Re: Hardiness and heat zones

**John MacGregor j*@earthlink.net**

on 6/26/03 4:04 PM, Larry Maupin at larrymaupin@sbcglobal.net wrote:

> Mostly correct - actually the USDA hardiness Zone numbers reflect average COLD
> HARDINESS temperatures, not temperature ranges. Although it has become
> generally accepted to say that a plant is hardy in Zones 5-8, which suggests
> that hotter than zone 8 is too hot for this plant, this info is not exactly
> correct as the system was originally intended. For more info read all about it
> at: <http://www.usna.usda.gov/Hardzone/index.html>

Larry,

Quoting from the web page you cite above:

> This map supersedes U.S. Department of Agriculture Miscellaneous Publication
> 814, "Plant Hardiness Zone Map," which was revised in 1965.  This 1990 version
> shows in detail the lowest temperatures that can be expected each year in the
> United States, Canada, and Mexico.  These temperatures are referred to as
> "average annual minimum temperatures" and are based on the lowest temperatures
> recorded for each of the years 1974 to 1986 in the United States and Canada
> and 1971 to 1984 in Mexico.

There are a great many more factors influencing the hardiness of a plant
apart from the lowest minimum temperatures it will endure (which is all that
the USDA map takes into consideration).  For one example, the amount of sun
a plant receives to harden it off before it is subjected to freezing is of
extreme importance, which is why many plants that are fine in parts of
northern California where temperatures are actually colder than they are in
England, will not make it even in Kent unless they are grown against a
south-facing wall and unless the summer and fall have been particularly
sunny.  Other limiting factors are the amount of moisture the plant receives
before, during, and after a freeze, the duration of the cold temperature,
the amount of snow cover, etc., etc., etc.  The USDA map addresses none of
these.

Again, quoting from the page cited above:

> Areas above an arbitrary elevation are traditionally considered unsuitable for
> plant cropping and do not bear appropriate zone designations.  There are also
> island zones that, because of elevation differences, are warmer or cooler than
> the surrounding areas and are given a different zone designation.  Note that
> many large urban areas carry a warmer zone designation than the surrounding
> countryside.  The map-contains as much detail as possible, considering the
vast amount of data on which it is based and its size.

Like the Sunset zone system, the USDA system has undergone refinement, but
it is limited by its own definition of its mission and by the need to
squeeze  so much information into a single small-scale map.

John MacGregor
South Pasadena, CA 91030
USDA zone 9   Sunset zones 21/23

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