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Frost-free dates

Log in a frozen riverI've always been confused by frost free dates, especially after asking two local extension agents for information and getting two very different answers.  So I was thrilled to stumble upon a tool that gives me actual data from local weather stations while also clearing up the mystery.

Frost free dates are like flood zones.  If you live in a wet region like ours, you'll want to head to the local authorities to find a map showing 10 year and 100 year flood zones.  The idea is that, on average, waters will reach the 10 year flood line once every decade and the 100 year flood line once per century.  Of course, this is a statistical tool, not a forecast, so you might get a 100 year flood three years running, or might not get one for 300 years.  But either way, you won't want to build your house in the 10 year flood zone, and probably shouldn't put it in the 100 year flood zone either.

Box elder seeds in the snowSimilarly, frost free dates are reported based on the percentage likelihood of seeing frost on a certain day in the spring.  Using data from our closest weather station, we have a 10% chance of seeing frost as late as May 16, compared to a 90% chance of seeing frost on April 12.  On average, our last frost falls on April 29.  So, in a way, those extension agents were both right.  One was being careful and giving me the date after which frost will nearly never occur while the other was more of a gambler and figured we could plant on the average last frost date.

Snowy cornfieldThe Dave's Garden tool also tells me the chances of seeing 28 degree frosts (which will nip our peach blossoms) and 24 degree frosts (which will kill unprotected broccoli.)  If I hadn't decided to change over to using soil temperature as my method of choosing planting dates, I can tell this tool would be worth its weight in gold.

(In case you're curious, the photos in this post are from our trip to Ohio, where there's still lots of snow on the ground.  You can see my review of Mound City over on our travel blog.)

Our chicken waterer never spills or fills with poop.

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Actually, the way you defined 10 and 100 year floods is a common misperception. "10 year flood" is actually a very poorly worded statement. What it means is that a flood of that magnitude has a 1 in 10 chance of happening in any given year. A 100 year flood has a 1% chance of happening in any given year.
Comment by Edward - If You Can Read, You Can Cook Tue Feb 15 11:59:54 2011
I've got a few US books on gardening, and they always go on about first frost date and last frost date. We don't get frosts here, so I've never been able to work out how to work with the systems in those books.
Comment by Darren (Green Change) Tue Feb 15 16:41:42 2011

Well, actually, according to wikipedia "A 100-year flood has approximately a 63.4% chance of occurring in any 100-year period, not a 100 percent chance of occurring." Not quite sure why that is.... But my point is that in both cases, these are probabilities, and you have to choose what your cutoff point is when dealing with probability.

(Not sure if I'm making sense. I'm fighting off a cold today and my head's a bit fuzzy...)

Comment by anna Tue Feb 15 16:50:33 2011
Darren --- Before I allow myself to moan over the fact that you can probably grow tomato bushes as perennials there, I always remind myself that frost is what gives autumn kale and carrots their sweetness!
Comment by anna Tue Feb 15 18:34:52 2011
Exponents can make ordinary seeming formulas turn numbers funny! I was just thinking earlier about what would the be combined likelihood of a 100-year flood happening in a 100 year period. Thanks for looking it up for me!
Comment by Edward - If You Can Read, You Can Cook Tue Feb 15 18:41:57 2011