A foot of snow. That’s what fell on my garden last week. Twelve inches of heavy, wet, icy snow covered our lawn, bent the branches on our trees, and broke the tender new shoots on my perennials. Yes, I had already planted annuals, but I put them in pots on our deck, which I hauled into the warm house when I saw the forecast. I managed to cover my lettuce and chard, which were already in the ground, but they’re reasonably hardy and did just fine, although they may still decide that they’ve endured a winter and it’s time to bloom, producing a flower stalk instead of the leaves I want. All things considered, however, we did well. Many of our friends and neighbors lost entire trees. I can’t complain.
One of the reasons my losses were so few was that I’m hesitant to plant too early. Several weeks of warm weather had lulled most gardeners in the area into planting tender veggies and flowers, but a snow this late isn’t all that unusual. In fact, for our elevation, our average frost date isn’t until May 21. That was the day we woke up to 12 inches of white.
What’s an average frost date? To a statistician, “average” can mean many things. The three most common are mean, median, and mode. Which one applies in this case?
The mean is the average you learned in school—add up all the numbers and then divide by the number of numbers. So to find the average of 3, 4, 7, 8, and 10, you add 3 + 4 + 7 + 8 + 10, then divide by 4. Adding 3 + 4 + 7 + 8 + 10 = 35, and 35 ÷ 5 = 7. So the mean is 7.
Median is the number in the middle. In this case, I’ve already lined the numbers up in numerical order. There are five numbers, and 7 is the middle one. So the median is 7.
Mode is the number that occurs the most. This list doesn’t have a mode, because every number is different. But consider another list: 5, 6, 6, 6, 8, 8, and 9. The number 6 occurs the most—three times—so the mode is 6.
In the case of an average last frost date, “average” means “mean.” Assign a number value of every day from January 1, which is 1, to December 31, which is (usually) 365. Now go through the weather records and jot down the number of the day that had the last frost for each year. (I’d guess our numbers range from the 120s to the 150s, which covers most of May.) Say you check the last 50 years. You add all the day numbers and divide by 50. The answer is the day of the average (mean) last frost. Technically, it should be called the mean last frost, which somehow seems appropriate!
Thankfully, we don’t need to do this ourselves. I like the calculator at Dave’s Garden, as it provides a bit more information than just a date:davesgarden.com/guides/freeze-frost-dates/. I entered my zip, and got these results (I love the repeated word “almost”):
Each winter, on average, your risk of frost is from September 5 through June 16.
Almost certainly, however, you will receive frost from September 28 through May 21.
You are almost guaranteed that you will not get frost from July 13 through August 14.
Your frost-free growing season is around 81 days.
And I wonder why it’s difficult to garden here?
Note again that this is an average. That means that we’re just as likely to have a last frost after that date as we are before that date. If you’re willing to gamble, you can plant on that date or earlier. I prefer to wait a bit longer.
There’s another aspect to timing your garden than just the minimum air temperature. Soil temperature can be just as important, or even more so. Most annuals and vegetables won’t grow if the soil is too cold. Setting them out too early just exposes them to the risk of freezing temperatures, wind, torrential downpours, and hail… not to mention pests and diseases. And since they’ll just sit there and sulk until their feet are warm, there’s no point. Yes, my house looks like a jungle in May, but I’m willing to crowd the transplants around the windows and under my LED lights until I know it’s reasonably safe.
Finally, it’s easier to be patient if you don’t start your seeds too early. A leggy squash or tomato can guilt you into putting them outside before you should. Pepper seedlings take about ten weeks. Tomatoes are fine with eight, unless you’re willing to keep moving them to bigger pots. Squash and cukes only need three weeks before they’re sprawling every which way. Write the seeding date on your calendar and don’t rush spring.
If the comments on our local gardening group website are any indication, a lot of people got too excited too soon. There are lots of rants, lots of tears, and the local garden centers are rapidly selling out of replacements. I’d rather look at all the snow (that’s still melting off our plants) than deal with fires this summer.