Killing Frost versus Hard Freeze

Interestingly enough, meteorologists consider the “first frost” the end of the growing season while we gardeners know that it often takes a “hard freeze” or a “killing freeze” to really end the growing season.  I ran into this distinction a week or two ago–maybe longer–when I was having an email “conversation” with a friend in Texas and I mentioned that we’d had our first frost.  In the back and forth that followed, I must have said something–or perhaps she did–about things still growing–and that’s how we had the discussion.

Interestingly enough, all of the weather-related data that one can look up will give the “first frost” and “last frost” dates for a place, but there aren’t really any places to go to look for dates of the first killing freeze in an area–and for many gardeners, this is what really matters.

What’s the distinction?  A first frost or killing frost will kill off tender vegetation.  Thus, anything that needs temperatures above about 40 degrees or so to do well–think tomatoes, peppers, basil, impatiens, for example–will be damaged or killed by this frost.

Why do I say 40 degrees?  Well, the ambient air temperatures are actually a wee bit higher than temperatures “at the surface,” as they say.  So if the temperature in the air drops to 40, it’s likely the temperature on your plants–depending on your particular location and “microclimate”–will be as much as 4 degrees lower.  36 degrees in the air is often a “killing frost.”  Go figure, right?

But as gardeners know, plenty of plants–perennials, woody shrubs, and even some “half-hardy annuals”, as they call them, take a frost just fine and continue to grow and even bloom right up until the first “hard” or “killing” freeze.

After my frost, I had roses, hydrangeas, torenias (an annual) and several other things still in bloom. I have no real photos because the hurricane winds beat the heck out of these plants–or I cut them back to spare them.

The difference in time period in my yard between “frost” and “freeze” is about a month.  This year the frost was on October 12 (the average date is October 9, so we were right around average).  My freeze occurred on November 6.  In warmer years, the difference can be longer.

And the warm period after that freeze is what’s known as “Indian Summer.”  No matter where you’re located, that doesn’t occur until November.  It’s a mistake to think it occurs in October–that again is just misinformation.

So with the difference between frost and freeze established, gardeners can cover some tender things and generally get a good month–or sometimes longer–out of the growing season!

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