Frost Dates: First and last frost dates for Great Falls, MT

ℹ️ How to read these charts for the spring

As spring unfolds, the risk of frost gradually decreases each day until your tender plants are finally safe. The probabilities shown in the chart help you make smart decisions about when to plant.

While frost typically forms at 32°F, tender plants can be damaged even when temperatures are slightly higher. Air temperatures are often measured several feet above the ground, but temperatures at plant level can be several degrees colder. Protected areas like porches help by blocking radiational cooling. Given these variables, we consider 36°F the "caution zone" for frost-sensitive plants.

Key dates for your spring planting:

  • By June 19, you have a 90% chance of being frost-free — this is your "safe date" for planting tender crops
  • Around May 31, there's a 50/50 chance of frost — check weather forecasts carefully if planting now
  • Before May 18, there's still an 80% risk of frost — too early for tomatoes and other tender plants unless you're prepared to protect them

If the 10-day forecast shows no temperatures below 40°F, you might safely plant earlier than the above dates suggest. Many gardeners watch for this weather window to get a head start on the growing season.

In the Spring
Temperature 10% 20% 30% 40% 50% 60% 70% 80% 90%
Last 16° Apr 12 Apr 7 Apr 4 Apr 1 Mar 29 Mar 26 Mar 23 Mar 19 Mar 14
Last 20° Apr 23 Apr 17 Apr 13 Apr 9 Apr 5 Apr 2 Mar 29 Mar 25 Mar 19
Last 24° May 4 Apr 28 Apr 24 Apr 20 Apr 17 Apr 13 Apr 10 Apr 5 Mar 30
Last 28° May 13 May 8 May 4 May 1 Apr 28 Apr 25 Apr 22 Apr 19 Apr 14
Last 32° Jun 1 May 27 May 23 May 20 May 17 May 14 May 11 May 7 May 2
Last 36° Jun 19 Jun 12 Jun 8 Jun 4 May 31 May 27 May 23 May 18 May 11

ℹ️ How to read these charts for the fall

As summer ends in Great Falls, MT, nighttime temperatures begin to drop, and your risk of frost increases with each passing day. Knowing when to expect frost helps you harvest sensitive crops and extend your growing season with protection methods.

The chart below shows the probability of experiencing specific temperatures on various dates. Here's what this means for your garden:

1
Early warning: Around September 13, there's a 20% chance of 32°F — start monitoring forecasts and have plant covers ready
2
Decision time: By September 22, there's a 50% chance of frost — harvest tomatoes, peppers, and other tender crops or prepare protection
3
Last chance: By October 1, there's an 80% chance of seeing 32°F — most tender plants will be damaged without protection
4
Another perspective: You have only a 1 in 5 chance of making it past October 1 without a freezing night

These probabilities help you time your end-of-season harvests and know when to apply protective measures like row covers, cloches, or cold frames to extend your harvest.

In the Fall
Temperature 10% 20% 30% 40% 50% 60% 70% 80% 90%
First 16° Oct 14 Oct 21 Oct 26 Oct 31 Nov 4 Nov 8 Nov 12 Nov 17 Nov 25
First 20° Oct 5 Oct 12 Oct 17 Oct 21 Oct 25 Oct 29 Nov 2 Nov 7 Nov 14
First 24° Sep 28 Oct 4 Oct 8 Oct 11 Oct 15 Oct 18 Oct 21 Oct 25 Oct 31
First 28° Sep 23 Sep 27 Sep 30 Oct 3 Oct 6 Oct 8 Oct 11 Oct 14 Oct 18
First 32° Sep 8 Sep 13 Sep 16 Sep 19 Sep 22 Sep 24 Sep 27 Oct 1 Oct 5
First 36° Aug 31 Sep 5 Sep 8 Sep 11 Sep 14 Sep 17 Sep 20 Sep 24 Sep 29

📍 Frost Dates for other nearby locations

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Data is provided by the NOAA National Centers for Environmental Information.