hardiness2

In garden situations where snow cover is intermittent or absent, this lack of internal
dormancy control may present a problem. Although Christmas rose is a fine garden
plant in many parts of the US, we have trouble growing in southern New England,
where it is often damaged by snowless cold following mild spells. Ironically, it is a
more satisfactory plant farther north where it is significantly colder but snow cover
comes earlier and stays longer.
Even the hardiest perennials begin to lose cold resistance once their dormancy
period has been satisfied and milder weather spurs them to undo some of their
midwinter preparations: antifreeze levels drop, cells rehydrate, and tender young
roots and shoots spring to life. Correctly timing the true onset of spring is critical for
plants. Grow too early and risk being killed by a late freeze; grow too late and risk
losing ground to competitors. Poorly timed deacclimation is sometimes a problem
when we cultivate plants from very different regions or climates. Though we
cultivate our northeastern trilliums very successfully, we lose the gorgeous Trillium
chloropetalum from California because it begins growing too early during our
unpredictable spring and is cut down by frost. The timing of deacclimation, like the
other stages of winter hardiness, is fine-tuned by natural selection. Those plants that
get it right year after year are the ones most likely to reproduce and pass on their
genetic calendar to the next generation.
In conclusion, to be truly winter hardy n your garden, a particular perennial needs to
possess the ability to read the approach of fall, build up sufficient countermeasures to
combat the damaging effects of ice crystals, winter dampness, and winter burn and to
also read the arrival of spring correctly. As the following graph illustrates, a
perennial can fail to survive the winter at any of these junctures. Species 1
acclimated well but lacked sufficient extreme cold tolerance to survive the
maximum winter low temperature hit in mid-January. Species 2 possessed sufficient
cold-tolerance to survive the entire season, while species 3 weathered the extreme
cold but deacclimated too quickly in spring and perished during a mid-April freeze.

Now this is an important point.
Hardiness zone ratings reflect only a
particular plant's ability to withstand
extreme low mid-winter
temperatures. Just because a fully
dormant individual rated as hardy to
zone 5 can withstand -20o F in
January, this does not mean it can
read daylength or cooling
temperatures correctly in fall or
emerge at the proper time in spring.
Winter hardiness zone ratings should
be used simply as a guide to eliminate
completely appropriate species. Even
so, remember that hardiness zone
ratings are assigned to species with no
regard for the variation in true
hardiness that it may exhibit over a
An Arisaema triphyllum 'Black Jack'
found in the wild in northern Florida
will be unlikely to possess zone 3
hardiness of another plant grown from
wild Vermont seed. Ilarge natural
range. . It is only through first hand
experience and a lot of dead plants
that you can gain a full understanding
of how a particular cultivar, species,
or genus will behave in your garden.