Frost Dates: First and last frost dates for Iowa City, IA

As your growing season comes to an end, the nightly temperatures for Iowa City, 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 October 4, and by October 26 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 October 10.
2. There is a 50% chance of being hit by a 32° frost starting around October 18
3. You have a 80% chance of seeing 32° by October 26
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? Frost can form on plants even when the air temperature is above 32°. This happens because of radiative cooling. On clear nights, surfaces like leaves and the ground radiate their heat to the cold night sky. As dew forms on these surfaces, those water droplets continue losing heat and can reach freezing even when the surrounding air is a few degrees warmer. This is why frost is more common on clear, calm nights (clouds act as insulation against this heat loss) and why low-lying areas frost first, since the coldest air settles near the ground. So many factors come into play, including wind, cloud cover, nearby structures, trees, and terrain. 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 15. 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 22, so we don't dare plant that early. We wait a few days and by May 1 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. If conditions change and a surprise frost does threaten, there's always things you can do to protect plants from frost.

The Victory Seed Company is family owned and operated, working to help fellow gardeners succeed by selling popular vegetable, herb and flower seeds. Since the 1990s they have been heavily involved with the preservation of rare, open-pollinated, heirloom seeds. Visit them at www.VictorySeeds.com.