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AHS Heat-Zones (was re:USDA zones)

-----Original Message-----
From: J Wintermute <jwintermute@idsonline.com>
Subject: Re: USDA zones (was Re: location)
>
>The American Horticultural Society's 1997 heat-zone map is more telling.  I
>don't think it's online (AHS sells it for about $15).  ...
>--Janet
>
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Following is an article I prepared for a local gardening pub awhile ago.  I
believe the USDA, AHS and Sunset zones are more useful for landscape and
perennial plant selection.  -Olin
==========================================
PLANT HEAT-ZONE RATINGS

The American Horticultural Society has developed a USA Plant Heat-Zone Map
that assigns geographical zones of plant tolerance to heat.  The AHS plant
heat-zones complement USDA plant cold-hardiness zones as an additional aid
in plant selection for a given region.  As gardeners know, not all plants
that are cold-hardy in USDA zone 9 (Phoenix AZ) can withstand our intense
summer heat;  it follows that not all plants that are heat-tolerant in the
new AHS heat-zone will also withstand the coldest days in USDA zone 9.  But
individual plants meeting both zone ratings for a given region should be
adaptable.

The map defines 12 color-coded zones based on the number of heat-days
occurring annually in each zone.  A heat-day occurs when the temperature
exceeds 86 degrees F.  The temperature 86 degrees F was chosen because plant
cellular proteins are damages at temperatures above this point.  Zone 1 Has
no heat-days (mountain ranges and much of Alaska), zone 2 has 1-7 heat-days,
zones 3 has 8-14, then it increases in 15 day increments up to zone 6 and in
30 day increments for the remaining zones.  Phoenix and the outlying
south-southwest-west areas fall in zones 11 (181 to 210 heat-days)and 12
(more than 210 heat-days);  the other outlying areas are in zone 10 (151-180
heat-days).  The temperatures are the 22-year statistical averages recorded
from 1974 through 1995.  AHS anticipates published plant profiles will
include both the USDA and AHS zone ratings within a few years.

The Sunset zones, which are true climatic zones, will continue to be the
more useful reference for us.  The USDA and AHS zones are sensitive to
temperature only and do not include other relevant factors such as humidity,
pH, temperature, exposure, wind, elevation, coastal-desert-mountain
influences, etc.

A full-color poster of the map is available for $14.95 from The American
Horticulture Society, 7931 East Blvd. Dr., Alexandria VA, 22308-1300, by
phone from AHS at 703-768-5700, by phone toll free at 800-777-7931, or on
the web at www.ahs.org.

There is also a book, “Heat-Zone Gardening”, by Dr. H. Marc Cathey, who
coordinated the establishment of the heat-zones.  The book contains a
picture gallery, with the temperature profiles, of 460 landscape plants and
herbs. One hundred twenty-five of these plants meet the criteria for both
the USDA and AHS zones for Phoenix.  The profiles list only the genus and
does not provide any details for plants with temperature tolerances that are
species dependent.  There are also a number of plants that are pretty easy
to grow here that do not meet the heat-zone criteria.  List price for the
book is $24.95 but it is available with various discounts at local
bookstores and on the web at www.amazon.com.   Possibly the best use of the
heat-zone ratings for our area is to help identify plants that are
marginally adapted.  Existing Sunset zone ratings appear to be more useful
for our desert climate than either the USDA or AHS zones.




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