulder usually arrives around November 3, and the last of the season around April 16. Those are the middle dates across the winters of 1991–2020 at Boulder, MT, 2.8 miles away: half of years saw snow earlier, half later. Snow fell in 81% of those winters. The last 32°F night of spring typically comes 52 days after the last snow.
Planting calendar for 59632
| When | What to do |
|---|---|
| March 29 – April 12 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| April 12 – April 26 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| May 10 – May 24 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| June 7 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by June 16 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by August 25 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| September 8 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 59632 is 3 miles from Boulder, MT, which stands at 4,904 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59632 in zone 5a, the band where the coldest night of the year averaged -20 to -15°F (-28.9 to -26.1°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.