Frost Dates: First and last frost dates for Knoxville, IA

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 May 10, you have a 90% chance of being frost-free — this is your "safe date" for planting tender crops
  • Around April 28, there's a 50/50 chance of frost — check weather forecasts carefully if planting now
  • Before April 21, 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 30 Mar 23 Mar 19 Mar 15 Mar 11 Mar 8 Mar 4 Feb 27 Feb 21
Last 20° Apr 8 Apr 2 Mar 29 Mar 25 Mar 22 Mar 19 Mar 15 Mar 11 Mar 5
Last 24° Apr 12 Apr 8 Apr 4 Apr 2 Mar 30 Mar 27 Mar 24 Mar 21 Mar 17
Last 28° Apr 18 Apr 15 Apr 12 Apr 10 Apr 8 Apr 5 Apr 3 Mar 31 Mar 28
Last 32° May 5 Apr 29 Apr 26 Apr 22 Apr 19 Apr 16 Apr 13 Apr 9 Apr 4
Last 36° May 10 May 6 May 3 May 1 Apr 28 Apr 26 Apr 24 Apr 21 Apr 17

ℹ️ How to read these charts for the fall

As summer ends in Knoxville, IA, 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 October 2, there's a 20% chance of 32°F — start monitoring forecasts and have plant covers ready
2
Decision time: By October 12, there's a 50% chance of frost — harvest tomatoes, peppers, and other tender crops or prepare protection
3
Last chance: By October 22, 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 October 22 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° Nov 7 Nov 13 Nov 17 Nov 20 Nov 24 Nov 27 Nov 30 Dec 4 Dec 10
First 20° Oct 27 Nov 1 Nov 6 Nov 9 Nov 13 Nov 16 Nov 20 Nov 24 Nov 30
First 24° Oct 19 Oct 25 Oct 29 Nov 1 Nov 5 Nov 8 Nov 11 Nov 16 Nov 21
First 28° Oct 9 Oct 15 Oct 18 Oct 22 Oct 25 Oct 28 Oct 31 Nov 4 Nov 9
First 32° Sep 26 Oct 2 Oct 5 Oct 9 Oct 12 Oct 15 Oct 18 Oct 22 Oct 27
First 36° Sep 20 Sep 25 Sep 28 Oct 1 Oct 3 Oct 6 Oct 9 Oct 12 Oct 16

📍 Frost Dates for other nearby locations

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