USDA Hardiness Zone 4a
Frost Dates for 59230 — Glasgow, Montana
NOAA 1991–2020 normals from Glasgow #2, MT (2,090 ft), 4 miles northeast.
Updated
In Glasgow, the last spring frost (32°F) is around May 3 in half of years and has passed by May 15 in nine years out of ten. The first fall frost comes around October 1, as early as September 19 one year in ten, leaving a typical freeze-free season of 151 days (NOAA, Glasgow #2, MT).
- Last spring frost
- May 3
- First fall frost
- October 1
- Growing season
- 151 days
- Hardiness zone
- 4a
50% probability
50% probability
freeze-free, 50%
-30 to -25°F (-34.4 to -31.7°C) coldest night
- First snow
- October 16
- Last snow
- April 16
- Winters with snow
- 100%
- Snow per year
- 43.7"
median, 1991–2020
median, 1991–2020
15 of 15 observed
NOAA normal
Frost risk by date in Glasgow
Half of springs at Glasgow #2, MT see their last 32°F night by May 3; only one in ten is past it by April 19, and one in ten still has one after May 15. Planting tender crops after May 8 leaves a 30% risk of a frost on them, after May 11 20%, and after May 15 10%. The 12 days between the even-odds date and the one-in-ten date is the narrowest 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 19 one year in ten, and holds off past October 15 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 6, 9 days before the 32°F one. The freeze-free season lasts 151 days in a typical year, longer than at 94% of the 68 Montana stations; it reaches 168 days one year in ten and falls short of 134 one year in ten. On average 175 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 11 days longer, to about May 14, and return around September 23. Severe cold of 16°F or below comes every winter, usually between November 6 and March 25.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 2 | May 6 | May 9 | May 11 | May 14 | May 16 | May 19 | May 22 | May 26 |
| 32°F | Apr 19 | Apr 25 | Apr 29 | May 1 | May 3 | May 6 | May 8 | May 11 | May 15 |
| 28°F | Apr 8 | Apr 13 | Apr 17 | Apr 20 | Apr 23 | Apr 26 | Apr 30 | May 2 | May 6 |
| 24°F | Mar 29 | Apr 2 | Apr 6 | Apr 9 | Apr 13 | Apr 16 | Apr 19 | Apr 23 | Apr 28 |
| 20°F | Mar 15 | Mar 22 | Mar 27 | Mar 31 | Apr 3 | Apr 5 | Apr 9 | Apr 13 | Apr 18 |
| 16°F | Mar 6 | Mar 11 | Mar 15 | Mar 20 | Mar 25 | Mar 29 | Apr 1 | Apr 4 | Apr 9 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 11 | Sep 16 | Sep 18 | Sep 21 | Sep 23 | Sep 25 | Sep 27 | Oct 1 | Oct 4 |
| 32°F | Sep 19 | Sep 23 | Sep 26 | Sep 28 | Oct 1 | Oct 4 | Oct 6 | Oct 10 | Oct 15 |
| 28°F | Sep 25 | Sep 30 | Oct 4 | Oct 7 | Oct 11 | Oct 14 | Oct 17 | Oct 21 | Oct 25 |
| 24°F | Oct 3 | Oct 10 | Oct 15 | Oct 19 | Oct 21 | Oct 24 | Oct 26 | Oct 30 | Nov 3 |
| 20°F | Oct 16 | Oct 21 | Oct 25 | Oct 28 | Oct 31 | Nov 1 | Nov 5 | Nov 8 | Nov 13 |
| 16°F | Oct 24 | Oct 29 | Nov 1 | Nov 3 | Nov 6 | Nov 9 | Nov 12 | Nov 16 | Nov 21 |
Freeze probabilities 1991–2020 at Glasgow #2, 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 59230 →Compare Glasgow with another place →
Weather by month in Glasgow
July is the warmest month at Glasgow #2, MT, averaging 87°F by day and 59°F at night; January is the coldest at 25°F and 4°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 62% of years.
Warmth for growing adds up to 2,492 growing degree days a year (base 50°F), more than at 90% of the 68 Montana stations. 31% of them (767) have built up by the end of June, and July adds the most, 705.
| Month | High °F | Low °F | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|
| Jan | 25 | 4 | 100% | 31 | 0 |
| Feb | 30 | 8 | 100% | 28 | 0 |
| Mar | 44 | 21 | 100% | 27 | 6 |
| Apr | 59 | 33 | 100% | 14 | 54 |
| May | 70 | 44 | 62% | 3 | 237 |
| Jun | 78 | 53 | 0% | 0 | 470 |
| Jul | 87 | 59 | 0% | 0 | 705 |
| Aug | 85 | 57 | 0% | 0 | 651 |
| Sep | 73 | 46 | 48% | 1 | 309 |
| Oct | 57 | 33 | 100% | 14 | 58 |
| Nov | 40 | 19 | 100% | 27 | 3 |
| Dec | 28 | 8 | 100% | 31 | 0 |
Monthly normals 1991–2020 at Glasgow #2, MT (2,090 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
When it snows in Glasgow, Montana
The first measurable snow (0.1 inch or more) in Glasgow usually arrives around October 16, and the last of the season around April 16. Those are the middle dates across the winters of 1991–2020 at Glasgow WFO, MT, 4.9 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 17 days after the last snow.
Planting calendar for 59230
| When | What to do |
|---|---|
| February 22 – March 8 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 8 – March 22 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| April 5 – April 19 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| May 3 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 59230 is 4 miles from Glasgow #2, MT, which stands at 2,090 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59230 in zone 4a, the band where the coldest night of the year averaged -30 to -25°F (-34.4 to -31.7°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.