The American Horticulture Society (AHS) Heat Zone Map
The significance of winter's lowest temperatures decreases as we shift from
places where winter freezes may kill many plants to areas where freezes merely
mean frost on lawns and windshields. Obviously, winter lows above 20°
F, and especially lows in the high 20s, are much less damaging than lower
temperatures are. But on the other hand, areas with mild winter temperatures
often have soaring summer temperatures. Gardeners have discovered that summer
high temperatures can limit plant survival just as surely as winter low
temperatures can. That's why the American Horticultural Society (AHS) published
a map (also created by Cathey) that takes heat into account. Called The Heat
Map, this 12-zone isotherm map indicates the average number of days each
year when given regions experience temperatures of 86° F or higher.
According to the AHS, that's the temperature at which many common plants
begin to suffer physiological damage. The zones range from 1 (one day or
less at 86° F or warmer) through 12 (210 days or more per year at 86°
F or
warmer). More
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The Sunset Garden Climate Zone Map
This map has more in common with the Koppen or Ecoregion Map than with
Humboldt's. Long the standard among gardeners who live in the 13 western
states, the map now extends to the Eastern Seaboard. Dividing the United
States into 45 garden zones, the Sunset map first and foremost recognizes
the difference between garden climates in the East and West. Sunset zones
1 through 24 are west of the 100th meridian, and zones 25 through 45 are
east of it. This zone-mapping system acknowledges the West's complex gardening
regions and recognizes that in many cases neither minimum nor maximum
temperatures determine a plant's survival. Like Koppen's zones, Sunset's
are based on regions where particular plants grow, not on regions that share
a feature such as temperature or heat; instead of matching a plant to an
established zone, the zone is created to match the plants.
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The National Gardening Association
The 14 regions of the National Gardening Zone Map descend directly from
the Koppen-type map that organizes regions of similiar native vegetation.
It's usefulness comes into play when the zones are correlated to the USDA,
average minimum-temperature zones. When a plant or activity is recommended
for a particular region and zone within that region, the gardener has
significantly more assurance of success compared to advice for the USDA Zone
alone. For this reason primarily, this approach has within the past year
become the most popular Internet-based zone
system.
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