Using the Heat-Zone Map
Use the AHS Plant Heat-Zone Map in the same way that you do the Hardiness
Map. Start by finding your town or city on the map. The larger versions of
the map have county outlines that may help you do this.
The
12 zones of the map indicate the average number of days each year that a
given region experiences "heat days"-temperatures over 86 degrees (30 degrees
Celsius). That is the point at which plants begin suffering physiological
damage from heat. The zones range from Zone 1 (less than one heat day) to
Zone 12 (more than 210 heat days).
Thousands
of garden plants have now been coded for heat tolerance, with more to come
in the near future. You will see the heat zone designations joining hardiness
zone designations in garden centers, references books, and
catalogs.
On
each plant, there will be four numbers. For example, a tulip may be 3-8,
8-1. If you live in USDA Zone 7 and AHS Zone 7, you will know that you can
leave tulips outdoors in your garden year-round. An ageratum may be 10-11,
12-1. It can withstand summer heat throughout the United States, but will
over winter only in our warmest zones. An English wallflower may be 5-8,
6-1. It is relatively cold hardy, but can't tolerate extreme summer
heat.
Gardeners
categorize plants using such tags as "annual" or "perennial," "temperate"
or "tropical," but these tags can obscure rather than illuminate our
understanding of exactly how plants sense and use the growth-regulating stimuli
sent by their environment.
Many
of the plants that we consider annuals-such as the petunia, coleus, snapdragon,
and vinca-are capable of living for years in a frost-free environment. The
Heat Map will differ from the Hardiness Map in assigning codes to "annuals,"
including vegetables and herbs, and ultimately field crops as
well.
Plants
vary in their ability to withstand heat, not only from species to species
but even among individual plants of the same species! Unusual seasons-fewer
or more hot days than normal-will invariably affect results in your garden.
And even more than with the hardiness zones, we expect gardeners to find
that many plants will survive outside their designated heat zone. This is
because so many other factors complicate a plant's reaction to
heat.
Most
important, the AHS Plant Heat-Zone ratings assume that adequate water is
supplied to the roots of the plant at all times. The accuracy of the zone
coding can be substantially distorted by a lack of water, even for a brief
period in the life of the plant.
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