Frost Dates: First and last frost dates for Deer Lodge, 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 31, you have a 90% chance of being frost-free — this is your "safe date" for planting tender crops
  • Around July 16, there's a 50/50 chance of frost — check weather forecasts carefully if planting now
  • Before July 5, 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° May 3 Apr 27 Apr 22 Apr 18 Apr 15 Apr 11 Apr 7 Apr 3 Mar 27
Last 20° May 14 May 10 May 7 May 4 May 2 Apr 29 Apr 26 Apr 23 Apr 19
Last 24° Jun 4 May 29 May 25 May 21 May 18 May 14 May 11 May 7 May 1
Last 28° Jul 5 Jun 27 Jun 22 Jun 17 Jun 12 Jun 8 Jun 3 May 29 May 21
Last 32° Jul 19 Jul 13 Jul 8 Jul 5 Jul 1 Jun 28 Jun 24 Jun 20 Jun 14
Last 36° Jul 31 Jul 26 Jul 22 Jul 19 Jul 16 Jul 12 Jul 9 Jul 5 Jun 30

ℹ️ How to read these charts for the fall

As summer ends in Deer Lodge, 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 August 15, there's a 20% chance of 32°F — start monitoring forecasts and have plant covers ready
2
Decision time: By August 25, there's a 50% chance of frost — harvest tomatoes, peppers, and other tender crops or prepare protection
3
Last chance: By September 5, 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 5 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° Sep 26 Oct 1 Oct 5 Oct 9 Oct 12 Oct 15 Oct 18 Oct 22 Oct 28
First 20° Sep 6 Sep 14 Sep 19 Sep 24 Sep 28 Oct 2 Oct 7 Oct 12 Oct 19
First 24° Sep 2 Sep 7 Sep 10 Sep 13 Sep 16 Sep 19 Sep 21 Sep 25 Sep 30
First 28° Aug 23 Aug 28 Aug 31 Sep 3 Sep 6 Sep 9 Sep 12 Sep 15 Sep 20
First 32° Aug 9 Aug 15 Aug 19 Aug 22 Aug 25 Aug 29 Sep 1 Sep 5 Sep 11
First 36° Jul 31 Aug 5 Aug 8 Aug 11 Aug 14 Aug 17 Aug 19 Aug 23 Aug 28

📍 Frost Dates for other nearby locations

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