Unfortunately, applying the
principles of the hardiness zone concept are not always
straight forward. If you do live in Zone 5 and they
predict -10 degrees one night, are your plants safe?
One factor to consider is that the official
weather forecast temperature is usually recorded at one
specific location and is normally taken from a thermometer
several feet off the ground above where lower growing plants
Also, if your landscape is in a "low
area at the bottom of a hill, the temperature may drop
several degrees lower than the prediction. On the other hand,
if your landscape is located near the top of the hill, odds
are better that the temperature did not go below -10 degrees.
Plants in an open, windswept site may suffer
more damage than those in a protected area. Cold, dry winds will cause desiccation (drying
out) of the plant tissue. This can make buds and twigs more
susceptible to low temperature damage.
The overall health of the plant going into
the winter will also be a factor in how much damage occurs. If the plant was
under-watered or under-nourished or suffered
site stresses the previous summer, it is more likely to be damaged by
Many of us "stretch the zone" and use plants
that are really suited to the next higher numbered hardiness
zone which does not get as cold.
These plants may work out fine for many years until a severely
cold winter happens. It only takes one night of
temperatures below the minimum for which your plants are
rated to cause damage or even death.
Another factor is that various tissues on a plant respond
differently to extreme cold. Generally, flower buds formed the
previous autumn are least hardy. If these are damaged or
killed, flowering shrubs or trees
that set their buds during the
fall may not be very showy in the spring following a
buds which will become leaves are hardier than flower buds.
Twigs are hardier than buds and major branches and the trunk
are the hardiest of the above ground parts of the plant.
What can you do if winter
temperatures are predicted to drop below the hardiness
level of your plants? For existing plants in your
answer is, "Nothing."
If you or someone in the past
selected and installed plants appropriate for your
hardiness zone, they should do just fine. However, if
you "stretched the zone" with more tender trees and
shrubs, only time will tell if they will survive.
For the future, use only plants known to be hardy
your particularly hardiness zone or lower. Keep plants as healthy and stress free as
possible during the growing season but do not stimulate lush
new growth in the fall by pruning or fertilizing late in the
If you feel a strong urge to grow
marginally hardy plants, located them in the high ground
on your site and/or provide some type of protection
against the raw winter winds.