|Snow just a few days before the Last Frost Date|
A search of the internet will give you a wheelbarrow load of information about the Last Frost Date and much of it is erroneous. I found one popular gardening site which defines the Average Last Frost Date as "the last day in the spring that you might have a killing frost." WRONG. A gardening site that places high on Google searches says the Average Last Frost Date is "the average date in which the last frost usually occurs in your area." Not technically accurate, and redundant, vague, and not very helpful.
When you ask your local Extension office or look to NOAA or the USDA, you can expect to get three different dates for the Last Frost Date and none of them are the date that you will definitely see no more freezing temperatures.
The first published date is the historical date when 10 percent of the time there are no more days below freezing (32F or 0C degrees). Note that it is a date when the temperature is below freezing, not just a frost. Frost can occur above the freezing point. In Colorado Springs, according to NOAA (CSU Extension is a few days later), the 10-percent Last Frost Date is April 24. It doesn't really mean much for most home gardeners except to say that there is a 90 percent chance there will still be freezing weather, so don't get your planting hopes up yet. It's a teaser date.
The second date is the Average Last Frost Date and is truly an average date. That's the point in the calendar that separates the historical weather data. Half of the freezing days occur before that date and half of the freezing days occur after that date. It is the definitive 50/50 point. It's not the average date that frost "usually" occurs, but a statistical point. If you like to play roulette, you can use that date in your planting calculations. Picking red or black in the casino gives the same odds as choosing to plant tender plants on that date and expecting them to see no more freezing temperatures. The Average Last Frost Date in Colorado Springs is May 4.
The third date is the one you will see most often as the Last Frost Date. That's the date when historical data show there will only be a 10 percent chance of any more days below freezing. It's important to note that the definition still allows for freezing temperatures. It gives you 90 percent probability of warm temperatures, but no guarantee. The Last Frost Date in Colorado Springs is May 15. That's the date that nurseries, gardening experts, and the local newspaper identify as the Last Frost Date. It's a pretty safe bet, about what most casinos pay out with their slot machines.
But the Last Frost Date really isn't the last frost date. Ever notice that there are a lot of people who lose money playing slot machines? There is still a 10 percent chance of freezing weather after the "official" Last Frost Date. Over time, you will see freezing temperatures after that date.
So what is the True Last Frost Date? That's a term I've coined to identify when you are 100 percent sure you won't have any more freezing temperatures. The problem is that it doesn't exist in any official record. And it is a very difficult date to find. NOAA offers a listing of record low temperatures and you can search your area for record lows for each month. It shows that Colorado Springs has seen a temperature of 32F degrees in June, but not on a specific date. I wasn't able to find any source that listed the specific True Last Frost Date for my area, or any other location. Luckily our local newspaper publishes monthly weather temperature charts at the beginning of each month and I pay attention.
Colorado Springs has seen a record low temperature of 32 degrees on June 1. I haven't seen it in the 14 years I've lived here, but it has happened. So, historically, June 1 is the True Last Frost Date for my home. You can do similar research to determine your True Last Frost Date. Why is this important and why doesn't anyone seem to know about it? There are experts that tell you this information, but you have to know how to decipher it.
Look at a seed packet of a warm season plant like pumpkins or melons and you may notice that they recommend planting two weeks after the Last Frost Date. Look at my True Last Frost Date above and you may notice it's just about two weeks after my official Last Frost Date. So you can estimate your True Last Frost Date to be about two weeks after your Last Frost Date.
Local, experienced gardeners wait until the Memorial Day weekend at the end of May to plant tomatoes and other warm season crops, a two week buffer. They've determined from experience when it's safe to plant.
Planting warm season plants after the True Last Frost Date is important because many of them can't handle any temperatures close to freezing. You can look at long-range weather forecasts to determine the best time to plant, but I think using the calendar gives you a better opportunity to develop your plan. Picking a date at least two weeks after your last Frost Date is a good estimate of your True Last Frost Date and as close to a 100 percent prediction as you can make. I call that a safe bet.