Frost Dates: First and last frost dates for Zortman, 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 5, you have a 90% chance of being frost-free — this is your "safe date" for planting tender crops
  • Around June 9, there's a 50/50 chance of frost — check weather forecasts carefully if planting now
  • Before May 23, 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 21 Apr 15 Apr 11 Apr 8 Apr 4 Apr 1 Mar 28 Mar 24 Mar 18
Last 20° May 19 May 9 May 2 Apr 26 Apr 21 Apr 15 Apr 9 Apr 3 Mar 24
Last 24° May 23 May 14 May 8 May 3 Apr 28 Apr 24 Apr 18 Apr 12 Apr 4
Last 28° Jun 7 May 29 May 22 May 16 May 11 May 6 Apr 30 Apr 24 Apr 15
Last 32° Jun 21 Jun 12 Jun 6 May 31 May 26 May 21 May 16 May 9 Apr 30
Last 36° Jul 5 Jun 26 Jun 20 Jun 14 Jun 9 Jun 4 May 29 May 23 May 14

ℹ️ How to read these charts for the fall

As summer ends in Zortman, 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 5, there's a 20% chance of 32°F — start monitoring forecasts and have plant covers ready
2
Decision time: By September 16, there's a 50% chance of frost — harvest tomatoes, peppers, and other tender crops or prepare protection
3
Last chance: By September 28, 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 28 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 5 Oct 11 Oct 15 Oct 18 Oct 21 Oct 24 Oct 28 Nov 1 Nov 6
First 20° Sep 27 Oct 2 Oct 5 Oct 8 Oct 10 Oct 13 Oct 16 Oct 19 Oct 24
First 24° Sep 19 Sep 23 Sep 26 Sep 28 Oct 1 Oct 3 Oct 5 Oct 8 Oct 12
First 28° Sep 11 Sep 15 Sep 18 Sep 21 Sep 23 Sep 26 Sep 28 Oct 2 Oct 6
First 32° Aug 30 Sep 5 Sep 9 Sep 13 Sep 16 Sep 20 Sep 23 Sep 28 Oct 4
First 36° Aug 17 Aug 23 Aug 27 Aug 31 Sep 4 Sep 7 Sep 11 Sep 15 Sep 22

📍 Frost Dates for other nearby locations

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