Frost Dates: First and last frost dates for Duluth Harbor Stn, MN

How to read these charts for the fall
As your growing season comes to an end, the nightly temperatures for Duluth Harbor Stn, MN 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 15, and by October 6 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 19.
  2. There is a 50% chance of being hit by a 32° frost starting around September 28
  3. You have a 80% chance of seeing 32° by October 6
  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 23 Oct 28 Oct 31 Nov 3 Nov 6 Nov 9 Nov 12 Nov 15 Nov 20
First 20° Oct 18 Oct 23 Oct 27 Oct 30 Nov 2 Nov 5 Nov 8 Nov 12 Nov 17
First 24° Oct 8 Oct 14 Oct 18 Oct 21 Oct 24 Oct 27 Oct 31 Nov 4 Nov 9
First 28° Sep 22 Sep 27 Oct 1 Oct 4 Oct 7 Oct 10 Oct 14 Oct 17 Oct 23
First 32° Sep 15 Sep 19 Sep 22 Sep 25 Sep 28 Sep 30 Oct 3 Oct 6 Oct 11
First 36° Sep 8 Sep 13 Sep 16 Sep 18 Sep 21 Sep 23 Sep 25 Sep 28 Oct 3

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 30. 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 May 10, so we don't dare plant that early. We wait a few days and by May 18 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 13 Apr 9 Apr 6 Apr 4 Apr 2 Mar 30 Mar 28 Mar 25 Mar 21
Last 20° Apr 20 Apr 16 Apr 13 Apr 11 Apr 9 Apr 7 Apr 4 Apr 1 Mar 28
Last 24° Apr 28 Apr 24 Apr 21 Apr 18 Apr 15 Apr 13 Apr 10 Apr 7 Apr 3
Last 28° May 12 May 7 May 3 Apr 30 Apr 28 Apr 25 Apr 22 Apr 18 Apr 13
Last 32° May 22 May 18 May 15 May 12 May 9 May 7 May 4 May 1 Apr 26
Last 36° May 30 May 26 May 23 May 20 May 18 May 16 May 13 May 10 May 6

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