What IS our annual Frost Date?

I mentioned in an earlier post that it’s important to know your annual spring frost date so you don’t plant too early. Most seedlings die off very quickly in a light frost situation and you don’t want all of your hard work to go to waste. But what do you do when your frost date differs depending on which source you rely upon?

For instance, I live in Chicago, Illinois and here are what three different websites have to say about our spring frost date:

The Old Farmers’ Almanac
April 20: chance of frost after this date = 50%
My grandmother might skin me alive for this, but these guys are off their rockers. I’m sure they have a plethora of data, but I think it’s the wrong data. They listed South Bend, Indiana’s frost date as April 26? How in the world did Chicago, a city with a much harsher climate than South Bend get a frost date almost an entire week earlier? And we’re only three days behind Indianapolis? I would hazard a guess that they’re relying a bit too much on their percentage of chance of frost after that date. But do you really want to chance killing off your plants? It’s not like they’re cheap to replace.

Victory Seed Company
April 25
They state on their website that this data is from NOAA (National Oceanic and Atmospheric Administration). If anyone would have the data—the CORRECT data—I think the Feds would. However, if you read along, you notice that the data is from 1988. Not that climate change would be that significant 20 years ago, but shouldn’t they maybe update their page? And this is a spit in the eye of global warming (which I do wholeheartedly believe in) since April 25 is still a mite early to be planting. How in the world did Chicago end up with a date of 4/25 when East St. Louis (all the way down at the southernmost part of the state) ended up with a spring frost date of 5/1? Sheesh! Does everyone in East St. Louis leave their refrigerators open?

NOAA
Okay, here’s where we get the real truth, with all of the probability numbers to back it up. I live right in between O’Hare and Midway airport where the government captures the weather information, so I’ll list both locations:

>50% probability that temps will drop to 32° F
O’Hare: April 25
Midway: April 20
>10% probability that temps will drop to 32° F
O’Hare: May 10
Midway: May 6

Since I’m in the middle, I’d say the dates are April 23 and May 8. I like being safe, so I’m not going to plant until May 8.