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  • From: N Sterman <TalkingPoints@PlantSoup.Com>
  • To: gardenwriters-on-gardening <gwl-g@lists.ibiblio.org>
  • Subject: Re: [gwl-g] Last Frost Date
  • Date: Thu, 27 Apr 2006 08:17:33 -0700

Hi Ellen,

Frost dates are calculated as 30 year averages.  Here in San Diego, they are published in  a compilation book entitled Climates of San Diego County: Agricultural Relationships.  That book contains topographical-style maps that indicate first and last frost dates for the county.  First frost maps are separated from last frost as not every area has the same first/last frost dates.

That book is a publication of the University of California Agricultural Extension Service; AND the Environmental Science Services Administration; AND the United States Weather Bureau.

So all three agencies were involved.  Our most recent edition was published in 1970 so, as many others pointed out, frost dates are extremely outdated.  That said, San Diego (and all of California) has such varied topography that we have tremendous variations over very small areas (like within a single suburban block) which translates to a kazillion microclimates.  

Bottom line is, when talking about frost dates, they are merely rules of thumb.  

Nan



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