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Plant Hardiness
Plant hardiness is an often misunderstood
topic deserving special discussion. It is not as simple
as it may appear.
The USDA Hardiness Zone map is based on average winter
low temperatures, and doesn't take into account rare
extremes. While the map is based on a tremendous
amount of data, it isn't perfect. Cold temperatures
are only one factor that affect plant hardiness.
Cold temperatures for one night are not the same
as cold temperatures for a period of weeks, even
though the same low temperature is reached in both
cases. In many cases, a low temperature of zero degrees
may cause cellular damage that will start to heal
if the temperature rises rapidly. If the temperatures
remain low for several days, cell damage may continue,
and result in the death of the plant.
Another phenomenon, seen in the cool areas of the
West Coast of the US is the difference in winter
hardiness due to a lack of summer heat. In many plants
native to warmer climates, summer heat causes increased
sugar production, which allows the plants to survive
more stress in the winter. In areas without summer
heat, a particular plant may only be hardy to 20
degrees F, while in an area with hot summers, the
same plant may easily be hardy to 0 degrees F.
Heat hardiness is an issue that has been discussed
recently, and while it is critical to those of
us in warmer zones, the AHS Heat Zone map is a
laughable excuse for a solution. Their heat zones
are based only on the number of days above 86 degrees
F. The heat map does not integrate data for humidity,
or the variance between day and night temperatures
which is far more important in determining how
far south a plant will grow.
For that reason, we have chosen the 1990 (most
recent) USDA Hardiness Map to help you in determining
your Zone. The map is clickable to show you more
detail for your region.
We are also fans of 'pushing the
zone' in our garden to try new and different plants.
If you are brave and like to try plants out of zone,
we would love to hear your results. Email
us to share your experiences.
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