Frost Dates: First and last frost dates for grinnell, ia
The charts on this page show the probabilities of receiving a certain temperature on a certain day. Some examples that might help:
- You have a small 20% chance of getting 32° by September 20.
- There is a 50% chance of being hit by a 32° frost starting around September 27
- You have a 80% chance of seeing 32° by October 5
- Said another way, you have a 1 in 5 chance at making it to that day without a 32° night.
Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
---|---|---|---|---|---|---|---|---|---|
First 16° | Oct 25 | Nov 1 | Nov 5 | Nov 9 | Nov 13 | Nov 17 | Nov 20 | Nov 25 | Dec 1 |
First 20° | Oct 16 | Oct 22 | Oct 26 | Oct 29 | Nov 1 | Nov 4 | Nov 8 | Nov 12 | Nov 17 |
First 24° | Oct 5 | Oct 10 | Oct 14 | Oct 18 | Oct 21 | Oct 24 | Oct 28 | Nov 1 | Nov 6 |
First 28° | Sep 26 | Sep 30 | Oct 3 | Oct 5 | Oct 8 | Oct 10 | Oct 13 | Oct 16 | Oct 20 |
First 32° | Sep 16 | Sep 20 | Sep 23 | Sep 25 | Sep 27 | Sep 30 | Oct 2 | Oct 5 | Oct 9 |
First 36° | Sep 9 | Sep 13 | Sep 16 | Sep 18 | Sep 20 | Sep 23 | Sep 25 | Sep 28 | Oct 2 |
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 June 1. 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 9, 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.
Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
---|---|---|---|---|---|---|---|---|---|
Last 16° | Apr 12 | Apr 7 | Apr 3 | Mar 30 | Mar 27 | Mar 24 | Mar 20 | Mar 16 | Mar 11 |
Last 20° | Apr 17 | Apr 13 | Apr 10 | Apr 8 | Apr 5 | Apr 3 | Apr 1 | Mar 29 | Mar 25 |
Last 24° | Apr 27 | Apr 22 | Apr 19 | Apr 16 | Apr 13 | Apr 10 | Apr 7 | Apr 4 | Mar 30 |
Last 28° | May 10 | May 5 | May 1 | Apr 28 | Apr 25 | Apr 23 | Apr 20 | Apr 16 | Apr 11 |
Last 32° | May 20 | May 15 | May 12 | May 9 | May 6 | May 3 | Apr 30 | Apr 26 | Apr 22 |
Last 36° | Jun 1 | May 27 | May 24 | May 21 | May 18 | May 15 | May 12 | May 9 | May 4 |
Data is provided by the NOAA National Centers for Environmental Information.