always been confused by frost
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.
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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.
Similarly, 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.
The 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.)