Newsgroups: rec.gardens
From: james@hplb.hpl.hp.com (James Harrison)
Subject: SUMMARY of zone info for non-US residents
Date: Thu, 10 Nov 1994 12:05:18 GMT

I had many responses to my question about how to work out which zone
you correspond to if you are not in North America. Here is a summary
of those which cover the general theory. Thanks also to those who
answered my specific question which I won't reproduce since it's
specific to my location...

Georgina J. Allbrook (GALLBROOK@waikato.ac.nz) forwarded an article by
Roger Case (rcase@RT66.com) who wrote:
> Some genius at the United States Department of Agriculture (USDA)
> came up with this zone system for rating hardiness.  It's based on
> the lowest approximate range of average annual minimum temperatures
> where a species will grow. Naturally, it uses good ol' Fahrenheit
> temperatures. (Hey, we'll go metric when we're good and ready. Now's
> not the time.) I guess it's simply easier to say "Zone 5" than to
> say "minus 20 degrees Fahrenheit to minus 10 degrees Fahrenheit
> approximate range of average annual minimum temperature" -- once you
> get everyone, nurseries, writers, consumers, etc., to agree on the
> system. But, I still try to give the range in addition to the zone
> number because that has a wee bit more meaning (unless you don't
> speak Fahrenheit; get out your calculator!).

> The table is simple:

> Zone     Approximate range of average annual minimum temperatures
>
>  1       below  -50 F   [Not even in Minnesota!]
>  2       -50 to -40 F
>  3       -40 to -30 F
>  4       -30 to -20 F
>  5       -20 to -10 F
>  6       -10 to   0 F
>  7         0 to  10 F
>  8        10 to  20 F
>  9        20 to  30 F
> 10        30 to  40 F

> But the maps are complex.  (and only for the US --the question Hud
> is responding to was from the UK and much like yours, o.e. "what are
> these zones referred to in the correspondence of this newsgroup and
> other gardening sources)

> As far as I know, it's just a US thing, though the zone maps I see
> always seem to include Canada. For the east, it's pretty easy to
> read the maps because the zones are in fairly broad bands. Out west,
> with mountains making things more interesting, the zones are really
> convoluted. To be most sure of your zone you need to consult a
> detailed map, or just know your climate well, even in the east where
> microclimates can push you a zone away from your nearby neighbors.

> Now, why they didn't just decide to rate hardiness with a simple
> number corresponding precisely to the average annual minimum
> temperature (or maybe the nearest multiple of 10, if we must be
> inaccurate) that a species could tolerate, I'll never know.

Georgina J. Allbrook (GALLBROOK@waikato.ac.nz) wrote:
>         I asked this question last week, and have had a few
> responses.  The crux of the matter is that the zone only refers to
> the minimum temperature and does not take into account the amount of
> rain or the maximum temperature.  I live in the North Island of New
> Zealand and as far as I can tell I live in zone 9 or 10.  This
> corresponds to Florida or California, I feel that this is incredible
> because it is much hotter in these states.  It is unfortunate that
> your posting is in Farenheit, the replies I got were in Celcius.
> The zones are as follows;

>         zone 1  -45
>              2  -45 - -40
>              3  -40 - -34
>              4  -34 - -28
>              5  -28 - -23
>              6  -23 - -17
>              7  -17 - -12
>              8  -12 - -6
>              9  -6  - -1
>             10  -1  -  4

Jerry Heverly(heverlyj@ccnet.com) wrote:
> According to my sources there is no one accepted Zone map.
> According to Wyman's Gardening Encyclopedia(MacMillan) and Taylor's
> Gardening Enclyclopedia(Houghton Mifflin, Cambridge) there are
> several discordant systems out there.

>         "The Hardiness Zone{Wyman says}...is based solely on average
> annual minimum temperatures.  It...has been in use...since 1938.
> {I}t was based on records summarized by the U.S. Weather Bureau over
> a 40-year period.  Data from Canada was supplied by the
> Meteorological Division, Department of Transport of the Canadian
> Government.  The U.S. and Canada are arbitrarily divided into 10
> zones, 9 of which are in the U.S.  These zones are based on 5-, 10-
> or 15-degree differences in the average minimum temperatures."
>         Taylor says: "While many refinements of method have been
> used for plotting such zones, perhaps the most readily workable one
> is the plan evolved by Alfred Rehder of considering the lowest mean
> temperature of the coldest month."
>         Apparently new maps were developed by a)the U.S. Dept of
> Agriculture in 1959-60 and b) by Canada somewhat later.  Neither map
> jived completely with the 1938 system and much confusion seems to
> have resulted.  The Canadians established nine new zones just within
> their own nation with no connection with the U.S. map.
>         The Wyman map is one number higher than the Taylor map(Wyman
> 10=Taylor 9, etc.).  What's REALLY confusing is that the two maps
> have dramatically different 'Average annual minimums'.  For instance
> my area of California(zone 8 Taylor, zone 9 Wyman) is listed as:
>         Wyman  30-40 degrees F.
>         Taylor 50-60 degrees F!
>         I know, from record-keeping done by students at my college,
> that Wyman is correct, Taylor incorrect.  (Wyman is dated 1971,
> Taylor 1938 with updates in 1959).
>         I started this reply thinking I could help you(and me) but,
> as you can see I'm more confused than when I started.  If someone
> wiser does not answer you before this weekend I intend to do more
> research on this subject at the Univ of Cal. Biosciences Library.
> Till then,