Frost Dates: First and last frost dates for Lewistown, 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 July 6, you have a 90% chance of being frost-free — this is your "safe date" for planting tender crops
  • Around June 15, there's a 50/50 chance of frost — check weather forecasts carefully if planting now
  • Before June 1, 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 17 Apr 12 Apr 9 Apr 6 Apr 3 Mar 31 Mar 28 Mar 25 Mar 20
Last 20° Apr 25 Apr 20 Apr 17 Apr 14 Apr 11 Apr 9 Apr 6 Apr 3 Mar 29
Last 24° May 7 May 2 Apr 28 Apr 25 Apr 23 Apr 20 Apr 17 Apr 13 Apr 9
Last 28° May 21 May 17 May 14 May 11 May 9 May 6 May 4 Apr 30 Apr 26
Last 32° Jun 10 Jun 4 Jun 1 May 28 May 25 May 22 May 19 May 15 May 10
Last 36° Jul 6 Jun 29 Jun 23 Jun 19 Jun 15 Jun 11 Jun 6 Jun 1 May 25

ℹ️ How to read these charts for the fall

As summer ends in Lewistown, 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 6, there's a 20% chance of 32°F — start monitoring forecasts and have plant covers ready
2
Decision time: By September 15, there's a 50% chance of frost — harvest tomatoes, peppers, and other tender crops or prepare protection
3
Last chance: By September 24, 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 September 24 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 10 Oct 16 Oct 20 Oct 24 Oct 27 Oct 30 Nov 3 Nov 7 Nov 13
First 20° Sep 28 Oct 4 Oct 9 Oct 13 Oct 16 Oct 20 Oct 24 Oct 28 Nov 3
First 24° Sep 22 Sep 28 Oct 2 Oct 6 Oct 9 Oct 13 Oct 16 Oct 20 Oct 26
First 28° Sep 12 Sep 17 Sep 21 Sep 24 Sep 26 Sep 29 Oct 2 Oct 5 Oct 10
First 32° Sep 2 Sep 6 Sep 10 Sep 12 Sep 15 Sep 18 Sep 20 Sep 24 Sep 28
First 36° Aug 17 Aug 23 Aug 27 Aug 31 Sep 4 Sep 7 Sep 11 Sep 16 Sep 22

📍 Frost Dates for other nearby locations

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