When Is the Last Frost?

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This last week of gorgeous spring weather has certainly brought out the crowds at the garden centers and home improvement stores. When I visited last weekend, carts full of geraniums, tomatoes, and other tender annuals were lined up at the checkout.

Today, the forecast is for snow. It was 30 degrees when I got up this morning. There was frost on the parked cars. As I type, big flakes are softly landing on the freshly turned soil out my window. I wondered how many of the people I’d seen at the store had gone home and planted their flowers, only to find them blackened after the sub-freezing night.

A bit of research will tell you that the average last frost date for Colorado Springs is May 15. The part most people forget is the important word, “average.” Somehow, we get the impression that it will not frost after this day, but that’s not at all what it means.

Some years, we have no frost at all in May. Other years we get frost in June. I vividly remember the year we received a heavy snowfall on the first day of summer vacation! Our kids were shocked, and furious that they’d missed out on a likely snow day.

The average last frost date is a helpful guide, but it’s not at all reliable. It’s just one statistic. For example, according to CSU Extension, if you live in Colorado Springs (at 6,170 feet elevation), the latest frost date on record was June 3, 1951. (I live outside of town at 7,000 feet, and had killing frosts later than that—in mid-June!—in 2008 and 2009.)

Another useful measurement is soil temperature. Be sure to measure in the root zone, six to twelve inches deep. Most veggie plants won’t do much growing until the soil reaches around 60°F. You can put a tomato in the ground in the middle of April,  swathed in plastic to protect the foliage, and it’ll still be sitting there a month later, the same size as when it was planted.

A better alternative is to pre-heat the soil, using clear or black plastic. (Clear plastic traps more heat, but allows weeds to grow underneath.) Then plant into the warmed soil. Of course, you’ll still need to protect the top growth until the weather stabilizes.

As a last resort, you can always wait until your average date, and then check the weather predictions. I’m not sure how reliable that is—Colorado forecasts are notorious for being works of fiction—but at least it’s one more variable you can add to your equation.

Hopefully you won’t be caught like I was several years ago. A Saturday in late May dawned warm and sunny. By ten o’clock it was 72° and I was happily working in the garden, setting out the tomato and pepper transplants that I had nurtured for the last two months. The sky was blue, the bird were chirping. Four hours later the temperature was in the mid-20s, and snow and sleet stung my face as I hastily unearthed my hapless seedlings, sticking them back into their plastic pots and hauling them into the house to wait out the storm.

When it comes to weather, Colorado keeps the gardener guessing!

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