Frost dates for Great Falls, Montana
In Great Falls, half of springs have had their last frost by May 6 (and 1 spring in 10 still has one after May 19), and half of falls have had their first frost by October 1, leaving about 147 frost-free days in a typical year. Here is how sure those dates are, what they mean for planting, and where they come from.
Frost dates for
Great Falls, Montana
Last spring frost
May 6
Half of springs have their last frost by this date. For a safer bet, wait until May 19: only 1 spring in 10 has a frost after that.
First fall frost
October 1
Half of falls have their first frost by this date. In 1 fall in 10 it comes by September 17.
Growing season
147 days
Frost-free days in a typical year. 9 years in 10 get at least 129.
Hardiness zone
4b
2023 USDA map: the coldest night of winter averages −25°F to −20°F. On the 2012 map it was 4a, half a zone colder. All 4 ZIP codes within about 6 miles of the city's center are in zone 4b.
Print a year planner for Great Falls
Frost risk through the year
How sure are these dates?
A frost date is a probability, not a promise. NOAA worked out, from 30 years of nightly lows, how often the last spring frost and the first fall frost fall on each date. The middle column is the typical year. The outer columns are the early and late years that come about once a decade.
| Early year 1 in 10 | Typical year 1 in 2 | Late year 1 in 10 | |
|---|---|---|---|
| Last frost in spring | April 23 | May 6 | May 19 |
| First frost in fall | September 17 | October 1 | October 16 |
The frost-free season between them runs 147 days in a typical year. In 1 year in 10 it is 166 days or longer, and in 1 year in 10 it is 129 days or shorter.
- Hard freeze (28°F): typically last on April 24 and first on October 11. It is a harder freeze than the 32°F frost line, cold enough to damage many garden plants; how hardy a crop is decides whether it comes through. (The National Weather Service's own hard-freeze threshold varies from office to office.)
- Frost possible (36°F at the station): typically last on May 19 and first on September 21. On clear, still nights the ground can be several degrees colder than the thermometer, so frost can form on plants when the official reading is in the mid-30s.
In 8 springs out of 10, the last frost in Great Falls comes between April 23 and May 19, a window of 26 days. That is narrower than at most NOAA stations (the typical window is 31 days), so the typical date is a steadier guide here than in most places.
Among Montana's 216 NOAA stations with a frost every year, 95 percent have their typical last frost later than Great Falls does.
Planting calendar for Great Falls
These dates count from the typical last and first frosts, using the planting rules published by Colorado State University Extension (spring), Virginia Cooperative Extension (fall crops) and University of Minnesota Extension (garlic, where winters are hard). Weather in any one year can run early or late, so check the forecast before you plant tender crops.
Your season is too short for a separate fall planting of cabbage and broccoli: sown late enough to be a fall crop, it wouldn't be ready before the typical first frost. Your spring planting is your harvest.
| When | What to do | Why then |
|---|---|---|
| February | ||
| February 18 to March 18 | Broccoli and cabbage Start seeds indoors | 5 to 7 weeks before planting them out |
| March | ||
| March 4 to April 1 | Cauliflower Start seeds indoors | 5 to 7 weeks before planting out |
| March 18 to April 8 | Eggplant Start seeds indoors | 6 to 9 weeks before planting out |
| March 25 to April 8 | Peppers Start seeds indoors | 6 to 8 weeks before planting out |
| April | ||
| April 1 to 15 | Tomatoes Start seeds indoors | 5 to 7 weeks before planting out |
| April 8 to 22 | Peas, spinach, lettuce, radishes, onion sets, turnips Sow outdoors, as early as | hardy crops: 2 to 4 weeks before the typical last frost |
| April 8 to 22 | Broccoli and cabbage Plant out, as early as | hardy crops: 2 to 4 weeks before the typical last frost |
| April 22 to May 6 | Beets, carrots, potatoes, Swiss chard Plant outdoors, as early as | semi-hardy crops: up to 2 weeks before the typical last frost |
| April 22 to May 6 | Cauliflower Plant out, as early as | semi-hardy: up to 2 weeks before the typical last frost |
| May | ||
| May 6 | Snap beans, sweet corn, cucumbers, summer squash Sow outdoors around | tender crops: around the typical last frost date |
| May 20 | Tomatoes, peppers, eggplant Plant out from | very tender: 2 weeks or more after the typical last frost |
| May 20 | Melons, winter squash, pumpkins, lima beans Sow outdoors from | very tender: 2 weeks or more after the typical last frost |
| July | ||
| July 5 | Snap beans for fall Last date to sow | 60 days to harvest, plus 2 weeks for slower fall growth, plus 2 weeks because frost kills it |
| July 9 | Carrots for fall Last date to sow | 70 days to harvest, plus 2 weeks for slower fall growth |
| July 10 | Cucumbers for fall Last date to sow | 55 days to harvest, plus 2 weeks for slower fall growth, plus 2 weeks because frost kills it |
| July 14 | Peas for fall Last date to sow | 65 days to harvest, plus 2 weeks for slower fall growth |
| July 15 | Summer squash for fall Last date to sow | 50 days to harvest, plus 2 weeks for slower fall growth, plus 2 weeks because frost kills it |
| July 19 | Leaf lettuce and beets for fall Last date to sow | 60 days to harvest, plus 2 weeks for slower fall growth |
| July 29 | Turnips and kohlrabi for fall Last date to sow | 50 days to harvest, plus 2 weeks for slower fall growth |
| August | ||
| August 8 | Spinach for fall Last date to sow | 40 days to harvest, plus 2 weeks for slower fall growth |
| August 18 | Radishes for fall Last date to sow | 30 days to harvest, plus 2 weeks for slower fall growth |
| September | ||
| September 17 | Tomatoes, peppers, squash, melons, cucumbers Watch the forecast from | in 1 fall in 10 the first frost comes by this date; pick tender crops before a frost |
| October | ||
| October 18 to 25 | Garlic Plant cloves | one or two weeks after the typical first hard freeze (28°F), our reading of the first killing frost |
Want more insurance? Shift every spring date 13 days later. That plans around May 19, the date after which only 1 spring in 10 has a frost.
For spring bulbs such as tulips and daffodils, plant once the soil has cooled to about 50 to 55°F and keep going until a couple of weeks before the ground freezes. Our guide to fall planting explains how to tell.
Where these numbers come from
These dates come from NOAA's weather station Great Falls 16st (station USC00243749), 1 mile from the center of Great Falls. It sits 170 feet higher, at 3,500 feet above sea level. NOAA rates this station's record as representative: at least 10 of the 30 years, with the gaps filled from nearby stations.
Our ZIP code lookup matches these 5 ZIP codes to the same station, so they get the same dates: 59401, 59404, 59405, 59412, 59414. Hardiness zones are worked out separately for each ZIP code.
The nearest other places with pages
- Missoula, Montana: last frost around May 16, first frost around September 26 (135 miles away)
See frost dates across Montana, or look up your own ZIP code for the station nearest you. Want the background? Read what a last frost date really means.