Hardiness
of plants is a term used to describe their ability to survive
adverse growing conditions. It is usually limited to discussions of
climatic adversity. Thus a plant's ability to tolerate cold, heat,
drought, or wind are typically considered measurements of hardiness.
In temperate latitudes, the term is most often used to describe
resistance to cold, or cold-hardiness and generally measured
by the lowest temperatures that a plant can withstand. The hardiness
of a plant is usually divided into three categories; tender,
half-hardy and hardy.  Plants vary a lot in their
tolerance of growing conditions. The selection or breeding of
varieties capable of withstanding particular climates forms an
important part of agriculture and horticulture. Plants can adapt to
some extent to changes in climate. Part of the work of nursery
growers of plants consists of hardening (or hardening off)
their plants, to prepare them for likely conditions in their later
life.
The hardiness of plants is
defined by their native extent's geographic location: longitude,
latitude and elevation. These attributes are often simplified to
define a
hardiness zone.
Zone maps as
guides to plant hardiness
There are several maps showing
plant hardiness zones, but the majority of gardeners in North
America use the US Department of Agriculture's (USDA) plant
hardiness zone map (shown here).
The USDA hardiness zone map
includes Canada
and is based on minimum winter temperatures.
The Canadian map is
based on a wider array of meteorological and horticultural
information, including observations of what plants grow where, but
doesn't include the
US.
For
both zone maps, here's the bottom line:
-
Just like a thermometer, the higher the number, the warmer the
region. For example, a plant rated as hardy to Zone 5 is not
recommended for the colder Zones 4 to 1, but can usually be
grown into the warmer zones.
-
The USDA hardiness areas have been divided into 11 zones, with
Zone 0 as the coldest. Other zones are progressively milder
going up to 11, which includes tropical parts of
Florida and
California.
Click here for the USDA hardiness
zone map.
-
The
Canadian zone map (shown here) also has Zone 0 as its
coldest, but here Zone 9 is the warmest.
-
If you are a Canadian gardener, it helps to know both your
Canadian and USDA zones because many plants imported from the
US
have tags listing the USDA hardiness zone. USDA zone numbers are
usually one lower: for example, if you garden in Canadian Zone
6, as I do, your USDA hardiness is Zone 5.
-
Hardiness is genetic. Some plants are naturally more tolerant of
cold temperatures than others, and some cultivars of the same
plant may be hardier because they were bred or selected from
plants that are tougher than their cousins.
-
The hardiness zone number will help you decide which woody
plants will grow in your climate. When it comes to perennials,
however, gardeners discover that they can grow plants that books
say aren't hardy in their region. Winter rain, ice and
freeze-thaw patterns can often kill plants in milder regions
that do fine in northerly regions that get a thick blanket of
snow all winter long.
-
Gardeners in the colder regions envy southern gardeners who can
grow plants like camellias outdoors. However, plants such as
peonies and daffodils actually need a period of cold weather to
thrive, which means they sulk in the hot and humid south. The
American Horticultural Society developed a
heat zone map to help southern gardeners determine what
plants they can grow
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