Frost Dates: First and last frost dates for Knoxville, 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 October 2.
- There is a 50% chance of being hit by a 32° frost starting around October 12
- You have a 80% chance of seeing 32° by October 22
- 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° | 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 |
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 10. 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 21, so we don't dare plant that early. We wait a few days and by April 28 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° | 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 |
Data is provided by the NOAA National Centers for Environmental Information.