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

As your growing season comes to an end, the nightly temperatures for Williamsburg, 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 24, and by October 14 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 28.
2. There is a 50% chance of being hit by a 32° frost starting around October 6
3. You have a 80% chance of seeing 32° by October 14
4. Said another way, you have a 1 in 5 chance at making it to that day without a 32° night.

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 22. 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 28, so we don't dare plant that early. We wait a few days and by May 7 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.