Last Frost – Counting the Days

How to Determine the Day in Your Area

The gardener’s mantra in winter is, “how many days until the last frost?” But if you ask three gardeners you know what the last frost date is in your area, I’ll bet you get three different answers. That’s because determining “average” last frost is a lot like betting. Scientists can’t really add up a bunch of dates and divide by the number of dates and get an “average.”  So they go for the technically more accurate “median” last frost and give you probabilities. You know probabilities….like the days that had an 80 percent probability of rain and the sun shone all day….or the picnic you planned on a day with 20% chance of precipitation and you got drenched in an afternoon thunderstorm. We just can’t definitively predict the weather.  And we can’t say….gee, global warming…I guess I can start my seeds early.

But good gamblers know how to hedge their bets….and that’s what we do with the last frost. We go to the official climate records at https://www.noaa.gov/

There, you select your state, then find your city and look in the third column that lists the 90% probability for temperatures above 32 degrees.  That means that in the last 100 years they were keeping records, the temperatures went below 32 after that date only ten of those years. Good odds.

Hedging your bets though means being ready just in case. Ten of those years it did freeze after the magic 90% probability date.  And there are other variables. All those temperatures were recorded 5 feet above ground…meaning it might have been colder on the ground where your tender little basil was. And you know how many little micro-climates there are in your yard….the south-facing bed next to the house, the bed on the north side that doesn’t warm up till June. So pick your date…but have your frost blanket ready just in case! Here in Boulder, Colorado the 90% probability date we use is May 14th, but the actual last frost back in 1951 was June 3rd!

Get your big calendar out and circle your last frost date in red. Trackback each week and write in how many weeks left until the last frost….and you’ll easily know if it’s time to plant that packet of seed yet.  If you’re really an organized person, you can clip your seed packets to the appropriate week to plant. Or write the date for sowing on the front of your seed packet.

Count the days.  Here in Boulder, it’s 11 weeks until the last frost. Too soon to start tomatoes but about right for starting perennials, cold hardy herbs and onions.