Frost Dates: First and last frost dates for harlan, ia

How to read these charts for the fall
As your growing season comes to an end, the nightly temperatures for harlan, ia start to go down, and therefore every day that goes by increases the chance that you'll get frost. Your risk of frost really begins around September 22, and by October 16 you're almost certain to have received at least one frost event.

The charts on this page show the probabilities of receiving a certain temperature on a certain day. Some examples that might help:

  1. You have a small 20% chance of getting 32° by September 27.
  2. There is a 50% chance of being hit by a 32° frost starting around October 7
  3. You have a 80% chance of seeing 32° by October 16
  4. Said another way, you have a 1 in 5 chance at making it to that day without a 32° night.
In the Fall
Temperature 10% 20% 30% 40% 50% 60% 70% 80% 90%
First 16° Oct 28 Nov 4 Nov 8 Nov 12 Nov 16 Nov 20 Nov 24 Nov 29 Dec 5
First 20° Oct 20 Oct 26 Oct 30 Nov 3 Nov 6 Nov 9 Nov 13 Nov 17 Nov 23
First 24° Oct 16 Oct 21 Oct 24 Oct 27 Oct 29 Nov 1 Nov 4 Nov 7 Nov 12
First 28° Oct 4 Oct 8 Oct 11 Oct 14 Oct 17 Oct 19 Oct 22 Oct 25 Oct 30
First 32° Sep 22 Sep 27 Oct 1 Oct 4 Oct 7 Oct 9 Oct 13 Oct 16 Oct 21
First 36° Sep 14 Sep 18 Sep 21 Sep 24 Sep 26 Sep 28 Oct 1 Oct 4 Oct 8

How to read these charts for the spring
As the growing season approaches, the days get longer, the weather begins to warm up, and the nights start being less cold. You're still getting frost, but the chance of nightly frosts gets less and less with each passing day. Eventually, the frost stops, and this is when your tender plants are fully safe. So, how do you plan for this? The probabilities on this page help you assess your risk of frost on any given day.

What's a safe temperature for tender plants? When the nightly temperature falls, frost can form, even above 32°, because the air is colder high above the ground and the frost can form up there and then fall down onto your garden and do some damage, even if it's 36° on the ground. So many factors come into play, including wind, concrete, houses, trees and other structures, etc etc etc. Because of all this, you might want to consider 36° as "the danger zone".

In your average springtime, you have a 90% chance that there will be no 36° nights by May 19. In other words, you can pretty much count on being safe from frost by that day. But we want to get those tomatoes in the ground as soon as possible, right? We see that there's still an 80% chance of 36° on April 27, so we don't dare plant that early. We wait a few days and by May 5 we are at the 50/50 point. At this point, we are close and we can start watching the weather forecast. If the upcoming week's forecast doesn't show below 40°, then it's probably okay to risk planting out your plants.

In the Spring
Temperature 10% 20% 30% 40% 50% 60% 70% 80% 90%
Last 16° Apr 3 Mar 29 Mar 25 Mar 21 Mar 18 Mar 15 Mar 11 Mar 7 Mar 2
Last 20° Apr 14 Apr 9 Apr 5 Apr 1 Mar 29 Mar 26 Mar 22 Mar 18 Mar 13
Last 24° Apr 16 Apr 12 Apr 9 Apr 7 Apr 5 Apr 2 Mar 31 Mar 28 Mar 24
Last 28° Apr 30 Apr 25 Apr 22 Apr 19 Apr 16 Apr 13 Apr 11 Apr 7 Apr 3
Last 32° May 12 May 7 May 3 Apr 30 Apr 27 Apr 24 Apr 21 Apr 18 Apr 13
Last 36° May 19 May 14 May 11 May 8 May 5 May 3 Apr 30 Apr 27 Apr 22

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