Playing the Odds with Frost Dates

Here in Massachusetts, I’ve often gone by the rough guideline of waiting until Memorial Day weekend to put out all but my cold-tolerant plants. It’s not a terrible guideline, but the date of Memorial Day does vary from year to year, from as early as May 25 to as late as May 31. It’s also too late for certain cool weather crops and can unnecessarily cut short your garden’s growing season.

Official frost dates are often given as a single number, such as this date given on the Old Farmer’s Almanac website.

farmers alm frost date

Here, the frost date is listed as April 16, which is quite a difference from the Memorial Day guideline.

A few things can be noted about this chart. (1) It uses weather data from South Weymouth. How close is Braintree’s weather data to South Weymouth? I don’t know, but I’d guess it’s a better comparison than Boston, which is in a slightly warmer growing zone. And (2) spring frost dates given as a single number are generally dates at which there is still a 50% chance of a later spring frost.

How lucky are you feeling at 50-50 odds?

I’d definitely feel okay about putting out kale at this time, which says nothing about my feelings toward kale. (It’s all right.) Kale can take the cold weather.

For a better sense of frost dates, I like this chart from garden.org. (Click here to see the full chart; excerpt is below.)

Temp (in Deg F) 10% 50% 90%
Last 16 Mar 25 Mar 10 Feb 24
Last 20 Mar 31 Mar 19 Mar 8
Last 24 Apr 12 Mar 30 Mar 17
Last 28 Apr 27 Apr 15 Apr 4
Last 32 May 15 Apr 30 Apr 16
Last 36 May 25 May 13 May 1

According to this chart, there’s only a 10% risk that temperatures will drop below 32 degrees F after May 15. Now we’re getting closer to my old rule of thumb. But note that there’s still a 10% risk of 36 degree weather after May 25, which could set back members of the nightshade family (e.g., tomatoes, potatoes, eggplant, peppers) for the rest of the summer. (Side Note: The chart at Garden.com seems to suggest that frost forms at 36 degrees. Not true, but for various reasons, your garden may experience colder temperatures than those measured at weather stations. Using the dates associated with 36 degrees gives you a little bit of insulation from frost.)

Click here if you’d like to see another chart, but don’t get too bogged down in the numbers. We’re looking for guidelines based on data, not one proven date.

Bottom line? “Frost date” is better expressed as a range of dates and probabilities.

Also think about…

  • How cold-tolerant your plants are.
  • How much risk you can handle and how sad you would feel if you froze that one special tomato plant variety you can’t get anywhere else.
  • How willing you are to take some extra measures to protect your plants if a frost bogey threatens crops in late spring/early fall.
  • How long you can hold your young plants in their containers. They’ll take some time to recover if they start to get pot bound before you put them out, which is another kind of risk.

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