Frost Dates: First and last frost dates for Portola, CA

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 29, you have a 90% chance of being frost-free — this is your "safe date" for planting tender crops
  • Around July 15, 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° Apr 17 Apr 7 Mar 30 Mar 24 Mar 18 Mar 12 Mar 6 Feb 26 Feb 16
Last 20° May 7 Apr 28 Apr 21 Apr 15 Apr 10 Apr 4 Mar 29 Mar 23 Mar 13
Last 24° May 25 May 17 May 12 May 7 May 2 Apr 27 Apr 22 Apr 17 Apr 9
Last 28° Jul 3 Jun 25 Jun 19 Jun 14 Jun 9 Jun 4 May 30 May 24 May 16
Last 32° Jul 18 Jul 10 Jul 5 Jul 1 Jun 26 Jun 22 Jun 18 Jun 13 Jun 5
Last 36° Jul 29 Jul 24 Jul 21 Jul 18 Jul 15 Jul 12 Jul 9 Jul 5 Jun 30

ℹ️ How to read these charts for the fall

As summer ends in Portola, CA, 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 29, there's a 50% chance of frost — harvest tomatoes, peppers, and other tender crops or prepare protection
3
Last chance: By September 12, 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 12 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 30 Nov 6 Nov 11 Nov 15 Nov 19 Nov 23 Nov 27 Dec 2 Dec 8
First 20° Oct 10 Oct 18 Oct 23 Oct 28 Nov 2 Nov 6 Nov 11 Nov 16 Nov 24
First 24° Sep 29 Oct 6 Oct 11 Oct 15 Oct 19 Oct 23 Oct 28 Nov 2 Nov 9
First 28° Aug 27 Sep 4 Sep 10 Sep 16 Sep 21 Sep 26 Oct 1 Oct 7 Oct 16
First 32° Aug 8 Aug 15 Aug 21 Aug 25 Aug 29 Sep 3 Sep 7 Sep 12 Sep 20
First 36° Aug 1 Aug 5 Aug 7 Aug 10 Aug 12 Aug 14 Aug 17 Aug 20 Aug 23

📍 Frost Dates for other nearby locations

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