This is a short note on something that I’m sure is obvious to most seasoned gardeners, but not to me. I’ll try to keep it short.
Edit: My wife informs me that this is more-or-less incomprehensible. Let me try to boil it down and save you the trouble of reading the full post.
A last-frost date tells you how likely you are to lose frost-sensitive plants if you blindly set them out on that date. For example, the Old Farmers Almanac gives you the “70th percentile” last frost date, meaning, if you set your plants out on that date, there’s a 30 percent chance a frost will come along and kill them.
But if you don’t just set them out blindly on that date, and instead pay attention to the seven-day forecast on that date, you actually have a much lower chance of having your plants killed by frost. Because, a) seven-day forecasts are pretty good and b) if frost is in the forecast, on that date, you’ll have the good sense not to set your plants out (duh).
The only real insight here is that, in effect, the labels attached to those last-frost dates — the likelihood you’ll lose your plants to frost — are wrong. (Or, really, they are only correct if you’re dumb enough to go ahead and plant with frost in the forecast). The forecast lets you see with good accuracy seven days into the future, nobody is dumb enough to set out their tomatoes with frost in the forecast, and the combination of those two factors means that your actual risk of having frost kill your plants is much lower than the label on that date would suggest.
In my case (Vienna, VA, Zone 7), what’s labeled as the 70th percentile last frost date (nominal 30% risk of having frost kill my plants) actually only carries a 10% risk of having frost kill my plants. And that’s because, on that date, you’ve got a real-time seven-day look into the future via the weather forecast.
I think that’s now all completely obvious. As I say, the only real insight here is that the labels on the dates are, in a practical sense, wrong. Those labels (e.g., 70th percentile) come from simply tabulating the last frost dates for the prior three decades. They don’t account for the fact that you’ll have the good sense to wait, if frost is in the forecast as of that date. And the second but important insight is that those labels are quite a bit wrong. Just having the common sense to check the seven-day forecast (in my location) cuts that nominal 30 percent risk of frost damage down to 10 percent.
Two caveats: The size of this “forecast” effect depends on where you live. In some areas, the spread between these percentile last-frost dates is larger than it is here in Vienna, VA. And, this doesn’t help you if the planting instructions tell you to (e.g.) plant two weeks before the last-frost date. That’s outside the forecast window (or, at least, the reasonably-accurate-forecast window.) This is really just about plants that should be planted directly after the last-frost date.
Back to the original and now totally unnecessary post. You might want to read the final addendum, because if you actually work through the logic here, the likelihood of frost killing your plants is even lower than what I just said.
Continue reading Post #G21-005: Your 70th percentile last frost date is actually your 90th percentile last frost date.