Frost Dates: First and last frost dates for Heron, 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 18, you have a 90% chance of being frost-free — this is your "safe date" for planting tender crops
  • Around June 27, there's a 50/50 chance of frost — check weather forecasts carefully if planting now
  • Before June 13, 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° Mar 29 Mar 22 Mar 16 Mar 11 Mar 7 Mar 3 Feb 26 Feb 20 Feb 13
Last 20° Apr 22 Apr 14 Apr 8 Apr 3 Mar 30 Mar 25 Mar 20 Mar 14 Mar 6
Last 24° May 5 Apr 28 Apr 23 Apr 19 Apr 15 Apr 11 Apr 7 Apr 2 Mar 26
Last 28° May 23 May 18 May 14 May 10 May 7 May 4 May 1 Apr 27 Apr 21
Last 32° Jun 24 Jun 17 Jun 11 Jun 7 Jun 2 May 29 May 24 May 19 May 11
Last 36° Jul 18 Jul 10 Jul 5 Jul 1 Jun 27 Jun 23 Jun 19 Jun 13 Jun 6

ℹ️ How to read these charts for the fall

As summer ends in Heron, 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 3, there's a 20% chance of 32°F — start monitoring forecasts and have plant covers ready
2
Decision time: By September 13, there's a 50% chance of frost — harvest tomatoes, peppers, and other tender crops or prepare protection
3
Last chance: By September 23, 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 23 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 24 Nov 2 Nov 9 Nov 15 Nov 21 Nov 26 Dec 2 Dec 9 Dec 18
First 20° Oct 8 Oct 16 Oct 22 Oct 27 Oct 31 Nov 5 Nov 10 Nov 16 Nov 24
First 24° Sep 24 Oct 1 Oct 6 Oct 10 Oct 14 Oct 18 Oct 23 Oct 28 Nov 4
First 28° Sep 15 Sep 20 Sep 24 Sep 27 Sep 30 Oct 3 Oct 6 Oct 10 Oct 15
First 32° Aug 29 Sep 3 Sep 7 Sep 10 Sep 13 Sep 16 Sep 20 Sep 23 Sep 29
First 36° Aug 9 Aug 15 Aug 19 Aug 22 Aug 26 Aug 29 Sep 1 Sep 6 Sep 11

📍 Frost Dates for other nearby locations

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