When the Last Frost Date Isn’t the Last Frost Date

The United States abounds with record high temperatures this month and spring arrived early in many places. I’ve been enjoying days that are 15 and even 20 degrees above normal for weeks; unusual is an understatement. At a time of year we normally cringe at the thought of more, freezing overnight temperatures, sunscreen and shade are now on my mind. This is a year when the projected official last frost date won’t even be close. Maybe.

Late spring frost on apricot blossoms

The last frost date for me is still more than two weeks away. That is, the official 90% last frost date is the 15th of May for my area. My garden is at a higher elevation than the city proper so I always add a few days to be safe, but it’s close. As I’ve written before (Mar 18, 2011, “Know Your Last Frost Date“; Mar 29, 2011, “What to Plant Before the Last Frost“; May 13, 2011, “What is the Last Frost Date?“), the “90% last frost date” is the one most often referred to by seed companies, the USDA, NOAA, and anyone else that throws out a date as the last one for gardeners to be concerned about.

Last year I got my last frost on May 15; the model proved true. This year the actual last frost in my garden should be different. In fact, I think the last frost happened yesterday.

Last year's snow in May

Last Frost Dates are determined by taking historical temperature data and finding a symbolic date that meets certain criteria. The “90% last frost date” is the date for a certain area when 90 percent of historical last frosts have already occurred. The “Average Last Frost Date” or “50% last frost date” is the point on the calendar when half of the historical last frosts have already happened. The third type of last frost date is the “10% last frost date”; by this date 10 percent of historical last frosts were final. The key to all of these dates is that they’re determined by using historical dates, days that occurred in the past.

For my garden zone, 10 percent of the last frost dates in years past happened by April 24, and 50 percent happened by May 4. Could this be another one of those kind of years?

As far back as I can remember, in my 15 years living here, the last frost happened in May. I always advise local gardeners to wait until late May to plant warm season plants. Many gardeners new to the area learn the hard way about planting too soon in our high-mountain, weather-crazy area.

This year is one of the ones that cause serious gardeners like me to start biting our nails and tearing out our hair. A look at the long-range weather forecast shows we’ll be well above freezing for the next 10 days. That puts us very close to the 90% last frost date with no worries. Historically, we can point to previous years like this one when no frost occurred in May.

Then again I can point to years when we had frosts in June. A month is a long time in the weather world. Something could be brewing off the coast of northern Alaska right now that won’t be revealed until two or three weeks from now. A blast of Arctic air might descend on my garden when I least expect it. Or not.

The last frost date is just a spot on the calendar for garden planning. It’s up to individual gardeners to determine what they do with it. With any mathematical model, some points of data lie well outside the norm. For a date to be considered average (technically the mean), half of statistical points happened before and half happened after. It’s nice that we gardeners have the 90% and 10% last frost dates at our disposal to develop a more precise planting plan.

As I look at my calendar, every day without frost puts me closer to the mathematical point when future frost isn’t likely to happen. I’ve passed the 10 percent point and should breeze past the 50 percent point with no problem. The 90 percent point looks very promising.

Does that mean I can relax and plant with no frost concerns? That’s a good question and one for which I wish I had the answer. Whatever I choose to do I have to be ready for the consequences… or rewards. Knowing how last frost dates are determined and using that knowledge will help me make an informed decision when it comes to planting.

I’ve just about convinced myself that this is one of those years when the last frost occurs early. Indications are good that warm days and nights ahead will help my plants. The ground is already warmer than it normally is at this point of the year. The weather trends seem positive.

But I’m not a gambler and I’m not ready to bet everything on a statistical probability. I’ll plant tomatoes this week almost a month ahead of normal, but I’ll enclose them in plastic season extenders. I’ll put some beans in the ground, but only in the beds with protective covering. I’ll delay planting squash and pumpkins until I’m absolutely sure the calendar coast is clear.

Usually I wouldn’t be considering such measures at the end of April, but this year is different. Sunny days are too inviting to sit back and wait for historical events to happen. Part of being a gardener is trying new things in the garden and by a small measure I’ll roll the dice a little. We all experience missteps and minor failures in gardening so I’m ready for the worst, but if my conjecture about the last frost date holds true I’ll be able to get a nice jump on the growing season.

I know my last frost date, all three of them actually. I know what they mean. I have experienced the ups and downs of weather’s effect on my garden. Also I’m anxious to get my garden growing. This year my last frost date won’t be an actual last frost date. That’s what I’ve determined based on the data at hand. Or at least that’s what I’m betting on.


3 responses to this post.

  1. We here in Napa had a heat wave and I thought the Last Frost Date was over. I was correct by a couple degrees. Plants ended up being fine. Just plant it. Go with your gut.


  2. I have Amish friends like brothers watch for full moon is almost always a frost In nW Pa I expect it memorial day weekend as we get rain and a frost. My baby plants been frosted and I drench them with water before direct sun and they are fine had garden in 4/15/13 this year which is unheard of


    • Rob, clear, cloudless nights tend to be cooler than cloudy ones so it makes sense that a night where you could see a full moon in spring might have a frost.


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